runtime/vm/heap/pages.h - sdk.git - Git at Google

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.

 #ifndef RUNTIME_VM_HEAP_PAGES_H_
 #define RUNTIME_VM_HEAP_PAGES_H_

 #include "platform/atomic.h"
 #include "vm/globals.h"
 #include "vm/heap/freelist.h"
 #include "vm/heap/spaces.h"
 #include "vm/lockers.h"
 #include "vm/ring_buffer.h"
 #include "vm/thread.h"
 #include "vm/virtual_memory.h"

 namespace dart {

 DECLARE_FLAG(bool, write_protect_code);

 // Forward declarations.
 class Heap;
 class JSONObject;
 class ObjectPointerVisitor;
 class ObjectSet;
 class ForwardingPage;
 class GCMarker;

 static constexpr intptr_t kOldPageSize = 512 * KB;
 static constexpr intptr_t kOldPageSizeInWords = kOldPageSize / kWordSize;
 static constexpr intptr_t kOldPageMask = ~(kOldPageSize - 1);

 static constexpr intptr_t kBitVectorWordsPerBlock = 1;
 static constexpr intptr_t kBlockSize =
     kObjectAlignment * kBitsPerWord * kBitVectorWordsPerBlock;
 static constexpr intptr_t kBlockMask = ~(kBlockSize - 1);
 static constexpr intptr_t kBlocksPerPage = kOldPageSize / kBlockSize;

 // A page containing old generation objects.
 class OldPage {
  public:
   enum PageType { kExecutable = 0, kData };

   OldPage* next() const { return next_; }
   void set_next(OldPage* next) { next_ = next; }

   bool Contains(uword addr) const { return memory_->Contains(addr); }
   intptr_t AliasOffset() const { return memory_->AliasOffset(); }

   uword object_start() const { return memory_->start() + ObjectStartOffset(); }
   uword object_end() const { return object_end_; }
   uword used_in_bytes() const { return used_in_bytes_; }
   void set_used_in_bytes(uword value) {
     ASSERT(Utils::IsAligned(value, kObjectAlignment));
     used_in_bytes_ = value;
   }

   ForwardingPage* forwarding_page() const { return forwarding_page_; }
   void AllocateForwardingPage();

   PageType type() const { return type_; }

   bool is_image_page() const { return !memory_->vm_owns_region(); }

   void VisitObjects(ObjectVisitor* visitor) const;
   void VisitObjectPointers(ObjectPointerVisitor* visitor) const;

   ObjectPtr FindObject(FindObjectVisitor* visitor) const;

   void WriteProtect(bool read_only);

   static intptr_t ObjectStartOffset() {
     return Utils::RoundUp(sizeof(OldPage), kMaxObjectAlignment);
   }

   // Warning: This does not work for objects on image pages because image pages
   // are not aligned. However, it works for objects on large pages, because
   // only one object is allocated per large page.
   static OldPage* Of(ObjectPtr obj) {
     ASSERT(obj->IsHeapObject());
     ASSERT(obj->IsOldObject());
     return reinterpret_cast<OldPage*>(static_cast<uword>(obj) & kOldPageMask);
   }

   // Warning: This does not work for addresses on image pages or on large pages.
   static OldPage* Of(uword addr) {
     return reinterpret_cast<OldPage*>(addr & kOldPageMask);
   }

   // Warning: This does not work for objects on image pages.
   static ObjectPtr ToExecutable(ObjectPtr obj) {
     OldPage* page = Of(obj);
     VirtualMemory* memory = page->memory_;
     const intptr_t alias_offset = memory->AliasOffset();
     if (alias_offset == 0) {
       return obj;  // Not aliased.
     }
     uword addr = UntaggedObject::ToAddr(obj);
     if (memory->Contains(addr)) {
       return UntaggedObject::FromAddr(addr + alias_offset);
     }
     // obj is executable.
     ASSERT(memory->ContainsAlias(addr));
     return obj;
   }

   // Warning: This does not work for objects on image pages.
   static ObjectPtr ToWritable(ObjectPtr obj) {
     OldPage* page = Of(obj);
     VirtualMemory* memory = page->memory_;
     const intptr_t alias_offset = memory->AliasOffset();
     if (alias_offset == 0) {
       return obj;  // Not aliased.
     }
     uword addr = UntaggedObject::ToAddr(obj);
     if (memory->ContainsAlias(addr)) {
       return UntaggedObject::FromAddr(addr - alias_offset);
     }
     // obj is writable.
     ASSERT(memory->Contains(addr));
     return obj;
   }

