diff a/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp b/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp
@@ -1,7 +1,8 @@
 /*
  * Copyright (c) 2016, 2021, Red Hat, Inc. All rights reserved.
+ * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.
@@ -22,257 +23,1184 @@
  *
  */
 
 #include "precompiled.hpp"
 #include "gc/shared/tlab_globals.hpp"
+#include "gc/shenandoah/shenandoahAffiliation.hpp"
 #include "gc/shenandoah/shenandoahFreeSet.hpp"
 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
 #include "gc/shenandoah/shenandoahHeapRegionSet.hpp"
 #include "gc/shenandoah/shenandoahMarkingContext.inline.hpp"
+#include "gc/shenandoah/shenandoahOldGeneration.hpp"
+#include "gc/shenandoah/shenandoahYoungGeneration.hpp"
+#include "gc/shenandoah/shenandoahSimpleBitMap.hpp"
+#include "gc/shenandoah/shenandoahSimpleBitMap.inline.hpp"
 #include "logging/logStream.hpp"
 #include "memory/resourceArea.hpp"
 #include "runtime/orderAccess.hpp"
 
+static const char* partition_name(ShenandoahFreeSetPartitionId t) {
+  switch (t) {
+    case ShenandoahFreeSetPartitionId::NotFree: return "NotFree";
+    case ShenandoahFreeSetPartitionId::Mutator: return "Mutator";
+    case ShenandoahFreeSetPartitionId::Collector: return "Collector";
+    case ShenandoahFreeSetPartitionId::OldCollector: return "OldCollector";
+    default:
+      ShouldNotReachHere();
+      return "Unrecognized";
+  }
+}
+
+class ShenandoahLeftRightIterator {
+private:
+  idx_t _idx;
+  idx_t _end;
+  ShenandoahRegionPartitions* _partitions;
+  ShenandoahFreeSetPartitionId _partition;
+public:
+  explicit ShenandoahLeftRightIterator(ShenandoahRegionPartitions* partitions, ShenandoahFreeSetPartitionId partition, bool use_empty = false)
+    : _idx(0), _end(0), _partitions(partitions), _partition(partition) {
+    _idx = use_empty ? _partitions->leftmost_empty(_partition) : _partitions->leftmost(_partition);
+    _end = use_empty ? _partitions->rightmost_empty(_partition) : _partitions->rightmost(_partition);
+  }
+
+  bool has_next() const {
+    if (_idx <= _end) {
+      assert(_partitions->in_free_set(_partition, _idx), "Boundaries or find_last_set_bit failed: " SSIZE_FORMAT, _idx);
+      return true;
+    }
+    return false;
+  }
+
+  idx_t current() const {
+    return _idx;
+  }
+
+  idx_t next() {
+    _idx = _partitions->find_index_of_next_available_region(_partition, _idx + 1);
+    return current();
+  }
+};
+
+class ShenandoahRightLeftIterator {
+private:
+  idx_t _idx;
+  idx_t _end;
+  ShenandoahRegionPartitions* _partitions;
+  ShenandoahFreeSetPartitionId _partition;
+public:
+  explicit ShenandoahRightLeftIterator(ShenandoahRegionPartitions* partitions, ShenandoahFreeSetPartitionId partition, bool use_empty = false)
+    : _idx(0), _end(0), _partitions(partitions), _partition(partition) {
+    _idx = use_empty ? _partitions->rightmost_empty(_partition) : _partitions->rightmost(_partition);
+    _end = use_empty ? _partitions->leftmost_empty(_partition) : _partitions->leftmost(_partition);
+  }
+
+  bool has_next() const {
+    if (_idx >= _end) {
+      assert(_partitions->in_free_set(_partition, _idx), "Boundaries or find_last_set_bit failed: " SSIZE_FORMAT, _idx);
+      return true;
+    }
+    return false;
+  }
+
+  idx_t current() const {
+    return _idx;
+  }
+
+  idx_t next() {
+    _idx = _partitions->find_index_of_previous_available_region(_partition, _idx - 1);
+    return current();
+  }
+};
+
+#ifndef PRODUCT
+void ShenandoahRegionPartitions::dump_bitmap() const {
+  log_debug(gc)("Mutator range [" SSIZE_FORMAT ", " SSIZE_FORMAT "], Collector range [" SSIZE_FORMAT ", " SSIZE_FORMAT
+                "], Old Collector range [" SSIZE_FORMAT ", " SSIZE_FORMAT "]",
+                _leftmosts[int(ShenandoahFreeSetPartitionId::Mutator)],
+                _rightmosts[int(ShenandoahFreeSetPartitionId::Mutator)],
+                _leftmosts[int(ShenandoahFreeSetPartitionId::Collector)],
+                _rightmosts[int(ShenandoahFreeSetPartitionId::Collector)],
+                _leftmosts[int(ShenandoahFreeSetPartitionId::OldCollector)],
+                _rightmosts[int(ShenandoahFreeSetPartitionId::OldCollector)]);
+  log_debug(gc)("Empty Mutator range [" SSIZE_FORMAT ", " SSIZE_FORMAT
+                "], Empty Collector range [" SSIZE_FORMAT ", " SSIZE_FORMAT
+                "], Empty Old Collecto range [" SSIZE_FORMAT ", " SSIZE_FORMAT "]",
+                _leftmosts_empty[int(ShenandoahFreeSetPartitionId::Mutator)],
+                _rightmosts_empty[int(ShenandoahFreeSetPartitionId::Mutator)],
+                _leftmosts_empty[int(ShenandoahFreeSetPartitionId::Collector)],
+                _rightmosts_empty[int(ShenandoahFreeSetPartitionId::Collector)],
+                _leftmosts_empty[int(ShenandoahFreeSetPartitionId::OldCollector)],
+                _rightmosts_empty[int(ShenandoahFreeSetPartitionId::OldCollector)]);
+
+  log_debug(gc)("%6s: %18s %18s %18s %18s", "index", "Mutator Bits", "Collector Bits", "Old Collector Bits", "NotFree Bits");
+  dump_bitmap_range(0, _max-1);
+}
+
+void ShenandoahRegionPartitions::dump_bitmap_range(idx_t start_region_idx, idx_t end_region_idx) const {
+  assert((start_region_idx >= 0) && (start_region_idx < (idx_t) _max), "precondition");
+  assert((end_region_idx >= 0) && (end_region_idx < (idx_t) _max), "precondition");
+  idx_t aligned_start = _membership[int(ShenandoahFreeSetPartitionId::Mutator)].aligned_index(start_region_idx);
+  idx_t aligned_end = _membership[int(ShenandoahFreeSetPartitionId::Mutator)].aligned_index(end_region_idx);
+  idx_t alignment = _membership[int(ShenandoahFreeSetPartitionId::Mutator)].alignment();
+  while (aligned_start <= aligned_end) {
+    dump_bitmap_row(aligned_start);
+    aligned_start += alignment;
+  }
+}
+
+void ShenandoahRegionPartitions::dump_bitmap_row(idx_t region_idx) const {
+  assert((region_idx >= 0) && (region_idx < (idx_t) _max), "precondition");
+  idx_t aligned_idx = _membership[int(ShenandoahFreeSetPartitionId::Mutator)].aligned_index(region_idx);
+  uintx mutator_bits = _membership[int(ShenandoahFreeSetPartitionId::Mutator)].bits_at(aligned_idx);
+  uintx collector_bits = _membership[int(ShenandoahFreeSetPartitionId::Collector)].bits_at(aligned_idx);
+  uintx old_collector_bits = _membership[int(ShenandoahFreeSetPartitionId::OldCollector)].bits_at(aligned_idx);
+  uintx free_bits = mutator_bits | collector_bits | old_collector_bits;
+  uintx notfree_bits =  ~free_bits;
+  log_debug(gc)(SSIZE_FORMAT_W(6) ": " SIZE_FORMAT_X_0 " 0x" SIZE_FORMAT_X_0 " 0x" SIZE_FORMAT_X_0 " 0x" SIZE_FORMAT_X_0,
+                aligned_idx, mutator_bits, collector_bits, old_collector_bits, notfree_bits);
+}
+#endif
+
+ShenandoahRegionPartitions::ShenandoahRegionPartitions(size_t max_regions, ShenandoahFreeSet* free_set) :
+    _max(max_regions),
+    _region_size_bytes(ShenandoahHeapRegion::region_size_bytes()),
+    _free_set(free_set),
+    _membership{ ShenandoahSimpleBitMap(max_regions), ShenandoahSimpleBitMap(max_regions) , ShenandoahSimpleBitMap(max_regions) }
+{
+  make_all_regions_unavailable();
+}
+
+inline bool ShenandoahFreeSet::can_allocate_from(ShenandoahHeapRegion *r) const {
+  return r->is_empty() || (r->is_trash() && !_heap->is_concurrent_weak_root_in_progress());
+}
+
+inline bool ShenandoahFreeSet::can_allocate_from(size_t idx) const {
+  ShenandoahHeapRegion* r = _heap->get_region(idx);
+  return can_allocate_from(r);
+}
+
+inline size_t ShenandoahFreeSet::alloc_capacity(ShenandoahHeapRegion *r) const {
+  if (r->is_trash()) {
+    // This would be recycled on allocation path
+    return ShenandoahHeapRegion::region_size_bytes();
+  } else {
+    return r->free();
+  }
+}
+
+inline size_t ShenandoahFreeSet::alloc_capacity(size_t idx) const {
+  ShenandoahHeapRegion* r = _heap->get_region(idx);
+  return alloc_capacity(r);
+}
+
+inline bool ShenandoahFreeSet::has_alloc_capacity(ShenandoahHeapRegion *r) const {
+  return alloc_capacity(r) > 0;
+}
+
+inline idx_t ShenandoahRegionPartitions::leftmost(ShenandoahFreeSetPartitionId which_partition) const {
+  assert (which_partition < NumPartitions, "selected free partition must be valid");
+  idx_t idx = _leftmosts[int(which_partition)];
+  if (idx >= _max) {
+    return _max;
+  } else {
+    // Cannot assert that membership[which_partition.is_set(idx) because this helper method may be used
+    // to query the original value of leftmost when leftmost must be adjusted because the interval representing
+    // which_partition is shrinking after the region that used to be leftmost is retired.
+    return idx;
+  }
+}
+
+inline idx_t ShenandoahRegionPartitions::rightmost(ShenandoahFreeSetPartitionId which_partition) const {
+  assert (which_partition < NumPartitions, "selected free partition must be valid");
+  idx_t idx = _rightmosts[int(which_partition)];
+  // Cannot assert that membership[which_partition.is_set(idx) because this helper method may be used
+  // to query the original value of leftmost when leftmost must be adjusted because the interval representing
+  // which_partition is shrinking after the region that used to be leftmost is retired.
+  return idx;
+}
+
+void ShenandoahRegionPartitions::make_all_regions_unavailable() {
+  for (size_t partition_id = 0; partition_id < IntNumPartitions; partition_id++) {
+    _membership[partition_id].clear_all();
+    _leftmosts[partition_id] = _max;
+    _rightmosts[partition_id] = -1;
+    _leftmosts_empty[partition_id] = _max;
+    _rightmosts_empty[partition_id] = -1;;
+    _capacity[partition_id] = 0;
+    _used[partition_id] = 0;
+  }
+  _region_counts[int(ShenandoahFreeSetPartitionId::Mutator)] = _region_counts[int(ShenandoahFreeSetPartitionId::Collector)] = 0;
+}
+
+void ShenandoahRegionPartitions::establish_mutator_intervals(idx_t mutator_leftmost, idx_t mutator_rightmost,
+                                                             idx_t mutator_leftmost_empty, idx_t mutator_rightmost_empty,
+                                                             size_t mutator_region_count, size_t mutator_used) {
+  _leftmosts[int(ShenandoahFreeSetPartitionId::Mutator)] = mutator_leftmost;
+  _rightmosts[int(ShenandoahFreeSetPartitionId::Mutator)] = mutator_rightmost;
+  _leftmosts_empty[int(ShenandoahFreeSetPartitionId::Mutator)] = mutator_leftmost_empty;
+  _rightmosts_empty[int(ShenandoahFreeSetPartitionId::Mutator)] = mutator_rightmost_empty;
+
+  _region_counts[int(ShenandoahFreeSetPartitionId::Mutator)] = mutator_region_count;
+  _used[int(ShenandoahFreeSetPartitionId::Mutator)] = mutator_used;
+  _capacity[int(ShenandoahFreeSetPartitionId::Mutator)] = mutator_region_count * _region_size_bytes;
+
+  _leftmosts[int(ShenandoahFreeSetPartitionId::Collector)] = _max;
+  _rightmosts[int(ShenandoahFreeSetPartitionId::Collector)] = -1;
+  _leftmosts_empty[int(ShenandoahFreeSetPartitionId::Collector)] = _max;
+  _rightmosts_empty[int(ShenandoahFreeSetPartitionId::Collector)] = -1;
+
+  _region_counts[int(ShenandoahFreeSetPartitionId::Collector)] = 0;
+  _used[int(ShenandoahFreeSetPartitionId::Collector)] = 0;
+  _capacity[int(ShenandoahFreeSetPartitionId::Collector)] = 0;
+}
+
+void ShenandoahRegionPartitions::establish_old_collector_intervals(idx_t old_collector_leftmost, idx_t old_collector_rightmost,
+                                                                   idx_t old_collector_leftmost_empty,
+                                                                   idx_t old_collector_rightmost_empty,
+                                                                   size_t old_collector_region_count, size_t old_collector_used) {
+  _leftmosts[int(ShenandoahFreeSetPartitionId::OldCollector)] = old_collector_leftmost;
+  _rightmosts[int(ShenandoahFreeSetPartitionId::OldCollector)] = old_collector_rightmost;
+  _leftmosts_empty[int(ShenandoahFreeSetPartitionId::OldCollector)] = old_collector_leftmost_empty;
+  _rightmosts_empty[int(ShenandoahFreeSetPartitionId::OldCollector)] = old_collector_rightmost_empty;
+
+  _region_counts[int(ShenandoahFreeSetPartitionId::OldCollector)] = old_collector_region_count;
+  _used[int(ShenandoahFreeSetPartitionId::OldCollector)] = old_collector_used;
+  _capacity[int(ShenandoahFreeSetPartitionId::OldCollector)] = old_collector_region_count * _region_size_bytes;
+}
+
+void ShenandoahRegionPartitions::increase_used(ShenandoahFreeSetPartitionId which_partition, size_t bytes) {
+  assert (which_partition < NumPartitions, "Partition must be valid");
+  _used[int(which_partition)] += bytes;
+  assert (_used[int(which_partition)] <= _capacity[int(which_partition)],
+          "Must not use (" SIZE_FORMAT ") more than capacity (" SIZE_FORMAT ") after increase by " SIZE_FORMAT,
+          _used[int(which_partition)], _capacity[int(which_partition)], bytes);
+}
+
+inline void ShenandoahRegionPartitions::shrink_interval_if_range_modifies_either_boundary(
+  ShenandoahFreeSetPartitionId partition, idx_t low_idx, idx_t high_idx) {
+  assert((low_idx <= high_idx) && (low_idx >= 0) && (high_idx < _max), "Range must span legal index values");
+  if (low_idx == leftmost(partition)) {
+    assert (!_membership[int(partition)].is_set(low_idx), "Do not shrink interval if region not removed");
+    if (high_idx + 1 == _max) {
+      _leftmosts[int(partition)] = _max;
+    } else {
+      _leftmosts[int(partition)] = find_index_of_next_available_region(partition, high_idx + 1);
+    }
+    if (_leftmosts_empty[int(partition)] < _leftmosts[int(partition)]) {
+      // This gets us closer to where we need to be; we'll scan further when leftmosts_empty is requested.
+      _leftmosts_empty[int(partition)] = _leftmosts[int(partition)];
+    }
+  }
+  if (high_idx == _rightmosts[int(partition)]) {
+    assert (!_membership[int(partition)].is_set(high_idx), "Do not shrink interval if region not removed");
+    if (low_idx == 0) {
+      _rightmosts[int(partition)] = -1;
+    } else {
+      _rightmosts[int(partition)] = find_index_of_previous_available_region(partition, low_idx - 1);
+    }
+    if (_rightmosts_empty[int(partition)] > _rightmosts[int(partition)]) {
+      // This gets us closer to where we need to be; we'll scan further when rightmosts_empty is requested.
+      _rightmosts_empty[int(partition)] = _rightmosts[int(partition)];
+    }
+  }
+  if (_leftmosts[int(partition)] > _rightmosts[int(partition)]) {
+    _leftmosts[int(partition)] = _max;
+    _rightmosts[int(partition)] = -1;
+    _leftmosts_empty[int(partition)] = _max;
+    _rightmosts_empty[int(partition)] = -1;
+  }
+}
+
+inline void ShenandoahRegionPartitions::shrink_interval_if_boundary_modified(ShenandoahFreeSetPartitionId partition, idx_t idx) {
+  shrink_interval_if_range_modifies_either_boundary(partition, idx, idx);
+}
+
+inline void ShenandoahRegionPartitions::expand_interval_if_boundary_modified(ShenandoahFreeSetPartitionId partition,
+                                                                             idx_t idx, size_t region_available) {
+  if (_leftmosts[int(partition)] > idx) {
+    _leftmosts[int(partition)] = idx;
+  }
+  if (_rightmosts[int(partition)] < idx) {
+    _rightmosts[int(partition)] = idx;
+  }
+  if (region_available == _region_size_bytes) {
+    if (_leftmosts_empty[int(partition)] > idx) {
+      _leftmosts_empty[int(partition)] = idx;
+    }
+    if (_rightmosts_empty[int(partition)] < idx) {
+      _rightmosts_empty[int(partition)] = idx;
+    }
+  }
+}
+
+void ShenandoahRegionPartitions::retire_range_from_partition(
+  ShenandoahFreeSetPartitionId partition, idx_t low_idx, idx_t high_idx) {
+
+  // Note: we may remove from free partition even if region is not entirely full, such as when available < PLAB::min_size()
+  assert ((low_idx < _max) && (high_idx < _max), "Both indices are sane: " SIZE_FORMAT " and " SIZE_FORMAT " < " SIZE_FORMAT,
+          low_idx, high_idx, _max);
+  assert (partition < NumPartitions, "Cannot remove from free partitions if not already free");
+
+  for (idx_t idx = low_idx; idx <= high_idx; idx++) {
+    assert (in_free_set(partition, idx), "Must be in partition to remove from partition");
+    _membership[int(partition)].clear_bit(idx);
+  }
+  _region_counts[int(partition)] -= high_idx + 1 - low_idx;
+  shrink_interval_if_range_modifies_either_boundary(partition, low_idx, high_idx);
+}
+
+void ShenandoahRegionPartitions::retire_from_partition(ShenandoahFreeSetPartitionId partition, idx_t idx, size_t used_bytes) {
+
+  // Note: we may remove from free partition even if region is not entirely full, such as when available < PLAB::min_size()
+  assert (idx < _max, "index is sane: " SIZE_FORMAT " < " SIZE_FORMAT, idx, _max);
+  assert (partition < NumPartitions, "Cannot remove from free partitions if not already free");
+  assert (in_free_set(partition, idx), "Must be in partition to remove from partition");
+
+  if (used_bytes < _region_size_bytes) {
+    // Count the alignment pad remnant of memory as used when we retire this region
+    increase_used(partition, _region_size_bytes - used_bytes);
+  }
+  _membership[int(partition)].clear_bit(idx);
+  shrink_interval_if_boundary_modified(partition, idx);
+  _region_counts[int(partition)]--;
+}
+
+void ShenandoahRegionPartitions::make_free(idx_t idx, ShenandoahFreeSetPartitionId which_partition, size_t available) {
+  assert (idx < _max, "index is sane: " SIZE_FORMAT " < " SIZE_FORMAT, idx, _max);
+  assert (membership(idx) == ShenandoahFreeSetPartitionId::NotFree, "Cannot make free if already free");
+  assert (which_partition < NumPartitions, "selected free partition must be valid");
+  assert (available <= _region_size_bytes, "Available cannot exceed region size");
+
+  _membership[int(which_partition)].set_bit(idx);
+  _capacity[int(which_partition)] += _region_size_bytes;
+  _used[int(which_partition)] += _region_size_bytes - available;
+  expand_interval_if_boundary_modified(which_partition, idx, available);
+  _region_counts[int(which_partition)]++;
+}
+
+bool ShenandoahRegionPartitions::is_mutator_partition(ShenandoahFreeSetPartitionId p) {
+  return (p == ShenandoahFreeSetPartitionId::Mutator);
+}
+
+bool ShenandoahRegionPartitions::is_young_collector_partition(ShenandoahFreeSetPartitionId p) {
+  return (p == ShenandoahFreeSetPartitionId::Collector);
+}
+
+bool ShenandoahRegionPartitions::is_old_collector_partition(ShenandoahFreeSetPartitionId p) {
+  return (p == ShenandoahFreeSetPartitionId::OldCollector);
+}
+
+bool ShenandoahRegionPartitions::available_implies_empty(size_t available_in_region) {
+  return (available_in_region == _region_size_bytes);
+}
+
+
+void ShenandoahRegionPartitions::move_from_partition_to_partition(idx_t idx, ShenandoahFreeSetPartitionId orig_partition,
+                                                                  ShenandoahFreeSetPartitionId new_partition, size_t available) {
+  ShenandoahHeapRegion* r = ShenandoahHeap::heap()->get_region(idx);
+  assert (idx < _max, "index is sane: " SIZE_FORMAT " < " SIZE_FORMAT, idx, _max);
+  assert (orig_partition < NumPartitions, "Original partition must be valid");
+  assert (new_partition < NumPartitions, "New partition must be valid");
+  assert (available <= _region_size_bytes, "Available cannot exceed region size");
+  assert (_membership[int(orig_partition)].is_set(idx), "Cannot move from partition unless in partition");
+  assert ((r != nullptr) && ((r->is_trash() && (available == _region_size_bytes)) ||
+                             (r->used() + available == _region_size_bytes)),
+          "Used: " SIZE_FORMAT " + available: " SIZE_FORMAT " should equal region size: " SIZE_FORMAT,
+          ShenandoahHeap::heap()->get_region(idx)->used(), available, _region_size_bytes);
+
+  // Expected transitions:
+  //  During rebuild:         Mutator => Collector
+  //                          Mutator empty => Collector
+  //                          Mutator empty => OldCollector
+  //  During flip_to_gc:      Mutator empty => Collector
+  //                          Mutator empty => OldCollector
+  // At start of update refs: Collector => Mutator
+  //                          OldCollector Empty => Mutator
+  assert ((is_mutator_partition(orig_partition) && is_young_collector_partition(new_partition)) ||
+          (is_mutator_partition(orig_partition) &&
+           available_implies_empty(available) && is_old_collector_partition(new_partition)) ||
+          (is_young_collector_partition(orig_partition) && is_mutator_partition(new_partition)) ||
+          (is_old_collector_partition(orig_partition)
+           && available_implies_empty(available) && is_mutator_partition(new_partition)),
+          "Unexpected movement between partitions, available: " SIZE_FORMAT ", _region_size_bytes: " SIZE_FORMAT
+          ", orig_partition: %s, new_partition: %s",
+          available, _region_size_bytes, partition_name(orig_partition), partition_name(new_partition));
+
+  size_t used = _region_size_bytes - available;
+  assert (_used[int(orig_partition)] >= used,
+          "Orig partition used: " SIZE_FORMAT " must exceed moved used: " SIZE_FORMAT " within region " SSIZE_FORMAT,
+          _used[int(orig_partition)], used, idx);
+
+  _membership[int(orig_partition)].clear_bit(idx);
+  _membership[int(new_partition)].set_bit(idx);
+
+  _capacity[int(orig_partition)] -= _region_size_bytes;
+  _used[int(orig_partition)] -= used;
+  shrink_interval_if_boundary_modified(orig_partition, idx);
+
+  _capacity[int(new_partition)] += _region_size_bytes;;
+  _used[int(new_partition)] += used;
+  expand_interval_if_boundary_modified(new_partition, idx, available);
+
+  _region_counts[int(orig_partition)]--;
+  _region_counts[int(new_partition)]++;
+}
+
+const char* ShenandoahRegionPartitions::partition_membership_name(idx_t idx) const {
+  return partition_name(membership(idx));
+}
+
+inline ShenandoahFreeSetPartitionId ShenandoahRegionPartitions::membership(idx_t idx) const {
+  assert (idx < _max, "index is sane: " SIZE_FORMAT " < " SIZE_FORMAT, idx, _max);
+  ShenandoahFreeSetPartitionId result = ShenandoahFreeSetPartitionId::NotFree;
+  for (uint partition_id = 0; partition_id < UIntNumPartitions; partition_id++) {
+    if (_membership[partition_id].is_set(idx)) {
+      assert(result == ShenandoahFreeSetPartitionId::NotFree, "Region should reside in only one partition");
+      result = (ShenandoahFreeSetPartitionId) partition_id;
+    }
+  }
+  return result;
+}
+
+#ifdef ASSERT
+inline bool ShenandoahRegionPartitions::partition_id_matches(idx_t idx, ShenandoahFreeSetPartitionId test_partition) const {
+  assert (idx < _max, "index is sane: " SIZE_FORMAT " < " SIZE_FORMAT, idx, _max);
+  assert (test_partition < ShenandoahFreeSetPartitionId::NotFree, "must be a valid partition");
+
+  return membership(idx) == test_partition;
+}
+#endif
+
+inline bool ShenandoahRegionPartitions::is_empty(ShenandoahFreeSetPartitionId which_partition) const {
+  assert (which_partition < NumPartitions, "selected free partition must be valid");
+  return (leftmost(which_partition) > rightmost(which_partition));
+}
+
+inline idx_t ShenandoahRegionPartitions::find_index_of_next_available_region(
+  ShenandoahFreeSetPartitionId which_partition, idx_t start_index) const {
+  idx_t rightmost_idx = rightmost(which_partition);
+  idx_t leftmost_idx = leftmost(which_partition);
+  if ((rightmost_idx < leftmost_idx) || (start_index > rightmost_idx)) return _max;
+  if (start_index < leftmost_idx) {
+    start_index = leftmost_idx;
+  }
+  idx_t result = _membership[int(which_partition)].find_first_set_bit(start_index, rightmost_idx + 1);
+  if (result > rightmost_idx) {
+    result = _max;
+  }
+  assert (result >= start_index, "Requires progress");
+  return result;
+}
+
+inline idx_t ShenandoahRegionPartitions::find_index_of_previous_available_region(
+  ShenandoahFreeSetPartitionId which_partition, idx_t last_index) const {
+  idx_t rightmost_idx = rightmost(which_partition);
+  idx_t leftmost_idx = leftmost(which_partition);
+  // if (leftmost_idx == max) then (last_index < leftmost_idx)
+  if (last_index < leftmost_idx) return -1;
+  if (last_index > rightmost_idx) {
+    last_index = rightmost_idx;
+  }
+  idx_t result = _membership[int(which_partition)].find_last_set_bit(-1, last_index);
+  if (result < leftmost_idx) {
+    result = -1;
+  }
+  assert (result <= last_index, "Requires progress");
+  return result;
+}
+
+inline idx_t ShenandoahRegionPartitions::find_index_of_next_available_cluster_of_regions(
+  ShenandoahFreeSetPartitionId which_partition, idx_t start_index, size_t cluster_size) const {
+  idx_t rightmost_idx = rightmost(which_partition);
+  idx_t leftmost_idx = leftmost(which_partition);
+  if ((rightmost_idx < leftmost_idx) || (start_index > rightmost_idx)) return _max;
+  idx_t result = _membership[int(which_partition)].find_first_consecutive_set_bits(start_index, rightmost_idx + 1, cluster_size);
+  if (result > rightmost_idx) {
+    result = _max;
+  }
+  assert (result >= start_index, "Requires progress");
+  return result;
+}
+
+inline idx_t ShenandoahRegionPartitions::find_index_of_previous_available_cluster_of_regions(
+  ShenandoahFreeSetPartitionId which_partition, idx_t last_index, size_t cluster_size) const {
+  idx_t leftmost_idx = leftmost(which_partition);
+  // if (leftmost_idx == max) then (last_index < leftmost_idx)
+  if (last_index < leftmost_idx) return -1;
+  idx_t result = _membership[int(which_partition)].find_last_consecutive_set_bits(leftmost_idx - 1, last_index, cluster_size);
+  if (result <= leftmost_idx) {
+    result = -1;
+  }
+  assert (result <= last_index, "Requires progress");
+  return result;
+}
+
+idx_t ShenandoahRegionPartitions::leftmost_empty(ShenandoahFreeSetPartitionId which_partition) {
+  assert (which_partition < NumPartitions, "selected free partition must be valid");
+  idx_t max_regions = _max;
+  if (_leftmosts_empty[int(which_partition)] == _max) {
+    return _max;
+  }
+  for (idx_t idx = find_index_of_next_available_region(which_partition, _leftmosts_empty[int(which_partition)]);
+       idx < max_regions; ) {
+    assert(in_free_set(which_partition, idx), "Boundaries or find_last_set_bit failed: " SSIZE_FORMAT, idx);
+    if (_free_set->alloc_capacity(idx) == _region_size_bytes) {
+      _leftmosts_empty[int(which_partition)] = idx;
+      return idx;
+    }
+    idx = find_index_of_next_available_region(which_partition, idx + 1);
+  }
+  _leftmosts_empty[int(which_partition)] = _max;
+  _rightmosts_empty[int(which_partition)] = -1;
+  return _max;
+}
+
+idx_t ShenandoahRegionPartitions::rightmost_empty(ShenandoahFreeSetPartitionId which_partition) {
+  assert (which_partition < NumPartitions, "selected free partition must be valid");
+  if (_rightmosts_empty[int(which_partition)] < 0) {
+    return -1;
+  }
+  for (idx_t idx = find_index_of_previous_available_region(which_partition, _rightmosts_empty[int(which_partition)]);
+       idx >= 0; ) {
+    assert(in_free_set(which_partition, idx), "Boundaries or find_last_set_bit failed: " SSIZE_FORMAT, idx);
+    if (_free_set->alloc_capacity(idx) == _region_size_bytes) {
+      _rightmosts_empty[int(which_partition)] = idx;
+      return idx;
+    }
+    idx = find_index_of_previous_available_region(which_partition, idx - 1);
+  }
+  _leftmosts_empty[int(which_partition)] = _max;
+  _rightmosts_empty[int(which_partition)] = -1;
+  return -1;
+}
+
+
+#ifdef ASSERT
+void ShenandoahRegionPartitions::assert_bounds() {
+
+  idx_t leftmosts[UIntNumPartitions];
+  idx_t rightmosts[UIntNumPartitions];
+  idx_t empty_leftmosts[UIntNumPartitions];
+  idx_t empty_rightmosts[UIntNumPartitions];
+
+  for (uint i = 0; i < UIntNumPartitions; i++) {
+    leftmosts[i] = _max;
+    empty_leftmosts[i] = _max;
+    rightmosts[i] = -1;
+    empty_rightmosts[i] = -1;
+  }
+
+  for (idx_t i = 0; i < _max; i++) {
+    ShenandoahFreeSetPartitionId partition = membership(i);
+    switch (partition) {
+      case ShenandoahFreeSetPartitionId::NotFree:
+        break;
+
+      case ShenandoahFreeSetPartitionId::Mutator:
+      case ShenandoahFreeSetPartitionId::Collector:
+      case ShenandoahFreeSetPartitionId::OldCollector:
+      {
+        size_t capacity = _free_set->alloc_capacity(i);
+        bool is_empty = (capacity == _region_size_bytes);
+        assert(capacity > 0, "free regions must have allocation capacity");
+        if (i < leftmosts[int(partition)]) {
+          leftmosts[int(partition)] = i;
+        }
+        if (is_empty && (i < empty_leftmosts[int(partition)])) {
+          empty_leftmosts[int(partition)] = i;
+        }
+        if (i > rightmosts[int(partition)]) {
+          rightmosts[int(partition)] = i;
+        }
+        if (is_empty && (i > empty_rightmosts[int(partition)])) {
+          empty_rightmosts[int(partition)] = i;
+        }
+        break;
+      }
+
+      default:
+        ShouldNotReachHere();
+    }
+  }
+
+  // Performance invariants. Failing these would not break the free partition, but performance would suffer.
+  assert (leftmost(ShenandoahFreeSetPartitionId::Mutator) <= _max,
+          "leftmost in bounds: "  SSIZE_FORMAT " < " SSIZE_FORMAT, leftmost(ShenandoahFreeSetPartitionId::Mutator),  _max);
+  assert (rightmost(ShenandoahFreeSetPartitionId::Mutator) < _max,
+          "rightmost in bounds: "  SSIZE_FORMAT " < " SSIZE_FORMAT, rightmost(ShenandoahFreeSetPartitionId::Mutator),  _max);
+
+  assert (leftmost(ShenandoahFreeSetPartitionId::Mutator) == _max
+          || partition_id_matches(leftmost(ShenandoahFreeSetPartitionId::Mutator), ShenandoahFreeSetPartitionId::Mutator),
+          "leftmost region should be free: " SSIZE_FORMAT,  leftmost(ShenandoahFreeSetPartitionId::Mutator));
+  assert (leftmost(ShenandoahFreeSetPartitionId::Mutator) == _max
+          || partition_id_matches(rightmost(ShenandoahFreeSetPartitionId::Mutator), ShenandoahFreeSetPartitionId::Mutator),
+          "rightmost region should be free: " SSIZE_FORMAT, rightmost(ShenandoahFreeSetPartitionId::Mutator));
+
+  // If Mutator partition is empty, leftmosts will both equal max, rightmosts will both equal zero.
+  // Likewise for empty region partitions.
+  idx_t beg_off = leftmosts[int(ShenandoahFreeSetPartitionId::Mutator)];
+  idx_t end_off = rightmosts[int(ShenandoahFreeSetPartitionId::Mutator)];
+  assert (beg_off >= leftmost(ShenandoahFreeSetPartitionId::Mutator),
+          "free regions before the leftmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          beg_off, leftmost(ShenandoahFreeSetPartitionId::Mutator));
+  assert (end_off <= rightmost(ShenandoahFreeSetPartitionId::Mutator),
+          "free regions past the rightmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          end_off, rightmost(ShenandoahFreeSetPartitionId::Mutator));
+
+  beg_off = empty_leftmosts[int(ShenandoahFreeSetPartitionId::Mutator)];
+  end_off = empty_rightmosts[int(ShenandoahFreeSetPartitionId::Mutator)];
+  assert (beg_off >= leftmost_empty(ShenandoahFreeSetPartitionId::Mutator),
+          "free empty regions before the leftmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          beg_off, leftmost_empty(ShenandoahFreeSetPartitionId::Mutator));
+  assert (end_off <= rightmost_empty(ShenandoahFreeSetPartitionId::Mutator),
+          "free empty regions past the rightmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          end_off, rightmost_empty(ShenandoahFreeSetPartitionId::Mutator));
+
+  // Performance invariants. Failing these would not break the free partition, but performance would suffer.
+  assert (leftmost(ShenandoahFreeSetPartitionId::Collector) <= _max, "leftmost in bounds: "  SSIZE_FORMAT " < " SSIZE_FORMAT,
+          leftmost(ShenandoahFreeSetPartitionId::Collector),  _max);
+  assert (rightmost(ShenandoahFreeSetPartitionId::Collector) < _max, "rightmost in bounds: "  SSIZE_FORMAT " < " SSIZE_FORMAT,
+          rightmost(ShenandoahFreeSetPartitionId::Collector),  _max);
+
+  assert (leftmost(ShenandoahFreeSetPartitionId::Collector) == _max
+          || partition_id_matches(leftmost(ShenandoahFreeSetPartitionId::Collector), ShenandoahFreeSetPartitionId::Collector),
+          "leftmost region should be free: " SSIZE_FORMAT,  leftmost(ShenandoahFreeSetPartitionId::Collector));
+  assert (leftmost(ShenandoahFreeSetPartitionId::Collector) == _max
+          || partition_id_matches(rightmost(ShenandoahFreeSetPartitionId::Collector), ShenandoahFreeSetPartitionId::Collector),
+          "rightmost region should be free: " SSIZE_FORMAT, rightmost(ShenandoahFreeSetPartitionId::Collector));
+
+  // If Collector partition is empty, leftmosts will both equal max, rightmosts will both equal zero.
+  // Likewise for empty region partitions.
+  beg_off = leftmosts[int(ShenandoahFreeSetPartitionId::Collector)];
+  end_off = rightmosts[int(ShenandoahFreeSetPartitionId::Collector)];
+  assert (beg_off >= leftmost(ShenandoahFreeSetPartitionId::Collector),
+          "free regions before the leftmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          beg_off, leftmost(ShenandoahFreeSetPartitionId::Collector));
+  assert (end_off <= rightmost(ShenandoahFreeSetPartitionId::Collector),
+          "free regions past the rightmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          end_off, rightmost(ShenandoahFreeSetPartitionId::Collector));
+
+  beg_off = empty_leftmosts[int(ShenandoahFreeSetPartitionId::Collector)];
+  end_off = empty_rightmosts[int(ShenandoahFreeSetPartitionId::Collector)];
+  assert (beg_off >= _leftmosts_empty[int(ShenandoahFreeSetPartitionId::Collector)],
+          "free empty regions before the leftmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          beg_off, leftmost_empty(ShenandoahFreeSetPartitionId::Collector));
+  assert (end_off <= _rightmosts_empty[int(ShenandoahFreeSetPartitionId::Collector)],
+          "free empty regions past the rightmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          end_off, rightmost_empty(ShenandoahFreeSetPartitionId::Collector));
+
+  // Performance invariants. Failing these would not break the free partition, but performance would suffer.
+  assert (leftmost(ShenandoahFreeSetPartitionId::OldCollector) <= _max, "leftmost in bounds: "  SSIZE_FORMAT " < " SSIZE_FORMAT,
+          leftmost(ShenandoahFreeSetPartitionId::OldCollector),  _max);
+  assert (rightmost(ShenandoahFreeSetPartitionId::OldCollector) < _max, "rightmost in bounds: "  SSIZE_FORMAT " < " SSIZE_FORMAT,
+          rightmost(ShenandoahFreeSetPartitionId::OldCollector),  _max);
+
+  assert (leftmost(ShenandoahFreeSetPartitionId::OldCollector) == _max
+          || partition_id_matches(leftmost(ShenandoahFreeSetPartitionId::OldCollector),
+                                  ShenandoahFreeSetPartitionId::OldCollector),
+          "leftmost region should be free: " SSIZE_FORMAT,  leftmost(ShenandoahFreeSetPartitionId::OldCollector));
+  assert (leftmost(ShenandoahFreeSetPartitionId::OldCollector) == _max
+          || partition_id_matches(rightmost(ShenandoahFreeSetPartitionId::OldCollector),
+                                  ShenandoahFreeSetPartitionId::OldCollector),
+          "rightmost region should be free: " SSIZE_FORMAT, rightmost(ShenandoahFreeSetPartitionId::OldCollector));
+
+  // If OldCollector partition is empty, leftmosts will both equal max, rightmosts will both equal zero.
+  // Likewise for empty region partitions.
+  beg_off = leftmosts[int(ShenandoahFreeSetPartitionId::OldCollector)];
+  end_off = rightmosts[int(ShenandoahFreeSetPartitionId::OldCollector)];
+  assert (beg_off >= leftmost(ShenandoahFreeSetPartitionId::OldCollector),
+          "free regions before the leftmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          beg_off, leftmost(ShenandoahFreeSetPartitionId::OldCollector));
+  assert (end_off <= rightmost(ShenandoahFreeSetPartitionId::OldCollector),
+          "free regions past the rightmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          end_off, rightmost(ShenandoahFreeSetPartitionId::OldCollector));
+
+  beg_off = empty_leftmosts[int(ShenandoahFreeSetPartitionId::OldCollector)];
+  end_off = empty_rightmosts[int(ShenandoahFreeSetPartitionId::OldCollector)];
+  assert (beg_off >= _leftmosts_empty[int(ShenandoahFreeSetPartitionId::OldCollector)],
+          "free empty regions before the leftmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          beg_off, leftmost_empty(ShenandoahFreeSetPartitionId::OldCollector));
+  assert (end_off <= _rightmosts_empty[int(ShenandoahFreeSetPartitionId::OldCollector)],
+          "free empty regions past the rightmost: " SSIZE_FORMAT ", bound " SSIZE_FORMAT,
+          end_off, rightmost_empty(ShenandoahFreeSetPartitionId::OldCollector));
+}
+#endif
+
 ShenandoahFreeSet::ShenandoahFreeSet(ShenandoahHeap* heap, size_t max_regions) :
   _heap(heap),
-  _mutator_free_bitmap(max_regions, mtGC),
-  _collector_free_bitmap(max_regions, mtGC),
-  _max(max_regions)
+  _partitions(max_regions, this),
+  _trash_regions(NEW_C_HEAP_ARRAY(ShenandoahHeapRegion*, max_regions, mtGC)),
+  _alloc_bias_weight(0)
 {
   clear_internal();
 }
 