   // 1 card = 128 slots.
   static const intptr_t kSlotsPerCardLog2 = 7;
   static const intptr_t kBytesPerCardLog2 =
       kCompressedWordSizeLog2 + kSlotsPerCardLog2;

   intptr_t card_table_size() const {
     return memory_->size() >> kBytesPerCardLog2;
   }

   static intptr_t card_table_offset() {
     return OFFSET_OF(OldPage, card_table_);
   }

   void RememberCard(ObjectPtr const* slot) {
     ASSERT(Contains(reinterpret_cast<uword>(slot)));
     if (card_table_ == NULL) {
       card_table_ = reinterpret_cast<uint8_t*>(
           calloc(card_table_size(), sizeof(uint8_t)));
     }
     intptr_t offset =
         reinterpret_cast<uword>(slot) - reinterpret_cast<uword>(this);
     intptr_t index = offset >> kBytesPerCardLog2;
     ASSERT((index >= 0) && (index < card_table_size()));
     card_table_[index] = 1;
   }
   bool IsCardRemembered(ObjectPtr const* slot) {
     ASSERT(Contains(reinterpret_cast<uword>(slot)));
     if (card_table_ == NULL) {
       return false;
     }
     intptr_t offset =
         reinterpret_cast<uword>(slot) - reinterpret_cast<uword>(this);
     intptr_t index = offset >> kBytesPerCardLog2;
     ASSERT((index >= 0) && (index < card_table_size()));
     return card_table_[index] != 0;
   }
 #if defined(DART_COMPRESSED_POINTERS)
   void RememberCard(CompressedObjectPtr const* slot) {
     ASSERT(Contains(reinterpret_cast<uword>(slot)));
     if (card_table_ == NULL) {
       card_table_ = reinterpret_cast<uint8_t*>(
           calloc(card_table_size(), sizeof(uint8_t)));
     }
     intptr_t offset =
         reinterpret_cast<uword>(slot) - reinterpret_cast<uword>(this);
     intptr_t index = offset >> kBytesPerCardLog2;
     ASSERT((index >= 0) && (index < card_table_size()));
     card_table_[index] = 1;
   }
   bool IsCardRemembered(CompressedObjectPtr const* slot) {
     ASSERT(Contains(reinterpret_cast<uword>(slot)));
     if (card_table_ == NULL) {
       return false;
     }
     intptr_t offset =
         reinterpret_cast<uword>(slot) - reinterpret_cast<uword>(this);
     intptr_t index = offset >> kBytesPerCardLog2;
     ASSERT((index >= 0) && (index < card_table_size()));
     return card_table_[index] != 0;
   }
 #endif
   void VisitRememberedCards(ObjectPointerVisitor* visitor);

  private:
   void set_object_end(uword value) {
     ASSERT((value & kObjectAlignmentMask) == kOldObjectAlignmentOffset);
     object_end_ = value;
   }

   // Returns NULL on OOM.
   static OldPage* Allocate(intptr_t size_in_words,
                            PageType type,
                            const char* name);

   // Deallocate the virtual memory backing this page. The page pointer to this
   // page becomes immediately inaccessible.
   void Deallocate();

   VirtualMemory* memory_;
   OldPage* next_;
   uword object_end_;
   uword used_in_bytes_;
   ForwardingPage* forwarding_page_;
   uint8_t* card_table_;  // Remembered set, not marking.
   PageType type_;

   friend class PageSpace;
   friend class GCCompactor;

   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(OldPage);
 };

 // The history holds the timing information of the last garbage collection
 // runs.
 class PageSpaceGarbageCollectionHistory {
  public:
   PageSpaceGarbageCollectionHistory() {}
   ~PageSpaceGarbageCollectionHistory() {}

   void AddGarbageCollectionTime(int64_t start, int64_t end);

   int GarbageCollectionTimeFraction();

   bool IsEmpty() const { return history_.Size() == 0; }

  private:
   struct Entry {
     int64_t start;
     int64_t end;
   };
   static const intptr_t kHistoryLength = 4;
   RingBuffer<Entry, kHistoryLength> history_;

   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(PageSpaceGarbageCollectionHistory);
 };

 // PageSpaceController controls the heap size.
 class PageSpaceController {
  public:
   // The heap is passed in for recording stats only. The controller does not
   // invoke GC by itself.
   PageSpaceController(Heap* heap,
                       int heap_growth_ratio,
                       int heap_growth_max,
                       int garbage_collection_time_ratio);
   ~PageSpaceController();

   // Returns whether growing to 'after' should trigger a GC.
   // This method can be called before allocation (e.g., pretenuring) or after
   // (e.g., promotion), as it does not change the state of the controller.
   bool ReachedHardThreshold(SpaceUsage after) const;
   bool ReachedSoftThreshold(SpaceUsage after) const;

   // Returns whether an idle GC is worthwhile.
   bool ReachedIdleThreshold(SpaceUsage current) const;

   // Should be called after each collection to update the controller state.
   void EvaluateGarbageCollection(SpaceUsage before,
                                  SpaceUsage after,
                                  int64_t start,
                                  int64_t end);
   void EvaluateAfterLoading(SpaceUsage after);
   void HintFreed(intptr_t size);

   void set_last_usage(SpaceUsage current) { last_usage_ = current; }

   void Enable() { is_enabled_ = true; }
   void Disable() { is_enabled_ = false; }
   bool is_enabled() { return is_enabled_; }

  private:
   friend class PageSpace;  // For MergeOtherPageSpaceController

   void RecordUpdate(SpaceUsage before, SpaceUsage after, const char* reason);
   void RecordUpdate(SpaceUsage before,
                     SpaceUsage after,
                     intptr_t growth_in_pages,
                     const char* reason);