-void ShenandoahFreeSet::increase_used(size_t num_bytes) {
+void ShenandoahFreeSet::add_promoted_in_place_region_to_old_collector(ShenandoahHeapRegion* region) {
   shenandoah_assert_heaplocked();
-  _used += num_bytes;
-
-  assert(_used <= _capacity, "must not use more than we have: used: " SIZE_FORMAT
-         ", capacity: " SIZE_FORMAT ", num_bytes: " SIZE_FORMAT, _used, _capacity, num_bytes);
+  size_t plab_min_size_in_bytes = ShenandoahGenerationalHeap::heap()->plab_min_size() * HeapWordSize;
+  size_t idx = region->index();
+  size_t capacity = alloc_capacity(region);
+  assert(_partitions.membership(idx) == ShenandoahFreeSetPartitionId::NotFree,
+         "Regions promoted in place should have been excluded from Mutator partition");
+  if (capacity >= plab_min_size_in_bytes) {
+    _partitions.make_free(idx, ShenandoahFreeSetPartitionId::OldCollector, capacity);
+    _heap->old_generation()->augment_promoted_reserve(capacity);
+  }
 }
 
-bool ShenandoahFreeSet::is_mutator_free(size_t idx) const {
-  assert (idx < _max, "index is sane: " SIZE_FORMAT " < " SIZE_FORMAT " (left: " SIZE_FORMAT ", right: " SIZE_FORMAT ")",
-          idx, _max, _mutator_leftmost, _mutator_rightmost);
-  return _mutator_free_bitmap.at(idx);
+HeapWord* ShenandoahFreeSet::allocate_from_partition_with_affiliation(ShenandoahAffiliation affiliation,
+                                                                      ShenandoahAllocRequest& req, bool& in_new_region) {
+
+  shenandoah_assert_heaplocked();
+  ShenandoahFreeSetPartitionId which_partition = req.is_old()? ShenandoahFreeSetPartitionId::OldCollector: ShenandoahFreeSetPartitionId::Collector;
+  if (_partitions.alloc_from_left_bias(which_partition)) {
+    ShenandoahLeftRightIterator iterator(&_partitions, which_partition, affiliation == ShenandoahAffiliation::FREE);
+    return allocate_with_affiliation(iterator, affiliation, req, in_new_region);
+  } else {
+    ShenandoahRightLeftIterator iterator(&_partitions, which_partition, affiliation == ShenandoahAffiliation::FREE);
+    return allocate_with_affiliation(iterator, affiliation, req, in_new_region);
+  }
 }
 
-bool ShenandoahFreeSet::is_collector_free(size_t idx) const {
-  assert (idx < _max, "index is sane: " SIZE_FORMAT " < " SIZE_FORMAT " (left: " SIZE_FORMAT ", right: " SIZE_FORMAT ")",
-          idx, _max, _collector_leftmost, _collector_rightmost);
-  return _collector_free_bitmap.at(idx);
+template<typename Iter>
+HeapWord* ShenandoahFreeSet::allocate_with_affiliation(Iter& iterator, ShenandoahAffiliation affiliation, ShenandoahAllocRequest& req, bool& in_new_region) {
+  for (idx_t idx = iterator.current(); iterator.has_next(); idx = iterator.next()) {
+    ShenandoahHeapRegion* r = _heap->get_region(idx);
+    if (r->affiliation() == affiliation) {
+      HeapWord* result = try_allocate_in(r, req, in_new_region);
+      if (result != nullptr) {
+        return result;
+      }
+    }
+  }
+  log_debug(gc, free)("Could not allocate collector region with affiliation: %s for request " PTR_FORMAT,
+                      shenandoah_affiliation_name(affiliation), p2i(&req));
+  return nullptr;
 }
 
 HeapWord* ShenandoahFreeSet::allocate_single(ShenandoahAllocRequest& req, bool& in_new_region) {
+  shenandoah_assert_heaplocked();
+
   // Scan the bitmap looking for a first fit.
   //
   // Leftmost and rightmost bounds provide enough caching to walk bitmap efficiently. Normally,
   // we would find the region to allocate at right away.
   //
-  // Allocations are biased: new application allocs go to beginning of the heap, and GC allocs
-  // go to the end. This makes application allocation faster, because we would clear lots
-  // of regions from the beginning most of the time.
+  // Allocations are biased: GC allocations are taken from the high end of the heap.  Regular (and TLAB)
+  // mutator allocations are taken from the middle of heap, below the memory reserved for Collector.
+  // Humongous mutator allocations are taken from the bottom of the heap.
   //
-  // Free set maintains mutator and collector views, and normally they allocate in their views only,
-  // unless we special cases for stealing and mixed allocations.
+  // Free set maintains mutator and collector partitions.  Normally, each allocates only from its partition,
+  // except in special cases when the collector steals regions from the mutator partition.
+
+  // Overwrite with non-zero (non-NULL) values only if necessary for allocation bookkeeping.
 
   switch (req.type()) {
     case ShenandoahAllocRequest::_alloc_tlab:
-    case ShenandoahAllocRequest::_alloc_shared: {
-
-      // Try to allocate in the mutator view
-      for (size_t idx = _mutator_leftmost; idx <= _mutator_rightmost; idx++) {
-        if (is_mutator_free(idx)) {
-          HeapWord* result = try_allocate_in(_heap->get_region(idx), req, in_new_region);
-          if (result != nullptr) {
-            return result;
-          }
-        }
-      }
-
-      // There is no recovery. Mutator does not touch collector view at all.
-      break;
-    }
+    case ShenandoahAllocRequest::_alloc_shared:
+      return allocate_for_mutator(req, in_new_region);
     case ShenandoahAllocRequest::_alloc_gclab:
-    case ShenandoahAllocRequest::_alloc_shared_gc: {
-      // size_t is unsigned, need to dodge underflow when _leftmost = 0
-
-      // Fast-path: try to allocate in the collector view first
-      for (size_t c = _collector_rightmost + 1; c > _collector_leftmost; c--) {
-        size_t idx = c - 1;
-        if (is_collector_free(idx)) {
-          HeapWord* result = try_allocate_in(_heap->get_region(idx), req, in_new_region);
-          if (result != nullptr) {
-            return result;
-          }
-        }
-      }
-
-      // No dice. Can we borrow space from mutator view?
-      if (!ShenandoahEvacReserveOverflow) {
-        return nullptr;
-      }
-
-      // Try to steal the empty region from the mutator view
-      for (size_t c = _mutator_rightmost + 1; c > _mutator_leftmost; c--) {
-        size_t idx = c - 1;
-        if (is_mutator_free(idx)) {
-          ShenandoahHeapRegion* r = _heap->get_region(idx);
-          if (can_allocate_from(r)) {
-            flip_to_gc(r);
-            HeapWord *result = try_allocate_in(r, req, in_new_region);
-            if (result != nullptr) {
-              return result;
-            }
-          }
-        }
-      }
-
-      // No dice. Do not try to mix mutator and GC allocations, because
-      // URWM moves due to GC allocations would expose unparsable mutator
-      // allocations.
-
-      break;
-    }
+    case ShenandoahAllocRequest::_alloc_plab:
+    case ShenandoahAllocRequest::_alloc_shared_gc:
+      return allocate_for_collector(req, in_new_region);
     default:
       ShouldNotReachHere();
   }
-
   return nullptr;
 }
 