   Heap* heap_;

   bool is_enabled_;

   // Usage after last evaluated GC or last enabled.
   SpaceUsage last_usage_;

   // If the garbage collector was not able to free more than heap_growth_ratio_
   // memory, then the heap is grown. Otherwise garbage collection is performed.
   const int heap_growth_ratio_;

   // The desired percent of heap in-use after a garbage collection.
   // Equivalent to \frac{100-heap_growth_ratio_}{100}.
   const double desired_utilization_;

   // Max number of pages we grow.
   const int heap_growth_max_;

   // If the relative GC time goes above garbage_collection_time_ratio_ %,
   // we grow the heap more aggressively.
   const int garbage_collection_time_ratio_;

   // Perform a stop-the-world GC when usage exceeds this amount.
   intptr_t hard_gc_threshold_in_words_;

   // Begin concurrent marking when usage exceeds this amount.
   intptr_t soft_gc_threshold_in_words_;

   // Run idle GC if time permits when usage exceeds this amount.
   intptr_t idle_gc_threshold_in_words_;

   PageSpaceGarbageCollectionHistory history_;

   DISALLOW_IMPLICIT_CONSTRUCTORS(PageSpaceController);
 };

 class PageSpace {
  public:
   enum GrowthPolicy { kControlGrowth, kForceGrowth };
   enum Phase {
     kDone,
     kMarking,
     kAwaitingFinalization,
     kSweepingLarge,
     kSweepingRegular
   };

   PageSpace(Heap* heap, intptr_t max_capacity_in_words);
   ~PageSpace();

   uword TryAllocate(intptr_t size,
                     OldPage::PageType type = OldPage::kData,
                     GrowthPolicy growth_policy = kControlGrowth) {
     bool is_protected =
         (type == OldPage::kExecutable) && FLAG_write_protect_code;
     bool is_locked = false;
     return TryAllocateInternal(size, &freelists_[type], type, growth_policy,
                                is_protected, is_locked);
   }

   void TryReleaseReservation();
   bool MarkReservation();
   void TryReserveForOOM();
   void VisitRoots(ObjectPointerVisitor* visitor);

   bool ReachedHardThreshold() const {
     return page_space_controller_.ReachedHardThreshold(usage_);
   }
   bool ReachedSoftThreshold() const {
     return page_space_controller_.ReachedSoftThreshold(usage_);
   }
   bool ReachedIdleThreshold() const {
     return page_space_controller_.ReachedIdleThreshold(usage_);
   }
   void EvaluateAfterLoading() {
     page_space_controller_.EvaluateAfterLoading(usage_);
   }
   void HintFreed(intptr_t size) { page_space_controller_.HintFreed(size); }

   int64_t UsedInWords() const { return usage_.used_in_words; }
   int64_t CapacityInWords() const {
     MutexLocker ml(&pages_lock_);
     return usage_.capacity_in_words;
   }
   void IncreaseCapacityInWords(intptr_t increase_in_words) {
     MutexLocker ml(&pages_lock_);
     IncreaseCapacityInWordsLocked(increase_in_words);
   }
   void IncreaseCapacityInWordsLocked(intptr_t increase_in_words) {
     DEBUG_ASSERT(pages_lock_.IsOwnedByCurrentThread());
     usage_.capacity_in_words += increase_in_words;
     UpdateMaxCapacityLocked();
   }

   void UpdateMaxCapacityLocked();
   void UpdateMaxUsed();

   int64_t ExternalInWords() const { return usage_.external_in_words; }
   SpaceUsage GetCurrentUsage() const {
     MutexLocker ml(&pages_lock_);
     return usage_;
   }
   int64_t ImageInWords() const {
     int64_t size = 0;
     MutexLocker ml(&pages_lock_);
     for (OldPage* page = image_pages_; page != nullptr; page = page->next()) {
       size += page->memory_->size();
     }
     return size >> kWordSizeLog2;
   }

   bool Contains(uword addr) const;
   bool ContainsUnsafe(uword addr) const;
   bool Contains(uword addr, OldPage::PageType type) const;
   bool DataContains(uword addr) const;
   bool IsValidAddress(uword addr) const { return Contains(addr); }

   void VisitObjects(ObjectVisitor* visitor) const;
   void VisitObjectsNoImagePages(ObjectVisitor* visitor) const;
   void VisitObjectsImagePages(ObjectVisitor* visitor) const;
   void VisitObjectPointers(ObjectPointerVisitor* visitor) const;

   void VisitRememberedCards(ObjectPointerVisitor* visitor) const;

   ObjectPtr FindObject(FindObjectVisitor* visitor,
                        OldPage::PageType type) const;

   // Collect the garbage in the page space using mark-sweep or mark-compact.
   void CollectGarbage(bool compact, bool finalize);

   void AddRegionsToObjectSet(ObjectSet* set) const;

   void InitGrowthControl() {
     page_space_controller_.set_last_usage(usage_);
     page_space_controller_.Enable();
   }

   void SetGrowthControlState(bool state) {
     if (state) {
       page_space_controller_.Enable();
     } else {
       page_space_controller_.Disable();
     }
   }

   bool GrowthControlState() { return page_space_controller_.is_enabled(); }

   // Note: Code pages are made executable/non-executable when 'read_only' is
   // true/false, respectively.
   void WriteProtect(bool read_only);
   void WriteProtectCode(bool read_only);

   bool ShouldStartIdleMarkSweep(int64_t deadline);
   bool ShouldPerformIdleMarkCompact(int64_t deadline);

   void AddGCTime(int64_t micros) { gc_time_micros_ += micros; }

   int64_t gc_time_micros() const { return gc_time_micros_; }

   void IncrementCollections() { collections_++; }

   intptr_t collections() const { return collections_; }

 #ifndef PRODUCT
   void PrintToJSONObject(JSONObject* object) const;
   void PrintHeapMapToJSONStream(IsolateGroup* isolate_group,
                                 JSONStream* stream) const;
 #endif  // PRODUCT

   void AllocateBlack(intptr_t size) {
     allocated_black_in_words_.fetch_add(size >> kWordSizeLog2);
   }

   void AllocatedExternal(intptr_t size) {
     ASSERT(size >= 0);
     intptr_t size_in_words = size >> kWordSizeLog2;
     usage_.external_in_words += size_in_words;
   }
   void FreedExternal(intptr_t size) {
     ASSERT(size >= 0);
     intptr_t size_in_words = size >> kWordSizeLog2;
     usage_.external_in_words -= size_in_words;
   }

   // Bulk data allocation.
   FreeList* DataFreeList(intptr_t i = 0) {
     return &freelists_[OldPage::kData + i];
   }
   void AcquireLock(FreeList* freelist);
   void ReleaseLock(FreeList* freelist);

   uword TryAllocateDataLocked(FreeList* freelist,
                               intptr_t size,
                               GrowthPolicy growth_policy) {
     bool is_protected = false;
     bool is_locked = true;
     return TryAllocateInternal(size, freelist, OldPage::kData, growth_policy,
                                is_protected, is_locked);
   }

   Monitor* tasks_lock() const { return &tasks_lock_; }
   intptr_t tasks() const { return tasks_; }
   void set_tasks(intptr_t val) {
     ASSERT(val >= 0);
     tasks_ = val;
   }
   intptr_t concurrent_marker_tasks() const { return concurrent_marker_tasks_; }
   void set_concurrent_marker_tasks(intptr_t val) {
     ASSERT(val >= 0);
     concurrent_marker_tasks_ = val;
   }
   Phase phase() const { return phase_; }
   void set_phase(Phase val) { phase_ = val; }

   // Attempt to allocate from bump block rather than normal freelist.
   uword TryAllocateDataBumpLocked(intptr_t size) {
     return TryAllocateDataBumpLocked(&freelists_[OldPage::kData], size);
   }
   uword TryAllocateDataBumpLocked(FreeList* freelist, intptr_t size);
   DART_FORCE_INLINE
   uword TryAllocatePromoLocked(FreeList* freelist, intptr_t size) {
     uword result = freelist->TryAllocateBumpLocked(size);
     if (result != 0) {
       return result;
     }
     return TryAllocatePromoLockedSlow(freelist, size);
   }
   uword TryAllocatePromoLockedSlow(FreeList* freelist, intptr_t size);
   ObjectPtr AllocateSnapshot(intptr_t size);

   void SetupImagePage(void* pointer, uword size, bool is_executable);

   // Return any bump allocation block to the freelist.
   void AbandonBumpAllocation();
   // Have threads release marking stack blocks, etc.
   void AbandonMarkingForShutdown();

   bool enable_concurrent_mark() const { return enable_concurrent_mark_; }
   void set_enable_concurrent_mark(bool enable_concurrent_mark) {
     enable_concurrent_mark_ = enable_concurrent_mark;
   }

   bool IsObjectFromImagePages(ObjectPtr object);

  private:
   // Ids for time and data records in Heap::GCStats.
   enum {
     // Time
     kConcurrentSweep = 0,
     kSafePoint = 1,
     kMarkObjects = 2,
     kResetFreeLists = 3,
     kSweepPages = 4,
     kSweepLargePages = 5,
     // Data
     kGarbageRatio = 0,
     kGCTimeFraction = 1,
     kPageGrowth = 2,
     kAllowedGrowth = 3
   };

   uword TryAllocateInternal(intptr_t size,
                             FreeList* freelist,
                             OldPage::PageType type,
                             GrowthPolicy growth_policy,
                             bool is_protected,
                             bool is_locked);
   uword TryAllocateInFreshPage(intptr_t size,
                                FreeList* freelist,
                                OldPage::PageType type,
                                GrowthPolicy growth_policy,
                                bool is_locked);
   uword TryAllocateInFreshLargePage(intptr_t size,
                                     OldPage::PageType type,
                                     GrowthPolicy growth_policy);

   void EvaluateConcurrentMarking(GrowthPolicy growth_policy);