-HeapWord* ShenandoahFreeSet::try_allocate_in(ShenandoahHeapRegion* r, ShenandoahAllocRequest& req, bool& in_new_region) {
-  assert (!has_no_alloc_capacity(r), "Performance: should avoid full regions on this path: " SIZE_FORMAT, r->index());
+HeapWord* ShenandoahFreeSet::allocate_for_mutator(ShenandoahAllocRequest &req, bool &in_new_region) {
+  update_allocation_bias();
 
-  if (_heap->is_concurrent_weak_root_in_progress() &&
-      r->is_trash()) {
+  if (_partitions.is_empty(ShenandoahFreeSetPartitionId::Mutator)) {
+    // There is no recovery. Mutator does not touch collector view at all.
     return nullptr;
   }
 
-  try_recycle_trashed(r);
+  // Try to allocate in the mutator view
+  if (_partitions.alloc_from_left_bias(ShenandoahFreeSetPartitionId::Mutator)) {
+    // Allocate from low to high memory.  This keeps the range of fully empty regions more tightly packed.
+    // Note that the most recently allocated regions tend not to be evacuated in a given GC cycle.  So this
+    // tends to accumulate "fragmented" uncollected regions in high memory.
+    ShenandoahLeftRightIterator iterator(&_partitions, ShenandoahFreeSetPartitionId::Mutator);
+    return allocate_from_regions(iterator, req, in_new_region);
+  }
 
-  in_new_region = r->is_empty();
+  // Allocate from high to low memory. This preserves low memory for humongous allocations.
+  ShenandoahRightLeftIterator iterator(&_partitions, ShenandoahFreeSetPartitionId::Mutator);
+  return allocate_from_regions(iterator, req, in_new_region);
+}
 
-  HeapWord* result = nullptr;
-  size_t size = req.size();
+void ShenandoahFreeSet::update_allocation_bias() {
+  if (_alloc_bias_weight-- <= 0) {
+    // We have observed that regions not collected in previous GC cycle tend to congregate at one end or the other
+    // of the heap.  Typically, these are the more recently engaged regions and the objects in these regions have not
+    // yet had a chance to die (and/or are treated as floating garbage).  If we use the same allocation bias on each
+    // GC pass, these "most recently" engaged regions for GC pass N will also be the "most recently" engaged regions
+    // for GC pass N+1, and the relatively large amount of live data and/or floating garbage introduced
+    // during the most recent GC pass may once again prevent the region from being collected.  We have found that
+    // alternating the allocation behavior between GC passes improves evacuation performance by 3-7% on certain
+    // benchmarks.  In the best case, this has the effect of consuming these partially consumed regions before
+    // the start of the next mark cycle so all of their garbage can be efficiently reclaimed.
+    //
+    // First, finish consuming regions that are already partially consumed so as to more tightly limit ranges of
+    // available regions.  Other potential benefits:
+    //  1. Eventual collection set has fewer regions because we have packed newly allocated objects into fewer regions
+    //  2. We preserve the "empty" regions longer into the GC cycle, reducing likelihood of allocation failures
+    //     late in the GC cycle.
+    idx_t non_empty_on_left = (_partitions.leftmost_empty(ShenandoahFreeSetPartitionId::Mutator)
+                               - _partitions.leftmost(ShenandoahFreeSetPartitionId::Mutator));
+    idx_t non_empty_on_right = (_partitions.rightmost(ShenandoahFreeSetPartitionId::Mutator)
+                                - _partitions.rightmost_empty(ShenandoahFreeSetPartitionId::Mutator));
+    _partitions.set_bias_from_left_to_right(ShenandoahFreeSetPartitionId::Mutator, (non_empty_on_right < non_empty_on_left));
+    _alloc_bias_weight = INITIAL_ALLOC_BIAS_WEIGHT;
+  }
+}
 
-  if (req.is_lab_alloc()) {
-    size_t free = align_down(r->free() >> LogHeapWordSize, MinObjAlignment);
-    if (size > free) {
-      size = free;
-    }
-    if (size >= req.min_size()) {
-      result = r->allocate(size, req.type());
-      assert (result != nullptr, "Allocation must succeed: free " SIZE_FORMAT ", actual " SIZE_FORMAT, free, size);
+template<typename Iter>
+HeapWord* ShenandoahFreeSet::allocate_from_regions(Iter& iterator, ShenandoahAllocRequest &req, bool &in_new_region) {
+  for (idx_t idx = iterator.current(); iterator.has_next(); idx = iterator.next()) {
+    ShenandoahHeapRegion* r = _heap->get_region(idx);
+    size_t min_size = (req.type() == ShenandoahAllocRequest::_alloc_tlab) ? req.min_size() : req.size();
+    if (alloc_capacity(r) >= min_size) {
+      HeapWord* result = try_allocate_in(r, req, in_new_region);
+      if (result != nullptr) {
+        return result;
+      }
     }
-  } else {
-    result = r->allocate(size, req.type());
   }
+  return nullptr;
+}
 
+HeapWord* ShenandoahFreeSet::allocate_for_collector(ShenandoahAllocRequest &req, bool &in_new_region) {
+  // Fast-path: try to allocate in the collector view first
+  HeapWord* result;
+  result = allocate_from_partition_with_affiliation(req.affiliation(), req, in_new_region);
   if (result != nullptr) {
-    // Allocation successful, bump stats:
-    if (req.is_mutator_alloc()) {
-      increase_used(size * HeapWordSize);
+    return result;
+  }
+
+  bool allow_new_region = can_allocate_in_new_region(req);
+  if (allow_new_region) {
+    // Try a free region that is dedicated to GC allocations.
+    result = allocate_from_partition_with_affiliation(ShenandoahAffiliation::FREE, req, in_new_region);
+    if (result != nullptr) {
+      return result;
     }
+  }
+
+  // No dice. Can we borrow space from mutator view?
+  if (!ShenandoahEvacReserveOverflow) {
+    return nullptr;
+  }
 
-    // Record actual allocation size
-    req.set_actual_size(size);
+  if (!allow_new_region && req.is_old() && (_heap->young_generation()->free_unaffiliated_regions() > 0)) {
+    // This allows us to flip a mutator region to old_collector
+    allow_new_region = true;
+  }
 
-    if (req.is_gc_alloc()) {
-      r->set_update_watermark(r->top());
-    }
+  // We should expand old-gen if this can prevent an old-gen evacuation failure.  We don't care so much about
+  // promotion failures since they can be mitigated in a subsequent GC pass.  Would be nice to know if this
+  // allocation request is for evacuation or promotion.  Individual threads limit their use of PLAB memory for
+  // promotions, so we already have an assurance that any additional memory set aside for old-gen will be used
+  // only for old-gen evacuations.
+  if (allow_new_region) {
+    // Try to steal an empty region from the mutator view.
+    result = try_allocate_from_mutator(req, in_new_region);
   }
 
-  if (result == nullptr || has_no_alloc_capacity(r)) {
-    // Region cannot afford this or future allocations. Retire it.
-    //
-    // While this seems a bit harsh, especially in the case when this large allocation does not
-    // fit, but the next small one would, we are risking to inflate scan times when lots of
-    // almost-full regions precede the fully-empty region where we want allocate the entire TLAB.
-    // TODO: Record first fully-empty region, and use that for large allocations
+  // This is it. Do not try to mix mutator and GC allocations, because adjusting region UWM
+  // due to GC allocations would expose unparsable mutator allocations.
+  return result;
+}
 
-    // Record the remainder as allocation waste
-    if (req.is_mutator_alloc()) {
-      size_t waste = r->free();
-      if (waste > 0) {
-        increase_used(waste);
-        _heap->notify_mutator_alloc_words(waste >> LogHeapWordSize, true);
+bool ShenandoahFreeSet::can_allocate_in_new_region(const ShenandoahAllocRequest& req) {
+  if (!_heap->mode()->is_generational()) {
+    return true;
+  }
+
+  assert(req.is_old() || req.is_young(), "Should request affiliation");
+  return (req.is_old() && _heap->old_generation()->free_unaffiliated_regions() > 0)
+         || (req.is_young() && _heap->young_generation()->free_unaffiliated_regions() > 0);
+}
+
+HeapWord* ShenandoahFreeSet::try_allocate_from_mutator(ShenandoahAllocRequest& req, bool& in_new_region) {
+  // The collector prefers to keep longer lived regions toward the right side of the heap, so it always
+  // searches for regions from right to left here.
+  ShenandoahRightLeftIterator iterator(&_partitions, ShenandoahFreeSetPartitionId::Mutator, true);
+  for (idx_t idx = iterator.current(); iterator.has_next(); idx = iterator.next()) {
+    ShenandoahHeapRegion* r = _heap->get_region(idx);
+    if (can_allocate_from(r)) {
+      if (req.is_old()) {
+        flip_to_old_gc(r);
+      } else {
+        flip_to_gc(r);
       }
+      // Region r is entirely empty.  If try_allocate_in fails on region r, something else is really wrong.
+      // Don't bother to retry with other regions.
+      log_debug(gc, free)("Flipped region " SIZE_FORMAT " to gc for request: " PTR_FORMAT, idx, p2i(&req));
+      return try_allocate_in(r, req, in_new_region);
     }
+  }
+
+  return nullptr;
+}
 
-    size_t num = r->index();
-    _collector_free_bitmap.clear_bit(num);
-    _mutator_free_bitmap.clear_bit(num);
-    // Touched the bounds? Need to update:
-    if (touches_bounds(num)) {
-      adjust_bounds();
+// This work method takes an argument corresponding to the number of bytes
+// free in a region, and returns the largest amount in heapwords that can be allocated
+// such that both of the following conditions are satisfied:
+//
+// 1. it is a multiple of card size
+// 2. any remaining shard may be filled with a filler object
+//
+// The idea is that the allocation starts and ends at card boundaries. Because
+// a region ('s end) is card-aligned, the remainder shard that must be filled is
+// at the start of the free space.
+//
+// This is merely a helper method to use for the purpose of such a calculation.
+size_t ShenandoahFreeSet::get_usable_free_words(size_t free_bytes) const {
+  // e.g. card_size is 512, card_shift is 9, min_fill_size() is 8
+  //      free is 514
+  //      usable_free is 512, which is decreased to 0
+  size_t usable_free = (free_bytes / CardTable::card_size()) << CardTable::card_shift();
+  assert(usable_free <= free_bytes, "Sanity check");
+  if ((free_bytes != usable_free) && (free_bytes - usable_free < ShenandoahHeap::min_fill_size() * HeapWordSize)) {
+    // After aligning to card multiples, the remainder would be smaller than
+    // the minimum filler object, so we'll need to take away another card's
+    // worth to construct a filler object.
+    if (usable_free >= CardTable::card_size()) {
+      usable_free -= CardTable::card_size();
+    } else {
+      assert(usable_free == 0, "usable_free is a multiple of card_size and card_size > min_fill_size");
     }
-    assert_bounds();
   }
-  return result;
+
+  return usable_free / HeapWordSize;
 }
 
-bool ShenandoahFreeSet::touches_bounds(size_t num) const {
-  return num == _collector_leftmost || num == _collector_rightmost || num == _mutator_leftmost || num == _mutator_rightmost;
+// Given a size argument, which is a multiple of card size, a request struct
+// for a PLAB, and an old region, return a pointer to the allocated space for
+// a PLAB which is card-aligned and where any remaining shard in the region
+// has been suitably filled by a filler object.
+// It is assumed (and assertion-checked) that such an allocation is always possible.
+HeapWord* ShenandoahFreeSet::allocate_aligned_plab(size_t size, ShenandoahAllocRequest& req, ShenandoahHeapRegion* r) {
+  assert(_heap->mode()->is_generational(), "PLABs are only for generational mode");
+  assert(r->is_old(), "All PLABs reside in old-gen");
+  assert(!req.is_mutator_alloc(), "PLABs should not be allocated by mutators.");
+  assert(is_aligned(size, CardTable::card_size_in_words()), "Align by design");
+
+  HeapWord* result = r->allocate_aligned(size, req, CardTable::card_size());
+  assert(result != nullptr, "Allocation cannot fail");
+  assert(r->top() <= r->end(), "Allocation cannot span end of region");
+  assert(is_aligned(result, CardTable::card_size_in_words()), "Align by design");
+  return result;
 }
 
-void ShenandoahFreeSet::recompute_bounds() {
-  // Reset to the most pessimistic case:
-  _mutator_rightmost = _max - 1;
-  _mutator_leftmost = 0;
-  _collector_rightmost = _max - 1;
-  _collector_leftmost = 0;
+HeapWord* ShenandoahFreeSet::try_allocate_in(ShenandoahHeapRegion* r, ShenandoahAllocRequest& req, bool& in_new_region) {
+  assert (has_alloc_capacity(r), "Performance: should avoid full regions on this path: " SIZE_FORMAT, r->index());
+  if (_heap->is_concurrent_weak_root_in_progress() && r->is_trash()) {
+    return nullptr;
+  }
+  HeapWord* result = nullptr;
+  try_recycle_trashed(r);
+  in_new_region = r->is_empty();
 
-  // ...and adjust from there
-  adjust_bounds();
-}
+  if (in_new_region) {
+    log_debug(gc)("Using new region (" SIZE_FORMAT ") for %s (" PTR_FORMAT ").",
+                       r->index(), ShenandoahAllocRequest::alloc_type_to_string(req.type()), p2i(&req));
+    assert(!r->is_affiliated(), "New region " SIZE_FORMAT " should be unaffiliated", r->index());
+    r->set_affiliation(req.affiliation());
+    if (r->is_old()) {
+      // Any OLD region allocated during concurrent coalesce-and-fill does not need to be coalesced and filled because
+      // all objects allocated within this region are above TAMS (and thus are implicitly marked).  In case this is an
+      // OLD region and concurrent preparation for mixed evacuations visits this region before the start of the next
+      // old-gen concurrent mark (i.e. this region is allocated following the start of old-gen concurrent mark but before
+      // concurrent preparations for mixed evacuations are completed), we mark this region as not requiring any
+      // coalesce-and-fill processing.
+      r->end_preemptible_coalesce_and_fill();
+      _heap->old_generation()->clear_cards_for(r);
+    }
+    _heap->generation_for(r->affiliation())->increment_affiliated_region_count();
 