   // Makes bump block walkable; do not call concurrently with mutator.
   void MakeIterable() const;

   void AddPageLocked(OldPage* page);
   void AddLargePageLocked(OldPage* page);
   void AddExecPageLocked(OldPage* page);
   void RemovePageLocked(OldPage* page, OldPage* previous_page);
   void RemoveLargePageLocked(OldPage* page, OldPage* previous_page);
   void RemoveExecPageLocked(OldPage* page, OldPage* previous_page);

   OldPage* AllocatePage(OldPage::PageType type, bool link = true);
   OldPage* AllocateLargePage(intptr_t size, OldPage::PageType type);

   void TruncateLargePage(OldPage* page, intptr_t new_object_size_in_bytes);
   void FreePage(OldPage* page, OldPage* previous_page);
   void FreeLargePage(OldPage* page, OldPage* previous_page);
   void FreePages(OldPage* pages);

   void CollectGarbageHelper(bool compact,
                             bool finalize,
                             int64_t pre_wait_for_sweepers,
                             int64_t pre_safe_point);
   void SweepLarge();
   void Sweep();
   void ConcurrentSweep(IsolateGroup* isolate_group);
   void Compact(Thread* thread);

   static intptr_t LargePageSizeInWordsFor(intptr_t size);

   bool CanIncreaseCapacityInWordsLocked(intptr_t increase_in_words) {
     if (max_capacity_in_words_ == 0) {
       // Unlimited.
       return true;
     }
     intptr_t free_capacity_in_words =
         (max_capacity_in_words_ - usage_.capacity_in_words);
     return ((free_capacity_in_words > 0) &&
             (increase_in_words <= free_capacity_in_words));
   }

   Heap* const heap_;

   // One list for executable pages at freelists_[OldPage::kExecutable].
   // FLAG_scavenger_tasks count of lists for data pages starting at
   // freelists_[OldPage::kData]. The sweeper inserts into the data page
   // freelists round-robin. The scavenger workers each use one of the data
   // page freelists without locking.
   const intptr_t num_freelists_;
   FreeList* freelists_;
   static constexpr intptr_t kOOMReservationSize = 32 * KB;
   FreeListElement* oom_reservation_ = nullptr;

   // Use ExclusivePageIterator for safe access to these.
   mutable Mutex pages_lock_;
   OldPage* pages_ = nullptr;
   OldPage* pages_tail_ = nullptr;
   OldPage* exec_pages_ = nullptr;
   OldPage* exec_pages_tail_ = nullptr;
   OldPage* large_pages_ = nullptr;
   OldPage* large_pages_tail_ = nullptr;
   OldPage* image_pages_ = nullptr;

   // Various sizes being tracked for this generation.
   intptr_t max_capacity_in_words_;

   // NOTE: The capacity component of usage_ is updated by the concurrent
   // sweeper. Use (Increase)CapacityInWords(Locked) for thread-safe access.
   SpaceUsage usage_;
   RelaxedAtomic<intptr_t> allocated_black_in_words_;

   // Keep track of running MarkSweep tasks.
   mutable Monitor tasks_lock_;
   intptr_t tasks_;
   intptr_t concurrent_marker_tasks_;
   Phase phase_;

 #if defined(DEBUG)
   Thread* iterating_thread_;
 #endif
   PageSpaceController page_space_controller_;
   GCMarker* marker_;

   int64_t gc_time_micros_;
   intptr_t collections_;
   intptr_t mark_words_per_micro_;

   bool enable_concurrent_mark_;

   friend class BasePageIterator;
   friend class ExclusivePageIterator;
   friend class ExclusiveCodePageIterator;
   friend class ExclusiveLargePageIterator;
   friend class HeapIterationScope;
   friend class HeapSnapshotWriter;
   friend class PageSpaceController;
   friend class ConcurrentSweeperTask;
   friend class GCCompactor;
   friend class CompactorTask;

   DISALLOW_IMPLICIT_CONSTRUCTORS(PageSpace);
 };

 }  // namespace dart

 #endif  // RUNTIME_VM_HEAP_PAGES_H_
	// Copyright (c) 2011, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.

	#ifndef RUNTIME_VM_HEAP_PAGES_H_
	#define RUNTIME_VM_HEAP_PAGES_H_

	#include "platform/atomic.h"
	#include "vm/globals.h"
	#include "vm/heap/freelist.h"
	#include "vm/heap/spaces.h"
	#include "vm/lockers.h"
	#include "vm/ring_buffer.h"
	#include "vm/thread.h"
	#include "vm/virtual_memory.h"

	namespace dart {

	DECLARE_FLAG(bool, write_protect_code);

	// Forward declarations.
	class Heap;
	class JSONObject;
	class ObjectPointerVisitor;
	class ObjectSet;
	class ForwardingPage;
	class GCMarker;

	static constexpr intptr_t kOldPageSize = 512 * KB;
	static constexpr intptr_t kOldPageSizeInWords = kOldPageSize / kWordSize;
	static constexpr intptr_t kOldPageMask = ~(kOldPageSize - 1);

	static constexpr intptr_t kBitVectorWordsPerBlock = 1;
	static constexpr intptr_t kBlockSize =
	kObjectAlignment * kBitsPerWord * kBitVectorWordsPerBlock;
	static constexpr intptr_t kBlockMask = ~(kBlockSize - 1);
	static constexpr intptr_t kBlocksPerPage = kOldPageSize / kBlockSize;

	// A page containing old generation objects.
	class OldPage {
	public:
	enum PageType { kExecutable = 0, kData };

	OldPage* next() const { return next_; }
	void set_next(OldPage* next) { next_ = next; }

	bool Contains(uword addr) const { return memory_->Contains(addr); }
	intptr_t AliasOffset() const { return memory_->AliasOffset(); }

	uword object_start() const { return memory_->start() + ObjectStartOffset(); }
	uword object_end() const { return object_end_; }
	uword used_in_bytes() const { return used_in_bytes_; }
	void set_used_in_bytes(uword value) {
	ASSERT(Utils::IsAligned(value, kObjectAlignment));
	used_in_bytes_ = value;
	}

	ForwardingPage* forwarding_page() const { return forwarding_page_; }
	void AllocateForwardingPage();

	PageType type() const { return type_; }

	bool is_image_page() const { return !memory_->vm_owns_region(); }

	void VisitObjects(ObjectVisitor* visitor) const;
	void VisitObjectPointers(ObjectPointerVisitor* visitor) const;

	ObjectPtr FindObject(FindObjectVisitor* visitor) const;

	void WriteProtect(bool read_only);

	static intptr_t ObjectStartOffset() {
	return Utils::RoundUp(sizeof(OldPage), kMaxObjectAlignment);
	}

	// Warning: This does not work for objects on image pages because image pages
	// are not aligned. However, it works for objects on large pages, because
	// only one object is allocated per large page.
	static OldPage* Of(ObjectPtr obj) {
	ASSERT(obj->IsHeapObject());
	ASSERT(obj->IsOldObject());
	return reinterpret_cast<OldPage*>(static_cast<uword>(obj) & kOldPageMask);
	}

	// Warning: This does not work for addresses on image pages or on large pages.
	static OldPage* Of(uword addr) {
	return reinterpret_cast<OldPage*>(addr & kOldPageMask);
	}

	// Warning: This does not work for objects on image pages.
	static ObjectPtr ToExecutable(ObjectPtr obj) {
	OldPage* page = Of(obj);
	VirtualMemory* memory = page->memory_;
	const intptr_t alias_offset = memory->AliasOffset();
	if (alias_offset == 0) {
	return obj; // Not aliased.
	}
	uword addr = UntaggedObject::ToAddr(obj);
	if (memory->Contains(addr)) {
	return UntaggedObject::FromAddr(addr + alias_offset);
	}
	// obj is executable.
	ASSERT(memory->ContainsAlias(addr));
	return obj;
	}

	// Warning: This does not work for objects on image pages.
	static ObjectPtr ToWritable(ObjectPtr obj) {
	OldPage* page = Of(obj);
	VirtualMemory* memory = page->memory_;
	const intptr_t alias_offset = memory->AliasOffset();
	if (alias_offset == 0) {
	return obj; // Not aliased.
	}
	uword addr = UntaggedObject::ToAddr(obj);
	if (memory->ContainsAlias(addr)) {
	return UntaggedObject::FromAddr(addr - alias_offset);
	}
	// obj is writable.
	ASSERT(memory->Contains(addr));
	return obj;
	}

	// 1 card = 128 slots.
	static const intptr_t kSlotsPerCardLog2 = 7;
	static const intptr_t kBytesPerCardLog2 =
	kCompressedWordSizeLog2 + kSlotsPerCardLog2;

	intptr_t card_table_size() const {
	return memory_->size() >> kBytesPerCardLog2;
	}

	static intptr_t card_table_offset() {
	return OFFSET_OF(OldPage, card_table_);
	}

	void RememberCard(ObjectPtr const* slot) {
	ASSERT(Contains(reinterpret_cast<uword>(slot)));
	if (card_table_ == NULL) {
	card_table_ = reinterpret_cast<uint8_t*>(
	calloc(card_table_size(), sizeof(uint8_t)));
	}
	intptr_t offset =
	reinterpret_cast<uword>(slot) - reinterpret_cast<uword>(this);
	intptr_t index = offset >> kBytesPerCardLog2;
	ASSERT((index >= 0) && (index < card_table_size()));
	card_table_[index] = 1;
	}
	bool IsCardRemembered(ObjectPtr const* slot) {
	ASSERT(Contains(reinterpret_cast<uword>(slot)));
	if (card_table_ == NULL) {
	return false;
	}
	intptr_t offset =
	reinterpret_cast<uword>(slot) - reinterpret_cast<uword>(this);
	intptr_t index = offset >> kBytesPerCardLog2;
	ASSERT((index >= 0) && (index < card_table_size()));
	return card_table_[index] != 0;
	}
	#if defined(DART_COMPRESSED_POINTERS)
	void RememberCard(CompressedObjectPtr const* slot) {
	ASSERT(Contains(reinterpret_cast<uword>(slot)));
	if (card_table_ == NULL) {
	card_table_ = reinterpret_cast<uint8_t*>(
	calloc(card_table_size(), sizeof(uint8_t)));
	}
	intptr_t offset =
	reinterpret_cast<uword>(slot) - reinterpret_cast<uword>(this);
	intptr_t index = offset >> kBytesPerCardLog2;
	ASSERT((index >= 0) && (index < card_table_size()));
	card_table_[index] = 1;
	}
	bool IsCardRemembered(CompressedObjectPtr const* slot) {
	ASSERT(Contains(reinterpret_cast<uword>(slot)));
	if (card_table_ == NULL) {
	return false;
	}
	intptr_t offset =
	reinterpret_cast<uword>(slot) - reinterpret_cast<uword>(this);
	intptr_t index = offset >> kBytesPerCardLog2;
	ASSERT((index >= 0) && (index < card_table_size()));
	return card_table_[index] != 0;
	}
	#endif
	void VisitRememberedCards(ObjectPointerVisitor* visitor);