-void ShenandoahFreeSet::adjust_bounds() {
-  // Rewind both mutator bounds until the next bit.
-  while (_mutator_leftmost < _max && !is_mutator_free(_mutator_leftmost)) {
-    _mutator_leftmost++;
+#ifdef ASSERT
+    ShenandoahMarkingContext* const ctx = _heap->complete_marking_context();
+    assert(ctx->top_at_mark_start(r) == r->bottom(), "Newly established allocation region starts with TAMS equal to bottom");
+    assert(ctx->is_bitmap_range_within_region_clear(ctx->top_bitmap(r), r->end()), "Bitmap above top_bitmap() must be clear");
+#endif
+    log_debug(gc)("Using new region (" SIZE_FORMAT ") for %s (" PTR_FORMAT ").",
+                       r->index(), ShenandoahAllocRequest::alloc_type_to_string(req.type()), p2i(&req));
+  } else {
+    assert(r->is_affiliated(), "Region " SIZE_FORMAT " that is not new should be affiliated", r->index());
+    if (r->affiliation() != req.affiliation()) {
+      assert(_heap->mode()->is_generational(), "Request for %s from %s region should only happen in generational mode.",
+             req.affiliation_name(), r->affiliation_name());
+      return nullptr;
+    }
   }
-  while (_mutator_rightmost > 0 && !is_mutator_free(_mutator_rightmost)) {
-    _mutator_rightmost--;
+
+  // req.size() is in words, r->free() is in bytes.
+  if (req.is_lab_alloc()) {
+    size_t adjusted_size = req.size();
+    size_t free = r->free();    // free represents bytes available within region r
+    if (req.type() == ShenandoahAllocRequest::_alloc_plab) {
+      // This is a PLAB allocation
+      assert(_heap->mode()->is_generational(), "PLABs are only for generational mode");
+      assert(_partitions.in_free_set(ShenandoahFreeSetPartitionId::OldCollector, r->index()),
+             "PLABS must be allocated in old_collector_free regions");
+
+      // Need to assure that plabs are aligned on multiple of card region
+      // Convert free from unaligned bytes to aligned number of words
+      size_t usable_free = get_usable_free_words(free);
+      if (adjusted_size > usable_free) {
+        adjusted_size = usable_free;
+      }
+      adjusted_size = align_down(adjusted_size, CardTable::card_size_in_words());
+      if (adjusted_size >= req.min_size()) {
+        result = allocate_aligned_plab(adjusted_size, req, r);
+        assert(result != nullptr, "allocate must succeed");
+        req.set_actual_size(adjusted_size);
+      } else {
+        // Otherwise, leave result == nullptr because the adjusted size is smaller than min size.
+        log_trace(gc, free)("Failed to shrink PLAB request (" SIZE_FORMAT ") in region " SIZE_FORMAT " to " SIZE_FORMAT
+                            " because min_size() is " SIZE_FORMAT, req.size(), r->index(), adjusted_size, req.min_size());
+      }
+    } else {
+      // This is a GCLAB or a TLAB allocation
+      // Convert free from unaligned bytes to aligned number of words
+      free = align_down(free >> LogHeapWordSize, MinObjAlignment);
+      if (adjusted_size > free) {
+        adjusted_size = free;
+      }
+      if (adjusted_size >= req.min_size()) {
+        result = r->allocate(adjusted_size, req);
+        assert (result != nullptr, "Allocation must succeed: free " SIZE_FORMAT ", actual " SIZE_FORMAT, free, adjusted_size);
+        req.set_actual_size(adjusted_size);
+      } else {
+        log_trace(gc, free)("Failed to shrink TLAB or GCLAB request (" SIZE_FORMAT ") in region " SIZE_FORMAT " to " SIZE_FORMAT
+                            " because min_size() is " SIZE_FORMAT, req.size(), r->index(), adjusted_size, req.min_size());
+      }
+    }
+  } else {
+    size_t size = req.size();
+    result = r->allocate(size, req);
+    if (result != nullptr) {
+      // Record actual allocation size
+      req.set_actual_size(size);
+    }
   }
-  // Rewind both collector bounds until the next bit.
-  while (_collector_leftmost < _max && !is_collector_free(_collector_leftmost)) {
-    _collector_leftmost++;
+
+  if (result != nullptr) {
+    // Allocation successful, bump stats:
+    if (req.is_mutator_alloc()) {
+      assert(req.is_young(), "Mutator allocations always come from young generation.");
+      _partitions.increase_used(ShenandoahFreeSetPartitionId::Mutator, req.actual_size() * HeapWordSize);
+    } else {
+      assert(req.is_gc_alloc(), "Should be gc_alloc since req wasn't mutator alloc");
+
+      // For GC allocations, we advance update_watermark because the objects relocated into this memory during
+      // evacuation are not updated during evacuation.  For both young and old regions r, it is essential that all
+      // PLABs be made parsable at the end of evacuation.  This is enabled by retiring all plabs at end of evacuation.
+      r->set_update_watermark(r->top());
+      if (r->is_old()) {
+        _partitions.increase_used(ShenandoahFreeSetPartitionId::OldCollector, req.actual_size() * HeapWordSize);
+        assert(req.type() != ShenandoahAllocRequest::_alloc_gclab, "old-gen allocations use PLAB or shared allocation");
+        // for plabs, we'll sort the difference between evac and promotion usage when we retire the plab
+      } else {
+        _partitions.increase_used(ShenandoahFreeSetPartitionId::Collector, req.actual_size() * HeapWordSize);
+      }
+    }
   }
-  while (_collector_rightmost > 0 && !is_collector_free(_collector_rightmost)) {
-    _collector_rightmost--;
+
+  static const size_t min_capacity = (size_t) (ShenandoahHeapRegion::region_size_bytes() * (1.0 - 1.0 / ShenandoahEvacWaste));
+  size_t ac = alloc_capacity(r);
+
+  if (((result == nullptr) && (ac < min_capacity)) || (alloc_capacity(r) < PLAB::min_size() * HeapWordSize)) {
+    // Regardless of whether this allocation succeeded, if the remaining memory is less than PLAB:min_size(), retire this region.
+    // Note that retire_from_partition() increases used to account for waste.
+
+    // Also, if this allocation request failed and the consumed within this region * ShenandoahEvacWaste > region size,
+    // then retire the region so that subsequent searches can find available memory more quickly.
+
+    size_t idx = r->index();
+    ShenandoahFreeSetPartitionId orig_partition;
+    if (req.is_mutator_alloc()) {
+      orig_partition = ShenandoahFreeSetPartitionId::Mutator;
+    } else if (req.type() == ShenandoahAllocRequest::_alloc_gclab) {
+      orig_partition = ShenandoahFreeSetPartitionId::Collector;
+    } else if (req.type() == ShenandoahAllocRequest::_alloc_plab) {
+      orig_partition = ShenandoahFreeSetPartitionId::OldCollector;
+    } else {
+      assert(req.type() == ShenandoahAllocRequest::_alloc_shared_gc, "Unexpected allocation type");
+      if (req.is_old()) {
+        orig_partition = ShenandoahFreeSetPartitionId::OldCollector;
+      } else {
+        orig_partition = ShenandoahFreeSetPartitionId::Collector;
+      }
+    }
+    _partitions.retire_from_partition(orig_partition, idx, r->used());
+    _partitions.assert_bounds();
   }
+  return result;
 }
 
 HeapWord* ShenandoahFreeSet::allocate_contiguous(ShenandoahAllocRequest& req) {
+  assert(req.is_mutator_alloc(), "All humongous allocations are performed by mutator");
   shenandoah_assert_heaplocked();
 
   size_t words_size = req.size();
-  size_t num = ShenandoahHeapRegion::required_regions(words_size * HeapWordSize);
+  idx_t num = ShenandoahHeapRegion::required_regions(words_size * HeapWordSize);
 
-  // No regions left to satisfy allocation, bye.
-  if (num > mutator_count()) {
+  assert(req.is_young(), "Humongous regions always allocated in YOUNG");
+  ShenandoahGeneration* generation = _heap->generation_for(req.affiliation());
+
+  // Check if there are enough regions left to satisfy allocation.
+  if (num > (idx_t) _partitions.count(ShenandoahFreeSetPartitionId::Mutator)) {
     return nullptr;
   }
 
+  idx_t start_range = _partitions.leftmost_empty(ShenandoahFreeSetPartitionId::Mutator);
+  idx_t end_range = _partitions.rightmost_empty(ShenandoahFreeSetPartitionId::Mutator) + 1;
+  idx_t last_possible_start = end_range - num;
+
   // Find the continuous interval of $num regions, starting from $beg and ending in $end,
   // inclusive. Contiguous allocations are biased to the beginning.
-
-  size_t beg = _mutator_leftmost;
-  size_t end = beg;
+  idx_t beg = _partitions.find_index_of_next_available_cluster_of_regions(ShenandoahFreeSetPartitionId::Mutator,
+                                                                          start_range, num);
+  if (beg > last_possible_start) {
+    // Hit the end, goodbye
+    return nullptr;
+  }
+  idx_t end = beg;
 
   while (true) {
-    if (end >= _max) {
-      // Hit the end, goodbye
-      return nullptr;
-    }
-
-    // If regions are not adjacent, then current [beg; end] is useless, and we may fast-forward.
-    // If region is not completely free, the current [beg; end] is useless, and we may fast-forward.
-    if (!is_mutator_free(end) || !can_allocate_from(_heap->get_region(end))) {
-      end++;
-      beg = end;
-      continue;
+    // We've confirmed num contiguous regions belonging to Mutator partition, so no need to confirm membership.
+    // If region is not completely free, the current [beg; end] is useless, and we may fast-forward.  If we can extend
+    // the existing range, we can exploit that certain regions are already known to be in the Mutator free set.
+    while (!can_allocate_from(_heap->get_region(end))) {
+      // region[end] is not empty, so we restart our search after region[end]
+      idx_t slide_delta = end + 1 - beg;
+      if (beg + slide_delta > last_possible_start) {
+        // no room to slide
+        return nullptr;
+      }
+      for (idx_t span_end = beg + num; slide_delta > 0; slide_delta--) {
+        if (!_partitions.in_free_set(ShenandoahFreeSetPartitionId::Mutator, span_end)) {
+          beg = _partitions.find_index_of_next_available_cluster_of_regions(ShenandoahFreeSetPartitionId::Mutator,
+                                                                            span_end + 1, num);
+          break;
+        } else {
+          beg++;
+          span_end++;
+        }
+      }
+      // Here, either beg identifies a range of num regions all of which are in the Mutator free set, or beg > last_possible_start
+      if (beg > last_possible_start) {
+        // Hit the end, goodbye
+        return nullptr;
+      }
+      end = beg;
     }
 
     if ((end - beg + 1) == num) {
       // found the match
       break;
@@ -280,13 +1208,12 @@
 
     end++;
   }
 
   size_t remainder = words_size & ShenandoahHeapRegion::region_size_words_mask();
-
   // Initialize regions:
-  for (size_t i = beg; i <= end; i++) {
+  for (idx_t i = beg; i <= end; i++) {
     ShenandoahHeapRegion* r = _heap->get_region(i);
     try_recycle_trashed(r);
 
     assert(i == beg || _heap->get_region(i - 1)->index() + 1 == r->index(), "Should be contiguous");
     assert(r->is_empty(), "Should be empty");
@@ -303,195 +1230,728 @@
       used_words = remainder;
     } else {
       used_words = ShenandoahHeapRegion::region_size_words();
     }
 
+    r->set_affiliation(req.affiliation());
+    r->set_update_watermark(r->bottom());
     r->set_top(r->bottom() + used_words);
-
-    _mutator_free_bitmap.clear_bit(r->index());
   }
-
-  // While individual regions report their true use, all humongous regions are
-  // marked used in the free set.
-  increase_used(ShenandoahHeapRegion::region_size_bytes() * num);
-
+  generation->increase_affiliated_region_count(num);
   if (remainder != 0) {
     // Record this remainder as allocation waste
     _heap->notify_mutator_alloc_words(ShenandoahHeapRegion::region_size_words() - remainder, true);
   }
 
-  // Allocated at left/rightmost? Move the bounds appropriately.
-  if (beg == _mutator_leftmost || end == _mutator_rightmost) {
-    adjust_bounds();
-  }
-  assert_bounds();
+  // retire_range_from_partition() will adjust bounds on Mutator free set if appropriate
+  _partitions.retire_range_from_partition(ShenandoahFreeSetPartitionId::Mutator, beg, end);
 
+  size_t total_humongous_size = ShenandoahHeapRegion::region_size_bytes() * num;
+  _partitions.increase_used(ShenandoahFreeSetPartitionId::Mutator, total_humongous_size);
+  _partitions.assert_bounds();
   req.set_actual_size(words_size);
-  return _heap->get_region(beg)->bottom();
-}
-
-bool ShenandoahFreeSet::can_allocate_from(ShenandoahHeapRegion *r) {
-  return r->is_empty() || (r->is_trash() && !_heap->is_concurrent_weak_root_in_progress());
-}
-
-size_t ShenandoahFreeSet::alloc_capacity(ShenandoahHeapRegion *r) {
-  if (r->is_trash()) {
-    // This would be recycled on allocation path
-    return ShenandoahHeapRegion::region_size_bytes();
-  } else {
-    return r->free();
+  if (remainder != 0) {
+    req.set_waste(ShenandoahHeapRegion::region_size_words() - remainder);
   }
+  return _heap->get_region(beg)->bottom();
 }
 
-bool ShenandoahFreeSet::has_no_alloc_capacity(ShenandoahHeapRegion *r) {
-  return alloc_capacity(r) == 0;
-}
-
-void ShenandoahFreeSet::try_recycle_trashed(ShenandoahHeapRegion *r) {
+void ShenandoahFreeSet::try_recycle_trashed(ShenandoahHeapRegion* r) {
   if (r->is_trash()) {
-    _heap->decrease_used(r->used());
     r->recycle();
   }
 }
 
 void ShenandoahFreeSet::recycle_trash() {
   // lock is not reentrable, check we don't have it
   shenandoah_assert_not_heaplocked();
-
+  size_t count = 0;
   for (size_t i = 0; i < _heap->num_regions(); i++) {
     ShenandoahHeapRegion* r = _heap->get_region(i);
     if (r->is_trash()) {
-      ShenandoahHeapLocker locker(_heap->lock());
-      try_recycle_trashed(r);
+      _trash_regions[count++] = r;
+    }
+  }
+
+  size_t total_batches = 0;
+  jlong batch_start_time = 0;
+  jlong recycle_trash_start_time = os::javaTimeNanos();    // This value will be treated as the initial batch_start_time
+  jlong batch_end_time = recycle_trash_start_time;
+  // Process as many batches as can be processed within 10 us.
+  static constexpr jlong deadline_ns = 10000;               // 10 us
+  size_t idx = 0;
+  jlong predicted_next_batch_end_time;
+  jlong batch_process_time_estimate = 0;
+  while (idx < count) {
+    if (idx > 0) {
+      os::naked_yield(); // Yield to allow allocators to take the lock, except on the first iteration
     }
-    SpinPause(); // allow allocators to take the lock
+    // Avoid another call to javaTimeNanos() if we already know time at which last batch ended
+    batch_start_time = batch_end_time;
+    const jlong deadline = batch_start_time + deadline_ns;
+
+    ShenandoahHeapLocker locker(_heap->lock());
+    do {
+      // Measurements on typical 2024 hardware suggest it typically requires between 1400 and 2000 ns to process a batch of
+      // 32 regions, assuming low contention with other threads.  Sometimes this goes higher, when mutator threads
+      // are contending for CPU cores and/or the heap lock.  On this hardware with a 10 us deadline, we expect 3-6 batches
+      // to be processed between yields most of the time.
+      //
+      // Note that deadline is enforced since the end of previous batch.  In the case that yield() or acquisition of heap lock
+      // takes a "long time", we will have less time to process regions, but we will always process at least one batch between
+      // yields.  Yielding more frequently when there is heavy contention for the heap lock or for CPU cores is considered the
+      // right thing to do.
+      const size_t REGIONS_PER_BATCH = 32;
+      size_t max_idx = MIN2(count, idx + REGIONS_PER_BATCH);
+      while (idx < max_idx) {
+        try_recycle_trashed(_trash_regions[idx++]);
+      }
+      total_batches++;
+      batch_end_time = os::javaTimeNanos();
+      // Estimate includes historic combination of yield times and heap lock acquisition times.
+      batch_process_time_estimate = (batch_end_time - recycle_trash_start_time) / total_batches;
+      predicted_next_batch_end_time = batch_end_time + batch_process_time_estimate;
+    } while ((idx < count) && (predicted_next_batch_end_time < deadline));
   }
 }
 
-void ShenandoahFreeSet::flip_to_gc(ShenandoahHeapRegion* r) {
+void ShenandoahFreeSet::flip_to_old_gc(ShenandoahHeapRegion* r) {
   size_t idx = r->index();
 
-  assert(_mutator_free_bitmap.at(idx), "Should be in mutator view");
+  assert(_partitions.partition_id_matches(idx, ShenandoahFreeSetPartitionId::Mutator), "Should be in mutator view");
   assert(can_allocate_from(r), "Should not be allocated");
 
-  _mutator_free_bitmap.clear_bit(idx);
-  _collector_free_bitmap.set_bit(idx);
-  _collector_leftmost = MIN2(idx, _collector_leftmost);
-  _collector_rightmost = MAX2(idx, _collector_rightmost);
+  ShenandoahGenerationalHeap* gen_heap = ShenandoahGenerationalHeap::heap();
+  size_t region_capacity = alloc_capacity(r);
+  _partitions.move_from_partition_to_partition(idx, ShenandoahFreeSetPartitionId::Mutator,
+                                               ShenandoahFreeSetPartitionId::OldCollector, region_capacity);
+  _partitions.assert_bounds();
+  _heap->old_generation()->augment_evacuation_reserve(region_capacity);
+  bool transferred = gen_heap->generation_sizer()->transfer_to_old(1);
+  if (!transferred) {
+    log_warning(gc, free)("Forcing transfer of " SIZE_FORMAT " to old reserve.", idx);
+    gen_heap->generation_sizer()->force_transfer_to_old(1);
+  }
+  // We do not ensure that the region is no longer trash, relying on try_allocate_in(), which always comes next,
+  // to recycle trash before attempting to allocate anything in the region.
+}
+
+void ShenandoahFreeSet::flip_to_gc(ShenandoahHeapRegion* r) {
+  size_t idx = r->index();
+
+  assert(_partitions.partition_id_matches(idx, ShenandoahFreeSetPartitionId::Mutator), "Should be in mutator view");
+  assert(can_allocate_from(r), "Should not be allocated");
 
-  _capacity -= alloc_capacity(r);
+  size_t ac = alloc_capacity(r);
+  _partitions.move_from_partition_to_partition(idx, ShenandoahFreeSetPartitionId::Mutator,
+                                               ShenandoahFreeSetPartitionId::Collector, ac);
+  _partitions.assert_bounds();
 
-  if (touches_bounds(idx)) {
-    adjust_bounds();
-  }
-  assert_bounds();
+  // We do not ensure that the region is no longer trash, relying on try_allocate_in(), which always comes next,
+  // to recycle trash before attempting to allocate anything in the region.
 }
 
 void ShenandoahFreeSet::clear() {
   shenandoah_assert_heaplocked();
   clear_internal();
 }
 
 void ShenandoahFreeSet::clear_internal() {
-  _mutator_free_bitmap.clear();
-  _collector_free_bitmap.clear();
-  _mutator_leftmost = _max;
-  _mutator_rightmost = 0;
-  _collector_leftmost = _max;
-  _collector_rightmost = 0;
-  _capacity = 0;
-  _used = 0;
+  _partitions.make_all_regions_unavailable();
+
+  _alloc_bias_weight = 0;
+  _partitions.set_bias_from_left_to_right(ShenandoahFreeSetPartitionId::Mutator, true);
+  _partitions.set_bias_from_left_to_right(ShenandoahFreeSetPartitionId::Collector, false);
+  _partitions.set_bias_from_left_to_right(ShenandoahFreeSetPartitionId::OldCollector, false);
 }
 
-void ShenandoahFreeSet::rebuild() {
-  shenandoah_assert_heaplocked();
-  clear();
+void ShenandoahFreeSet::find_regions_with_alloc_capacity(size_t &young_cset_regions, size_t &old_cset_regions,
+                                                         size_t &first_old_region, size_t &last_old_region,
+                                                         size_t &old_region_count) {
+  clear_internal();
 
-  for (size_t idx = 0; idx < _heap->num_regions(); idx++) {
+  first_old_region = _heap->num_regions();
+  last_old_region = 0;
+  old_region_count = 0;
+  old_cset_regions = 0;
+  young_cset_regions = 0;
+
+  size_t region_size_bytes = _partitions.region_size_bytes();
+  size_t max_regions = _partitions.max_regions();
+
+  size_t mutator_leftmost = max_regions;
+  size_t mutator_rightmost = 0;
+  size_t mutator_leftmost_empty = max_regions;
+  size_t mutator_rightmost_empty = 0;
+  size_t mutator_regions = 0;
+  size_t mutator_used = 0;
+
+  size_t old_collector_leftmost = max_regions;
+  size_t old_collector_rightmost = 0;
+  size_t old_collector_leftmost_empty = max_regions;
+  size_t old_collector_rightmost_empty = 0;
+  size_t old_collector_regions = 0;
+  size_t old_collector_used = 0;
+
+  size_t num_regions = _heap->num_regions();
+  for (size_t idx = 0; idx < num_regions; idx++) {
     ShenandoahHeapRegion* region = _heap->get_region(idx);
+    if (region->is_trash()) {
+      // Trashed regions represent regions that had been in the collection partition but have not yet been "cleaned up".
+      // The cset regions are not "trashed" until we have finished update refs.
+      if (region->is_old()) {
+        old_cset_regions++;
+      } else {
+        assert(region->is_young(), "Trashed region should be old or young");
+        young_cset_regions++;
+      }
+    } else if (region->is_old()) {
+      // count both humongous and regular regions, but don't count trash (cset) regions.
+      old_region_count++;
+      if (first_old_region > idx) {
+        first_old_region = idx;
+      }
+      last_old_region = idx;
+    }
     if (region->is_alloc_allowed() || region->is_trash()) {
-      assert(!region->is_cset(), "Shouldn't be adding those to the free set");
+      assert(!region->is_cset(), "Shouldn't be adding cset regions to the free set");
+
+      // Do not add regions that would almost surely fail allocation
+      size_t ac = alloc_capacity(region);
+      if (ac > PLAB::min_size() * HeapWordSize) {
+        if (region->is_trash() || !region->is_old()) {
+          // Both young and old collected regions (trashed) are placed into the Mutator set
+          _partitions.raw_assign_membership(idx, ShenandoahFreeSetPartitionId::Mutator);
+          if (idx < mutator_leftmost) {
+            mutator_leftmost = idx;
+          }
+          if (idx > mutator_rightmost) {
+            mutator_rightmost = idx;
+          }
+          if (ac == region_size_bytes) {
+            if (idx < mutator_leftmost_empty) {
+              mutator_leftmost_empty = idx;
+            }
+            if (idx > mutator_rightmost_empty) {
+              mutator_rightmost_empty = idx;
+            }
+          }
+          mutator_regions++;
+          mutator_used += (region_size_bytes - ac);
+        } else {
+          // !region->is_trash() && region is_old()
+          _partitions.raw_assign_membership(idx, ShenandoahFreeSetPartitionId::OldCollector);
+          if (idx < old_collector_leftmost) {
+            old_collector_leftmost = idx;
+          }
+          if (idx > old_collector_rightmost) {
+            old_collector_rightmost = idx;
+          }
+          if (ac == region_size_bytes) {
+            if (idx < old_collector_leftmost_empty) {
+              old_collector_leftmost_empty = idx;
+            }
+            if (idx > old_collector_rightmost_empty) {
+              old_collector_rightmost_empty = idx;
+            }
+          }
+          old_collector_regions++;
+          old_collector_used += (region_size_bytes - ac);
+        }
+      }
+    }
+  }
+  log_debug(gc)("  At end of prep_to_rebuild, mutator_leftmost: " SIZE_FORMAT
+                ", mutator_rightmost: " SIZE_FORMAT
+                ", mutator_leftmost_empty: " SIZE_FORMAT
+                ", mutator_rightmost_empty: " SIZE_FORMAT
+                ", mutator_regions: " SIZE_FORMAT
+                ", mutator_used: " SIZE_FORMAT,
+                mutator_leftmost, mutator_rightmost, mutator_leftmost_empty, mutator_rightmost_empty,
+                mutator_regions, mutator_used);
+
+  log_debug(gc)("  old_collector_leftmost: " SIZE_FORMAT
+                ", old_collector_rightmost: " SIZE_FORMAT
+                ", old_collector_leftmost_empty: " SIZE_FORMAT
+                ", old_collector_rightmost_empty: " SIZE_FORMAT
+                ", old_collector_regions: " SIZE_FORMAT
+                ", old_collector_used: " SIZE_FORMAT,
+                old_collector_leftmost, old_collector_rightmost, old_collector_leftmost_empty, old_collector_rightmost_empty,
+                old_collector_regions, old_collector_used);
+
+  idx_t rightmost_idx = (mutator_leftmost == max_regions)? -1: (idx_t) mutator_rightmost;
+  idx_t rightmost_empty_idx = (mutator_leftmost_empty == max_regions)? -1: (idx_t) mutator_rightmost_empty;
+  _partitions.establish_mutator_intervals(mutator_leftmost, rightmost_idx, mutator_leftmost_empty, rightmost_empty_idx,
+                                          mutator_regions, mutator_used);
+  rightmost_idx = (old_collector_leftmost == max_regions)? -1: (idx_t) old_collector_rightmost;
+  rightmost_empty_idx = (old_collector_leftmost_empty == max_regions)? -1: (idx_t) old_collector_rightmost_empty;
+  _partitions.establish_old_collector_intervals(old_collector_leftmost, rightmost_idx, old_collector_leftmost_empty,
+                                                rightmost_empty_idx, old_collector_regions, old_collector_used);
+  log_debug(gc)("  After find_regions_with_alloc_capacity(), Mutator range [" SSIZE_FORMAT ", " SSIZE_FORMAT "],"
+                "  Old Collector range [" SSIZE_FORMAT ", " SSIZE_FORMAT "]",
+                _partitions.leftmost(ShenandoahFreeSetPartitionId::Mutator),
+                _partitions.rightmost(ShenandoahFreeSetPartitionId::Mutator),
+                _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector),
+                _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector));
+}
+
+// Returns number of regions transferred, adds transferred bytes to var argument bytes_transferred
+size_t ShenandoahFreeSet::transfer_empty_regions_from_collector_set_to_mutator_set(ShenandoahFreeSetPartitionId which_collector,
+                                                                                   size_t max_xfer_regions,
+                                                                                   size_t& bytes_transferred) {
+  shenandoah_assert_heaplocked();
+  const size_t region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
+  size_t transferred_regions = 0;
+  ShenandoahLeftRightIterator iterator(&_partitions, which_collector, true);
+  idx_t rightmost = _partitions.rightmost_empty(which_collector);
+  for (idx_t idx = iterator.current(); transferred_regions < max_xfer_regions && iterator.has_next(); idx = iterator.next()) {
+    // Note: can_allocate_from() denotes that region is entirely empty
+    if (can_allocate_from(idx)) {
+      _partitions.move_from_partition_to_partition(idx, which_collector, ShenandoahFreeSetPartitionId::Mutator, region_size_bytes);
+      transferred_regions++;
+      bytes_transferred += region_size_bytes;
+    }
+  }
+  return transferred_regions;
+}
 
-      // Do not add regions that would surely fail allocation
-      if (has_no_alloc_capacity(region)) continue;
+// Returns number of regions transferred, adds transferred bytes to var argument bytes_transferred
+size_t ShenandoahFreeSet::transfer_non_empty_regions_from_collector_set_to_mutator_set(ShenandoahFreeSetPartitionId which_collector,
+                                                                                       size_t max_xfer_regions,
+                                                                                       size_t& bytes_transferred) {
+  shenandoah_assert_heaplocked();
+  size_t transferred_regions = 0;
+  ShenandoahLeftRightIterator iterator(&_partitions, which_collector, false);
+  for (idx_t idx = iterator.current(); transferred_regions < max_xfer_regions && iterator.has_next(); idx = iterator.next()) {
+    size_t ac = alloc_capacity(idx);
+    if (ac > 0) {
+      _partitions.move_from_partition_to_partition(idx, which_collector, ShenandoahFreeSetPartitionId::Mutator, ac);
+      transferred_regions++;
+      bytes_transferred += ac;
+    }
+  }
+  return transferred_regions;
+}
 
-      _capacity += alloc_capacity(region);
-      assert(_used <= _capacity, "must not use more than we have");
+void ShenandoahFreeSet::move_regions_from_collector_to_mutator(size_t max_xfer_regions) {
+  size_t collector_xfer = 0;
+  size_t old_collector_xfer = 0;
+
+  // Process empty regions within the Collector free partition
+  if ((max_xfer_regions > 0) &&
+      (_partitions.leftmost_empty(ShenandoahFreeSetPartitionId::Collector)
+       <= _partitions.rightmost_empty(ShenandoahFreeSetPartitionId::Collector))) {
+    ShenandoahHeapLocker locker(_heap->lock());
+    max_xfer_regions -=
+      transfer_empty_regions_from_collector_set_to_mutator_set(ShenandoahFreeSetPartitionId::Collector, max_xfer_regions,
+                                                               collector_xfer);
+  }
 
-      assert(!is_mutator_free(idx), "We are about to add it, it shouldn't be there already");
-      _mutator_free_bitmap.set_bit(idx);
+  // Process empty regions within the OldCollector free partition
+  if ((max_xfer_regions > 0) &&
+      (_partitions.leftmost_empty(ShenandoahFreeSetPartitionId::OldCollector)
+       <= _partitions.rightmost_empty(ShenandoahFreeSetPartitionId::OldCollector))) {
+    ShenandoahHeapLocker locker(_heap->lock());
+    size_t old_collector_regions =
+      transfer_empty_regions_from_collector_set_to_mutator_set(ShenandoahFreeSetPartitionId::OldCollector, max_xfer_regions,
+                                                               old_collector_xfer);
+    max_xfer_regions -= old_collector_regions;
+    if (old_collector_regions > 0) {
+      ShenandoahGenerationalHeap::cast(_heap)->generation_sizer()->transfer_to_young(old_collector_regions);
     }
   }
 
-  // Evac reserve: reserve trailing space for evacuations
-  size_t to_reserve = _heap->max_capacity() / 100 * ShenandoahEvacReserve;
-  size_t reserved = 0;
+  // If there are any non-empty regions within Collector partition, we can also move them to the Mutator free partition
+  if ((max_xfer_regions > 0) && (_partitions.leftmost(ShenandoahFreeSetPartitionId::Collector)
+                                 <= _partitions.rightmost(ShenandoahFreeSetPartitionId::Collector))) {
+    ShenandoahHeapLocker locker(_heap->lock());
+    max_xfer_regions -=
+      transfer_non_empty_regions_from_collector_set_to_mutator_set(ShenandoahFreeSetPartitionId::Collector, max_xfer_regions,
+                                                                   collector_xfer);
+  }
 
-  for (size_t idx = _heap->num_regions() - 1; idx > 0; idx--) {
-    if (reserved >= to_reserve) break;
+  size_t total_xfer = collector_xfer + old_collector_xfer;
+  log_info(gc, ergo)("At start of update refs, moving " SIZE_FORMAT "%s to Mutator free set from Collector Reserve ("
+                     SIZE_FORMAT "%s) and from Old Collector Reserve (" SIZE_FORMAT "%s)",
+                     byte_size_in_proper_unit(total_xfer), proper_unit_for_byte_size(total_xfer),
+                     byte_size_in_proper_unit(collector_xfer), proper_unit_for_byte_size(collector_xfer),
+                     byte_size_in_proper_unit(old_collector_xfer), proper_unit_for_byte_size(old_collector_xfer));
+}
 