	private:
	void set_object_end(uword value) {
	ASSERT((value & kObjectAlignmentMask) == kOldObjectAlignmentOffset);
	object_end_ = value;
	}

	// Returns NULL on OOM.
	static OldPage* Allocate(intptr_t size_in_words,
	PageType type,
	const char* name);

	// Deallocate the virtual memory backing this page. The page pointer to this
	// page becomes immediately inaccessible.
	void Deallocate();

	VirtualMemory* memory_;
	OldPage* next_;
	uword object_end_;
	uword used_in_bytes_;
	ForwardingPage* forwarding_page_;
	uint8_t* card_table_; // Remembered set, not marking.
	PageType type_;

	friend class PageSpace;
	friend class GCCompactor;

	DISALLOW_ALLOCATION();
	DISALLOW_IMPLICIT_CONSTRUCTORS(OldPage);
	};

	// The history holds the timing information of the last garbage collection
	// runs.
	class PageSpaceGarbageCollectionHistory {
	public:
	PageSpaceGarbageCollectionHistory() {}
	~PageSpaceGarbageCollectionHistory() {}

	void AddGarbageCollectionTime(int64_t start, int64_t end);

	int GarbageCollectionTimeFraction();

	bool IsEmpty() const { return history_.Size() == 0; }

	private:
	struct Entry {
	int64_t start;
	int64_t end;
	};
	static const intptr_t kHistoryLength = 4;
	RingBuffer<Entry, kHistoryLength> history_;

	DISALLOW_ALLOCATION();
	DISALLOW_COPY_AND_ASSIGN(PageSpaceGarbageCollectionHistory);
	};

	// PageSpaceController controls the heap size.
	class PageSpaceController {
	public:
	// The heap is passed in for recording stats only. The controller does not
	// invoke GC by itself.
	PageSpaceController(Heap* heap,
	int heap_growth_ratio,
	int heap_growth_max,
	int garbage_collection_time_ratio);
	~PageSpaceController();

	// Returns whether growing to 'after' should trigger a GC.
	// This method can be called before allocation (e.g., pretenuring) or after
	// (e.g., promotion), as it does not change the state of the controller.
	bool ReachedHardThreshold(SpaceUsage after) const;
	bool ReachedSoftThreshold(SpaceUsage after) const;

	// Returns whether an idle GC is worthwhile.
	bool ReachedIdleThreshold(SpaceUsage current) const;

	// Should be called after each collection to update the controller state.
	void EvaluateGarbageCollection(SpaceUsage before,
	SpaceUsage after,
	int64_t start,
	int64_t end);
	void EvaluateAfterLoading(SpaceUsage after);
	void HintFreed(intptr_t size);

	void set_last_usage(SpaceUsage current) { last_usage_ = current; }

	void Enable() { is_enabled_ = true; }
	void Disable() { is_enabled_ = false; }
	bool is_enabled() { return is_enabled_; }

	private:
	friend class PageSpace; // For MergeOtherPageSpaceController

	void RecordUpdate(SpaceUsage before, SpaceUsage after, const char* reason);
	void RecordUpdate(SpaceUsage before,
	SpaceUsage after,
	intptr_t growth_in_pages,
	const char* reason);

	Heap* heap_;

	bool is_enabled_;

	// Usage after last evaluated GC or last enabled.
	SpaceUsage last_usage_;

	// If the garbage collector was not able to free more than heap_growth_ratio_
	// memory, then the heap is grown. Otherwise garbage collection is performed.
	const int heap_growth_ratio_;

	// The desired percent of heap in-use after a garbage collection.
	// Equivalent to \frac{100-heap_growth_ratio_}{100}.
	const double desired_utilization_;

	// Max number of pages we grow.
	const int heap_growth_max_;

	// If the relative GC time goes above garbage_collection_time_ratio_ %,
	// we grow the heap more aggressively.
	const int garbage_collection_time_ratio_;

	// Perform a stop-the-world GC when usage exceeds this amount.
	intptr_t hard_gc_threshold_in_words_;

	// Begin concurrent marking when usage exceeds this amount.
	intptr_t soft_gc_threshold_in_words_;

	// Run idle GC if time permits when usage exceeds this amount.
	intptr_t idle_gc_threshold_in_words_;

	PageSpaceGarbageCollectionHistory history_;