-    ShenandoahHeapRegion* region = _heap->get_region(idx);
-    if (_mutator_free_bitmap.at(idx) && can_allocate_from(region)) {
-      _mutator_free_bitmap.clear_bit(idx);
-      _collector_free_bitmap.set_bit(idx);
-      size_t ac = alloc_capacity(region);
-      _capacity -= ac;
-      reserved += ac;
+
+// Overwrite arguments to represent the amount of memory in each generation that is about to be recycled
+void ShenandoahFreeSet::prepare_to_rebuild(size_t &young_cset_regions, size_t &old_cset_regions,
+                                           size_t &first_old_region, size_t &last_old_region, size_t &old_region_count) {
+  shenandoah_assert_heaplocked();
+  // This resets all state information, removing all regions from all sets.
+  clear();
+  log_debug(gc, free)("Rebuilding FreeSet");
+
+  // This places regions that have alloc_capacity into the old_collector set if they identify as is_old() or the
+  // mutator set otherwise.  All trashed (cset) regions are affiliated young and placed in mutator set.
+  find_regions_with_alloc_capacity(young_cset_regions, old_cset_regions, first_old_region, last_old_region, old_region_count);
+}
+
+void ShenandoahFreeSet::establish_generation_sizes(size_t young_region_count, size_t old_region_count) {
+  assert(young_region_count + old_region_count == ShenandoahHeap::heap()->num_regions(), "Sanity");
+  if (ShenandoahHeap::heap()->mode()->is_generational()) {
+    ShenandoahGenerationalHeap* heap = ShenandoahGenerationalHeap::heap();
+    ShenandoahOldGeneration* old_gen = heap->old_generation();
+    ShenandoahYoungGeneration* young_gen = heap->young_generation();
+    size_t region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
+
+    size_t original_old_capacity = old_gen->max_capacity();
+    size_t new_old_capacity = old_region_count * region_size_bytes;
+    size_t new_young_capacity = young_region_count * region_size_bytes;
+    old_gen->set_capacity(new_old_capacity);
+    young_gen->set_capacity(new_young_capacity);
+
+    if (new_old_capacity > original_old_capacity) {
+      size_t region_count = (new_old_capacity - original_old_capacity) / region_size_bytes;
+      log_info(gc, ergo)("Transfer " SIZE_FORMAT " region(s) from %s to %s, yielding increased size: " PROPERFMT,
+                         region_count, young_gen->name(), old_gen->name(), PROPERFMTARGS(new_old_capacity));
+    } else if (new_old_capacity < original_old_capacity) {
+      size_t region_count = (original_old_capacity - new_old_capacity) / region_size_bytes;
+      log_info(gc, ergo)("Transfer " SIZE_FORMAT " region(s) from %s to %s, yielding increased size: " PROPERFMT,
+                         region_count, old_gen->name(), young_gen->name(), PROPERFMTARGS(new_young_capacity));
+    }
+    // This balances generations, so clear any pending request to balance.
+    old_gen->set_region_balance(0);
+  }
+}
+
+void ShenandoahFreeSet::finish_rebuild(size_t young_cset_regions, size_t old_cset_regions, size_t old_region_count,
+                                       bool have_evacuation_reserves) {
+  shenandoah_assert_heaplocked();
+  size_t young_reserve(0), old_reserve(0);
+
+  if (_heap->mode()->is_generational()) {
+    compute_young_and_old_reserves(young_cset_regions, old_cset_regions, have_evacuation_reserves,
+                                   young_reserve, old_reserve);
+  } else {
+    young_reserve = (_heap->max_capacity() / 100) * ShenandoahEvacReserve;
+    old_reserve = 0;
+  }
+
+  // Move some of the mutator regions in the Collector and OldCollector partitions in order to satisfy
+  // young_reserve and old_reserve.
+  reserve_regions(young_reserve, old_reserve, old_region_count);
+  size_t young_region_count = _heap->num_regions() - old_region_count;
+  establish_generation_sizes(young_region_count, old_region_count);
+  establish_old_collector_alloc_bias();
+  _partitions.assert_bounds();
+  log_status();
+}
+
+void ShenandoahFreeSet::compute_young_and_old_reserves(size_t young_cset_regions, size_t old_cset_regions,
+                                                       bool have_evacuation_reserves,
+                                                       size_t& young_reserve_result, size_t& old_reserve_result) const {
+  shenandoah_assert_generational();
+  const size_t region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
+
+  ShenandoahOldGeneration* const old_generation = _heap->old_generation();
+  size_t old_available = old_generation->available();
+  size_t old_unaffiliated_regions = old_generation->free_unaffiliated_regions();
+  ShenandoahYoungGeneration* const young_generation = _heap->young_generation();
+  size_t young_capacity = young_generation->max_capacity();
+  size_t young_unaffiliated_regions = young_generation->free_unaffiliated_regions();
+
+  // Add in the regions we anticipate to be freed by evacuation of the collection set
+  old_unaffiliated_regions += old_cset_regions;
+  young_unaffiliated_regions += young_cset_regions;
+
+  // Consult old-region balance to make adjustments to current generation capacities and availability.
+  // The generation region transfers take place after we rebuild.
+  const ssize_t old_region_balance = old_generation->get_region_balance();
+  if (old_region_balance != 0) {
+#ifdef ASSERT
+    if (old_region_balance > 0) {
+      assert(old_region_balance <= checked_cast<ssize_t>(old_unaffiliated_regions), "Cannot transfer regions that are affiliated");
+    } else {
+      assert(0 - old_region_balance <= checked_cast<ssize_t>(young_unaffiliated_regions), "Cannot transfer regions that are affiliated");
+    }
+#endif
+
+    ssize_t xfer_bytes = old_region_balance * checked_cast<ssize_t>(region_size_bytes);
+    old_available -= xfer_bytes;
+    old_unaffiliated_regions -= old_region_balance;
+    young_capacity += xfer_bytes;
+    young_unaffiliated_regions += old_region_balance;
+  }
+
+  // All allocations taken from the old collector set are performed by GC, generally using PLABs for both
+  // promotions and evacuations.  The partition between which old memory is reserved for evacuation and
+  // which is reserved for promotion is enforced using thread-local variables that prescribe intentions for
+  // each PLAB's available memory.
+  if (have_evacuation_reserves) {
+    // We are rebuilding at the end of final mark, having already established evacuation budgets for this GC pass.
+    const size_t promoted_reserve = old_generation->get_promoted_reserve();
+    const size_t old_evac_reserve = old_generation->get_evacuation_reserve();
+    young_reserve_result = young_generation->get_evacuation_reserve();
+    old_reserve_result = promoted_reserve + old_evac_reserve;
+    assert(old_reserve_result <= old_available,
+           "Cannot reserve (" SIZE_FORMAT " + " SIZE_FORMAT") more OLD than is available: " SIZE_FORMAT,
+           promoted_reserve, old_evac_reserve, old_available);
+  } else {
+    // We are rebuilding at end of GC, so we set aside budgets specified on command line (or defaults)
+    young_reserve_result = (young_capacity * ShenandoahEvacReserve) / 100;
+    // The auto-sizer has already made old-gen large enough to hold all anticipated evacuations and promotions.
+    // Affiliated old-gen regions are already in the OldCollector free set.  Add in the relevant number of
+    // unaffiliated regions.
+    old_reserve_result = old_available;
+  }
+
+  // Old available regions that have less than PLAB::min_size() of available memory are not placed into the OldCollector
+  // free set.  Because of this, old_available may not have enough memory to represent the intended reserve.  Adjust
+  // the reserve downward to account for this possibility. This loss is part of the reason why the original budget
+  // was adjusted with ShenandoahOldEvacWaste and ShenandoahOldPromoWaste multipliers.
+  if (old_reserve_result >
+      _partitions.capacity_of(ShenandoahFreeSetPartitionId::OldCollector) + old_unaffiliated_regions * region_size_bytes) {
+    old_reserve_result =
+      _partitions.capacity_of(ShenandoahFreeSetPartitionId::OldCollector) + old_unaffiliated_regions * region_size_bytes;
+  }
+
+  if (young_reserve_result > young_unaffiliated_regions * region_size_bytes) {
+    young_reserve_result = young_unaffiliated_regions * region_size_bytes;
+  }
+}
+
+// Having placed all regions that have allocation capacity into the mutator set if they identify as is_young()
+// or into the old collector set if they identify as is_old(), move some of these regions from the mutator set
+// into the collector set or old collector set in order to assure that the memory available for allocations within
+// the collector set is at least to_reserve and the memory available for allocations within the old collector set
+// is at least to_reserve_old.
+void ShenandoahFreeSet::reserve_regions(size_t to_reserve, size_t to_reserve_old, size_t &old_region_count) {
+  for (size_t i = _heap->num_regions(); i > 0; i--) {
+    size_t idx = i - 1;
+    ShenandoahHeapRegion* r = _heap->get_region(idx);
+    if (!_partitions.in_free_set(ShenandoahFreeSetPartitionId::Mutator, idx)) {
+      continue;
+    }
+
+    size_t ac = alloc_capacity(r);
+    assert (ac > 0, "Membership in free set implies has capacity");
+    assert (!r->is_old() || r->is_trash(), "Except for trash, mutator_is_free regions should not be affiliated OLD");
+
+    bool move_to_old_collector = _partitions.available_in(ShenandoahFreeSetPartitionId::OldCollector) < to_reserve_old;
+    bool move_to_collector = _partitions.available_in(ShenandoahFreeSetPartitionId::Collector) < to_reserve;
+
+    if (!move_to_collector && !move_to_old_collector) {
+      // We've satisfied both to_reserve and to_reserved_old
+      break;
+    }
+
+    if (move_to_old_collector) {
+      // We give priority to OldCollector partition because we desire to pack OldCollector regions into higher
+      // addresses than Collector regions.  Presumably, OldCollector regions are more "stable" and less likely to
+      // be collected in the near future.
+      if (r->is_trash() || !r->is_affiliated()) {
+        // OLD regions that have available memory are already in the old_collector free set.
+        _partitions.move_from_partition_to_partition(idx, ShenandoahFreeSetPartitionId::Mutator,
+                                                     ShenandoahFreeSetPartitionId::OldCollector, ac);
+        log_debug(gc)("  Shifting region " SIZE_FORMAT " from mutator_free to old_collector_free", idx);
+        log_debug(gc)("  Shifted Mutator range [" SSIZE_FORMAT ", " SSIZE_FORMAT "],"
+                      "  Old Collector range [" SSIZE_FORMAT ", " SSIZE_FORMAT "]",
+                      _partitions.leftmost(ShenandoahFreeSetPartitionId::Mutator),
+                      _partitions.rightmost(ShenandoahFreeSetPartitionId::Mutator),
+                      _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector),
+                      _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector));
+        old_region_count++;
+        continue;
+      }
+    }
+
+    if (move_to_collector) {
+      // Note: In a previous implementation, regions were only placed into the survivor space (collector_is_free) if
+      // they were entirely empty.  This has the effect of causing new Mutator allocation to reside next to objects
+      // that have already survived at least one GC, mixing ephemeral with longer-lived objects in the same region.
+      // Any objects that have survived a GC are less likely to immediately become garbage, so a region that contains
+      // survivor objects is less likely to be selected for the collection set.  This alternative implementation allows
+      // survivor regions to continue accumulating other survivor objects, and makes it more likely that ephemeral objects
+      // occupy regions comprised entirely of ephemeral objects.  These regions are highly likely to be included in the next
+      // collection set, and they are easily evacuated because they have low density of live objects.
+      _partitions.move_from_partition_to_partition(idx, ShenandoahFreeSetPartitionId::Mutator,
+                                                   ShenandoahFreeSetPartitionId::Collector, ac);
+      log_debug(gc)("  Shifting region " SIZE_FORMAT " from mutator_free to collector_free", idx);
+      log_debug(gc)("  Shifted Mutator range [" SSIZE_FORMAT ", " SSIZE_FORMAT "],"
+                    "  Collector range [" SSIZE_FORMAT ", " SSIZE_FORMAT "]",
+                    _partitions.leftmost(ShenandoahFreeSetPartitionId::Mutator),
+                    _partitions.rightmost(ShenandoahFreeSetPartitionId::Mutator),
+                    _partitions.leftmost(ShenandoahFreeSetPartitionId::Collector),
+                    _partitions.rightmost(ShenandoahFreeSetPartitionId::Collector));
+    }
+  }
+
+  if (LogTarget(Info, gc, free)::is_enabled()) {
+    size_t old_reserve = _partitions.available_in(ShenandoahFreeSetPartitionId::OldCollector);
+    if (old_reserve < to_reserve_old) {
+      log_info(gc, free)("Wanted " PROPERFMT " for old reserve, but only reserved: " PROPERFMT,
+                         PROPERFMTARGS(to_reserve_old), PROPERFMTARGS(old_reserve));
+    }
+    size_t reserve = _partitions.available_in(ShenandoahFreeSetPartitionId::Collector);
+    if (reserve < to_reserve) {
+      log_debug(gc)("Wanted " PROPERFMT " for young reserve, but only reserved: " PROPERFMT,
+                    PROPERFMTARGS(to_reserve), PROPERFMTARGS(reserve));
+    }
+  }
+}
+
+void ShenandoahFreeSet::establish_old_collector_alloc_bias() {
+  ShenandoahHeap* heap = ShenandoahHeap::heap();
+  shenandoah_assert_heaplocked();
+
+  idx_t left_idx = _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector);
+  idx_t right_idx = _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector);
+  idx_t middle = (left_idx + right_idx) / 2;
+  size_t available_in_first_half = 0;
+  size_t available_in_second_half = 0;
+
+  for (idx_t index = left_idx; index < middle; index++) {
+    if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::OldCollector, index)) {
+      ShenandoahHeapRegion* r = heap->get_region((size_t) index);
+      available_in_first_half += r->free();
+    }
+  }
+  for (idx_t index = middle; index <= right_idx; index++) {
+    if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::OldCollector, index)) {
+      ShenandoahHeapRegion* r = heap->get_region(index);
+      available_in_second_half += r->free();
     }
   }
 
-  recompute_bounds();
-  assert_bounds();
+  // We desire to first consume the sparsely distributed regions in order that the remaining regions are densely packed.
+  // Densely packing regions reduces the effort to search for a region that has sufficient memory to satisfy a new allocation
+  // request.  Regions become sparsely distributed following a Full GC, which tends to slide all regions to the front of the
+  // heap rather than allowing survivor regions to remain at the high end of the heap where we intend for them to congregate.
+  _partitions.set_bias_from_left_to_right(ShenandoahFreeSetPartitionId::OldCollector,
+                                          (available_in_second_half > available_in_first_half));
+}
+
+void ShenandoahFreeSet::log_status_under_lock() {
+  // Must not be heap locked, it acquires heap lock only when log is enabled
+  shenandoah_assert_not_heaplocked();
+  if (LogTarget(Info, gc, free)::is_enabled()
+      DEBUG_ONLY(|| LogTarget(Debug, gc, free)::is_enabled())) {
+    ShenandoahHeapLocker locker(_heap->lock());
+    log_status();
+  }
 }
 
 void ShenandoahFreeSet::log_status() {
   shenandoah_assert_heaplocked();
 
-  LogTarget(Info, gc, ergo) lt;
+#ifdef ASSERT
+  // Dump of the FreeSet details is only enabled if assertions are enabled
+  if (LogTarget(Debug, gc, free)::is_enabled()) {
+#define BUFFER_SIZE 80
+
+    char buffer[BUFFER_SIZE];
+    for (uint i = 0; i < BUFFER_SIZE; i++) {
+      buffer[i] = '\0';
+    }
+
+    log_debug(gc)("FreeSet map legend:"
+                       " M:mutator_free C:collector_free O:old_collector_free"
+                       " H:humongous ~:retired old _:retired young");
+    log_debug(gc)(" mutator free range [" SIZE_FORMAT ".." SIZE_FORMAT "] allocating from %s, "
+                  " collector free range [" SIZE_FORMAT ".." SIZE_FORMAT "], "
+                  "old collector free range [" SIZE_FORMAT ".." SIZE_FORMAT "] allocates from %s",
+                  _partitions.leftmost(ShenandoahFreeSetPartitionId::Mutator),
+                  _partitions.rightmost(ShenandoahFreeSetPartitionId::Mutator),
+                  _partitions.alloc_from_left_bias(ShenandoahFreeSetPartitionId::Mutator)? "left to right": "right to left",
+                  _partitions.leftmost(ShenandoahFreeSetPartitionId::Collector),
+                  _partitions.rightmost(ShenandoahFreeSetPartitionId::Collector),
+                  _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector),
+                  _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector),
+                  _partitions.alloc_from_left_bias(ShenandoahFreeSetPartitionId::OldCollector)? "left to right": "right to left");
+
+    for (uint i = 0; i < _heap->num_regions(); i++) {
+      ShenandoahHeapRegion *r = _heap->get_region(i);
+      uint idx = i % 64;
+      if ((i != 0) && (idx == 0)) {
+        log_debug(gc)(" %6u: %s", i-64, buffer);
+      }
+      if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::Mutator, i)) {
+        size_t capacity = alloc_capacity(r);
+        assert(!r->is_old() || r->is_trash(), "Old regions except trash regions should not be in mutator_free set");
+        buffer[idx] = (capacity == ShenandoahHeapRegion::region_size_bytes()) ? 'M' : 'm';
+      } else if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::Collector, i)) {
+        size_t capacity = alloc_capacity(r);
+        assert(!r->is_old() || r->is_trash(), "Old regions except trash regions should not be in collector_free set");
+        buffer[idx] = (capacity == ShenandoahHeapRegion::region_size_bytes()) ? 'C' : 'c';
+      } else if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::OldCollector, i)) {
+        size_t capacity = alloc_capacity(r);
+        buffer[idx] = (capacity == ShenandoahHeapRegion::region_size_bytes()) ? 'O' : 'o';
+      } else if (r->is_humongous()) {
+        if (r->is_old()) {
+          buffer[idx] = 'H';
+        } else {
+          buffer[idx] = 'h';
+        }
+      } else {
+        if (r->is_old()) {
+          buffer[idx] = '~';
+        } else {
+          buffer[idx] = '_';
+        }
+      }
+    }
+    uint remnant = _heap->num_regions() % 64;
+    if (remnant > 0) {
+      buffer[remnant] = '\0';
+    } else {
+      remnant = 64;
+    }
+    log_debug(gc)(" %6u: %s", (uint) (_heap->num_regions() - remnant), buffer);
+  }
+#endif
+
+  LogTarget(Info, gc, free) lt;
   if (lt.is_enabled()) {
     ResourceMark rm;
     LogStream ls(lt);
 
     {
-      size_t last_idx = 0;
+      idx_t last_idx = 0;
       size_t max = 0;
       size_t max_contig = 0;
       size_t empty_contig = 0;
 
       size_t total_used = 0;
       size_t total_free = 0;
       size_t total_free_ext = 0;
 
-      for (size_t idx = _mutator_leftmost; idx <= _mutator_rightmost; idx++) {
-        if (is_mutator_free(idx)) {
+      for (idx_t idx = _partitions.leftmost(ShenandoahFreeSetPartitionId::Mutator);
+           idx <= _partitions.rightmost(ShenandoahFreeSetPartitionId::Mutator); idx++) {
+        if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::Mutator, idx)) {
           ShenandoahHeapRegion *r = _heap->get_region(idx);
           size_t free = alloc_capacity(r);
-
           max = MAX2(max, free);
-
           if (r->is_empty()) {
             total_free_ext += free;
             if (last_idx + 1 == idx) {
               empty_contig++;
             } else {
               empty_contig = 1;
             }
           } else {
             empty_contig = 0;
           }
-
           total_used += r->used();
           total_free += free;
-
           max_contig = MAX2(max_contig, empty_contig);
           last_idx = idx;
         }
       }
 
       size_t max_humongous = max_contig * ShenandoahHeapRegion::region_size_bytes();
       size_t free = capacity() - used();
 
+      // Since certain regions that belonged to the Mutator free partition at the time of most recent rebuild may have been
+      // retired, the sum of used and capacities within regions that are still in the Mutator free partition may not match
+      // my internally tracked values of used() and free().
+      assert(free == total_free, "Free memory should match");
       ls.print("Free: " SIZE_FORMAT "%s, Max: " SIZE_FORMAT "%s regular, " SIZE_FORMAT "%s humongous, ",
                byte_size_in_proper_unit(total_free),    proper_unit_for_byte_size(total_free),
                byte_size_in_proper_unit(max),           proper_unit_for_byte_size(max),
                byte_size_in_proper_unit(max_humongous), proper_unit_for_byte_size(max_humongous)
       );
@@ -504,204 +1964,158 @@
         frag_ext = 0;
       }
       ls.print(SIZE_FORMAT "%% external, ", frag_ext);
 
       size_t frag_int;
-      if (mutator_count() > 0) {
-        frag_int = (100 * (total_used / mutator_count()) / ShenandoahHeapRegion::region_size_bytes());
+      if (_partitions.count(ShenandoahFreeSetPartitionId::Mutator) > 0) {
+        frag_int = (100 * (total_used / _partitions.count(ShenandoahFreeSetPartitionId::Mutator))
+                    / ShenandoahHeapRegion::region_size_bytes());
       } else {
         frag_int = 0;
       }
       ls.print(SIZE_FORMAT "%% internal; ", frag_int);
+      ls.print("Used: " SIZE_FORMAT "%s, Mutator Free: " SIZE_FORMAT,
+               byte_size_in_proper_unit(total_used), proper_unit_for_byte_size(total_used),
+               _partitions.count(ShenandoahFreeSetPartitionId::Mutator));
     }
 
     {
       size_t max = 0;
       size_t total_free = 0;
+      size_t total_used = 0;
 
-      for (size_t idx = _collector_leftmost; idx <= _collector_rightmost; idx++) {
-        if (is_collector_free(idx)) {
+      for (idx_t idx = _partitions.leftmost(ShenandoahFreeSetPartitionId::Collector);
+           idx <= _partitions.rightmost(ShenandoahFreeSetPartitionId::Collector); idx++) {
+        if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::Collector, idx)) {
           ShenandoahHeapRegion *r = _heap->get_region(idx);
           size_t free = alloc_capacity(r);
           max = MAX2(max, free);
           total_free += free;
+          total_used += r->used();
         }
       }
+      ls.print(" Collector Reserve: " SIZE_FORMAT "%s, Max: " SIZE_FORMAT "%s; Used: " SIZE_FORMAT "%s",
+               byte_size_in_proper_unit(total_free), proper_unit_for_byte_size(total_free),
+               byte_size_in_proper_unit(max),        proper_unit_for_byte_size(max),
+               byte_size_in_proper_unit(total_used), proper_unit_for_byte_size(total_used));
+    }
 
-      ls.print_cr("Reserve: " SIZE_FORMAT "%s, Max: " SIZE_FORMAT "%s",
+    if (_heap->mode()->is_generational()) {
+      size_t max = 0;
+      size_t total_free = 0;
+      size_t total_used = 0;
+
+      for (idx_t idx = _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector);
+           idx <= _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector); idx++) {
+        if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::OldCollector, idx)) {
+          ShenandoahHeapRegion *r = _heap->get_region(idx);
+          size_t free = alloc_capacity(r);
+          max = MAX2(max, free);
+          total_free += free;
+          total_used += r->used();
+        }
+      }
+      ls.print_cr(" Old Collector Reserve: " SIZE_FORMAT "%s, Max: " SIZE_FORMAT "%s; Used: " SIZE_FORMAT "%s",
                   byte_size_in_proper_unit(total_free), proper_unit_for_byte_size(total_free),
-                  byte_size_in_proper_unit(max),        proper_unit_for_byte_size(max));
+                  byte_size_in_proper_unit(max),        proper_unit_for_byte_size(max),
+                  byte_size_in_proper_unit(total_used), proper_unit_for_byte_size(total_used));
     }
   }
 }
 
 HeapWord* ShenandoahFreeSet::allocate(ShenandoahAllocRequest& req, bool& in_new_region) {
   shenandoah_assert_heaplocked();
-  assert_bounds();
-
-  if (req.size() > ShenandoahHeapRegion::humongous_threshold_words()) {
+  if (ShenandoahHeapRegion::requires_humongous(req.size())) {
     switch (req.type()) {
       case ShenandoahAllocRequest::_alloc_shared:
       case ShenandoahAllocRequest::_alloc_shared_gc:
         in_new_region = true;
         return allocate_contiguous(req);
+      case ShenandoahAllocRequest::_alloc_plab:
       case ShenandoahAllocRequest::_alloc_gclab:
       case ShenandoahAllocRequest::_alloc_tlab:
         in_new_region = false;
-        assert(false, "Trying to allocate TLAB larger than the humongous threshold: " SIZE_FORMAT " > " SIZE_FORMAT,
-               req.size(), ShenandoahHeapRegion::humongous_threshold_words());
+        assert(false, "Trying to allocate TLAB in humongous region: " SIZE_FORMAT, req.size());
         return nullptr;
       default:
         ShouldNotReachHere();
         return nullptr;
     }
   } else {
     return allocate_single(req, in_new_region);
   }
 }
 
-size_t ShenandoahFreeSet::unsafe_peek_free() const {
-  // Deliberately not locked, this method is unsafe when free set is modified.
-
-  for (size_t index = _mutator_leftmost; index <= _mutator_rightmost; index++) {
-    if (index < _max && is_mutator_free(index)) {
-      ShenandoahHeapRegion* r = _heap->get_region(index);
-      if (r->free() >= MinTLABSize) {
-        return r->free();
-      }
-    }
+void ShenandoahFreeSet::print_on(outputStream* out) const {
+  out->print_cr("Mutator Free Set: " SIZE_FORMAT "", _partitions.count(ShenandoahFreeSetPartitionId::Mutator));
+  ShenandoahLeftRightIterator mutator(const_cast<ShenandoahRegionPartitions*>(&_partitions), ShenandoahFreeSetPartitionId::Mutator);
+  for (idx_t index = mutator.current(); mutator.has_next(); index = mutator.next()) {
+    _heap->get_region(index)->print_on(out);
   }
 
-  // It appears that no regions left
-  return 0;
-}
-
-void ShenandoahFreeSet::print_on(outputStream* out) const {
-  out->print_cr("Mutator Free Set: " SIZE_FORMAT "", mutator_count());
-  for (size_t index = _mutator_leftmost; index <= _mutator_rightmost; index++) {
-    if (is_mutator_free(index)) {
-      _heap->get_region(index)->print_on(out);
-    }
+  out->print_cr("Collector Free Set: " SIZE_FORMAT "", _partitions.count(ShenandoahFreeSetPartitionId::Collector));
+  ShenandoahLeftRightIterator collector(const_cast<ShenandoahRegionPartitions*>(&_partitions), ShenandoahFreeSetPartitionId::Collector);
+  for (idx_t index = collector.current(); collector.has_next(); index = collector.next()) {
+    _heap->get_region(index)->print_on(out);
   }
-  out->print_cr("Collector Free Set: " SIZE_FORMAT "", collector_count());
-  for (size_t index = _collector_leftmost; index <= _collector_rightmost; index++) {
-    if (is_collector_free(index)) {
-      _heap->get_region(index)->print_on(out);
+
+  if (_heap->mode()->is_generational()) {
+    out->print_cr("Old Collector Free Set: " SIZE_FORMAT "", _partitions.count(ShenandoahFreeSetPartitionId::OldCollector));
+    for (idx_t index = _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector);
+         index <= _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector); index++) {
+      if (_partitions.in_free_set(ShenandoahFreeSetPartitionId::OldCollector, index)) {
+        _heap->get_region(index)->print_on(out);
+      }
     }
   }
 }
 
-/*
- * Internal fragmentation metric: describes how fragmented the heap regions are.
- *
- * It is derived as:
- *
- *               sum(used[i]^2, i=0..k)
- *   IF = 1 - ------------------------------
- *              C * sum(used[i], i=0..k)
- *
- * ...where k is the number of regions in computation, C is the region capacity, and
- * used[i] is the used space in the region.
- *
- * The non-linearity causes IF to be lower for the cases where the same total heap
- * used is densely packed. For example:
- *   a) Heap is completely full  => IF = 0
- *   b) Heap is half full, first 50% regions are completely full => IF = 0
- *   c) Heap is half full, each region is 50% full => IF = 1/2
- *   d) Heap is quarter full, first 50% regions are completely full => IF = 0
- *   e) Heap is quarter full, each region is 25% full => IF = 3/4
- *   f) Heap has one small object per each region => IF =~ 1
- */
 double ShenandoahFreeSet::internal_fragmentation() {
   double squared = 0;
   double linear = 0;
 
-  for (size_t index = _mutator_leftmost; index <= _mutator_rightmost; index++) {
-    if (is_mutator_free(index)) {
-      ShenandoahHeapRegion* r = _heap->get_region(index);
-      size_t used = r->used();
-      squared += used * used;
-      linear += used;
-    }
+  ShenandoahLeftRightIterator iterator(&_partitions, ShenandoahFreeSetPartitionId::Mutator);
+  for (idx_t index = iterator.current(); iterator.has_next(); index = iterator.next()) {
+    ShenandoahHeapRegion* r = _heap->get_region(index);
+    size_t used = r->used();
+    squared += used * used;
+    linear += used;
   }
 
   if (linear > 0) {
     double s = squared / (ShenandoahHeapRegion::region_size_bytes() * linear);
     return 1 - s;
   } else {
     return 0;
   }
 }
 
-/*
- * External fragmentation metric: describes how fragmented the heap is.
- *
- * It is derived as:
- *
- *   EF = 1 - largest_contiguous_free / total_free
- *
- * For example:
- *   a) Heap is completely empty => EF = 0
- *   b) Heap is completely full => EF = 0
- *   c) Heap is first-half full => EF = 1/2
- *   d) Heap is half full, full and empty regions interleave => EF =~ 1
- */
 double ShenandoahFreeSet::external_fragmentation() {
-  size_t last_idx = 0;
+  idx_t last_idx = 0;
   size_t max_contig = 0;
   size_t empty_contig = 0;
-
   size_t free = 0;
 
-  for (size_t index = _mutator_leftmost; index <= _mutator_rightmost; index++) {
-    if (is_mutator_free(index)) {
-      ShenandoahHeapRegion* r = _heap->get_region(index);
-      if (r->is_empty()) {
-        free += ShenandoahHeapRegion::region_size_bytes();
-        if (last_idx + 1 == index) {
-          empty_contig++;
-        } else {
-          empty_contig = 1;
-        }
+  ShenandoahLeftRightIterator iterator(&_partitions, ShenandoahFreeSetPartitionId::Mutator);
+  for (idx_t index = iterator.current(); iterator.has_next(); index = iterator.next()) {
+    ShenandoahHeapRegion* r = _heap->get_region(index);
+    if (r->is_empty()) {
+      free += ShenandoahHeapRegion::region_size_bytes();
+      if (last_idx + 1 == index) {
+        empty_contig++;
       } else {
-        empty_contig = 0;
+        empty_contig = 1;
       }
-
-      max_contig = MAX2(max_contig, empty_contig);
-      last_idx = index;
+    } else {
+      empty_contig = 0;
     }
+    max_contig = MAX2(max_contig, empty_contig);
+    last_idx = index;
   }
 
   if (free > 0) {
     return 1 - (1.0 * max_contig * ShenandoahHeapRegion::region_size_bytes() / free);
   } else {
     return 0;
   }
 }
 
-#ifdef ASSERT
-void ShenandoahFreeSet::assert_bounds() const {
-  // Performance invariants. Failing these would not break the free set, but performance
-  // would suffer.
-  assert (_mutator_leftmost <= _max, "leftmost in bounds: "  SIZE_FORMAT " < " SIZE_FORMAT, _mutator_leftmost,  _max);
-  assert (_mutator_rightmost < _max, "rightmost in bounds: " SIZE_FORMAT " < " SIZE_FORMAT, _mutator_rightmost, _max);
-
-  assert (_mutator_leftmost == _max || is_mutator_free(_mutator_leftmost),  "leftmost region should be free: " SIZE_FORMAT,  _mutator_leftmost);
-  assert (_mutator_rightmost == 0   || is_mutator_free(_mutator_rightmost), "rightmost region should be free: " SIZE_FORMAT, _mutator_rightmost);
-
-  size_t beg_off = _mutator_free_bitmap.find_first_set_bit(0);
-  size_t end_off = _mutator_free_bitmap.find_first_set_bit(_mutator_rightmost + 1);
-  assert (beg_off >= _mutator_leftmost, "free regions before the leftmost: " SIZE_FORMAT ", bound " SIZE_FORMAT, beg_off, _mutator_leftmost);
-  assert (end_off == _max,      "free regions past the rightmost: " SIZE_FORMAT ", bound " SIZE_FORMAT,  end_off, _mutator_rightmost);
-
-  assert (_collector_leftmost <= _max, "leftmost in bounds: "  SIZE_FORMAT " < " SIZE_FORMAT, _collector_leftmost,  _max);
-  assert (_collector_rightmost < _max, "rightmost in bounds: " SIZE_FORMAT " < " SIZE_FORMAT, _collector_rightmost, _max);
-
-  assert (_collector_leftmost == _max || is_collector_free(_collector_leftmost),  "leftmost region should be free: " SIZE_FORMAT,  _collector_leftmost);
-  assert (_collector_rightmost == 0   || is_collector_free(_collector_rightmost), "rightmost region should be free: " SIZE_FORMAT, _collector_rightmost);
-
-  beg_off = _collector_free_bitmap.find_first_set_bit(0);
-  end_off = _collector_free_bitmap.find_first_set_bit(_collector_rightmost + 1);
-  assert (beg_off >= _collector_leftmost, "free regions before the leftmost: " SIZE_FORMAT ", bound " SIZE_FORMAT, beg_off, _collector_leftmost);
-  assert (end_off == _max,      "free regions past the rightmost: " SIZE_FORMAT ", bound " SIZE_FORMAT,  end_off, _collector_rightmost);
-}
-#endif