	DISALLOW_IMPLICIT_CONSTRUCTORS(PageSpaceController);
	};

	class PageSpace {
	public:
	enum GrowthPolicy { kControlGrowth, kForceGrowth };
	enum Phase {
	kDone,
	kMarking,
	kAwaitingFinalization,
	kSweepingLarge,
	kSweepingRegular
	};

	PageSpace(Heap* heap, intptr_t max_capacity_in_words);
	~PageSpace();

	uword TryAllocate(intptr_t size,
	OldPage::PageType type = OldPage::kData,
	GrowthPolicy growth_policy = kControlGrowth) {
	bool is_protected =
	(type == OldPage::kExecutable) && FLAG_write_protect_code;
	bool is_locked = false;
	return TryAllocateInternal(size, &freelists_[type], type, growth_policy,
	is_protected, is_locked);
	}

	void TryReleaseReservation();
	bool MarkReservation();
	void TryReserveForOOM();
	void VisitRoots(ObjectPointerVisitor* visitor);

	bool ReachedHardThreshold() const {
	return page_space_controller_.ReachedHardThreshold(usage_);
	}
	bool ReachedSoftThreshold() const {
	return page_space_controller_.ReachedSoftThreshold(usage_);
	}
	bool ReachedIdleThreshold() const {
	return page_space_controller_.ReachedIdleThreshold(usage_);
	}
	void EvaluateAfterLoading() {
	page_space_controller_.EvaluateAfterLoading(usage_);
	}
	void HintFreed(intptr_t size) { page_space_controller_.HintFreed(size); }

	int64_t UsedInWords() const { return usage_.used_in_words; }
	int64_t CapacityInWords() const {
	MutexLocker ml(&pages_lock_);
	return usage_.capacity_in_words;
	}
	void IncreaseCapacityInWords(intptr_t increase_in_words) {
	MutexLocker ml(&pages_lock_);
	IncreaseCapacityInWordsLocked(increase_in_words);
	}
	void IncreaseCapacityInWordsLocked(intptr_t increase_in_words) {
	DEBUG_ASSERT(pages_lock_.IsOwnedByCurrentThread());
	usage_.capacity_in_words += increase_in_words;
	UpdateMaxCapacityLocked();
	}

	void UpdateMaxCapacityLocked();
	void UpdateMaxUsed();

	int64_t ExternalInWords() const { return usage_.external_in_words; }
	SpaceUsage GetCurrentUsage() const {
	MutexLocker ml(&pages_lock_);
	return usage_;
	}
	int64_t ImageInWords() const {
	int64_t size = 0;
	MutexLocker ml(&pages_lock_);
	for (OldPage* page = image_pages_; page != nullptr; page = page->next()) {
	size += page->memory_->size();
	}
	return size >> kWordSizeLog2;
	}

	bool Contains(uword addr) const;
	bool ContainsUnsafe(uword addr) const;
	bool Contains(uword addr, OldPage::PageType type) const;
	bool DataContains(uword addr) const;
	bool IsValidAddress(uword addr) const { return Contains(addr); }

	void VisitObjects(ObjectVisitor* visitor) const;
	void VisitObjectsNoImagePages(ObjectVisitor* visitor) const;
	void VisitObjectsImagePages(ObjectVisitor* visitor) const;
	void VisitObjectPointers(ObjectPointerVisitor* visitor) const;

	void VisitRememberedCards(ObjectPointerVisitor* visitor) const;

	ObjectPtr FindObject(FindObjectVisitor* visitor,
	OldPage::PageType type) const;

	// Collect the garbage in the page space using mark-sweep or mark-compact.
	void CollectGarbage(bool compact, bool finalize);

	void AddRegionsToObjectSet(ObjectSet* set) const;

	void InitGrowthControl() {
	page_space_controller_.set_last_usage(usage_);
	page_space_controller_.Enable();
	}

	void SetGrowthControlState(bool state) {
	if (state) {
	page_space_controller_.Enable();
	} else {
	page_space_controller_.Disable();
	}
	}

	bool GrowthControlState() { return page_space_controller_.is_enabled(); }

	// Note: Code pages are made executable/non-executable when 'read_only' is
	// true/false, respectively.
	void WriteProtect(bool read_only);
	void WriteProtectCode(bool read_only);

	bool ShouldStartIdleMarkSweep(int64_t deadline);
	bool ShouldPerformIdleMarkCompact(int64_t deadline);

	void AddGCTime(int64_t micros) { gc_time_micros_ += micros; }

	int64_t gc_time_micros() const { return gc_time_micros_; }

	void IncrementCollections() { collections_++; }

	intptr_t collections() const { return collections_; }

	#ifndef PRODUCT
	void PrintToJSONObject(JSONObject* object) const;
	void PrintHeapMapToJSONStream(IsolateGroup* isolate_group,
	JSONStream* stream) const;
	#endif // PRODUCT

	void AllocateBlack(intptr_t size) {
	allocated_black_in_words_.fetch_add(size >> kWordSizeLog2);
	}

	void AllocatedExternal(intptr_t size) {
	ASSERT(size >= 0);
	intptr_t size_in_words = size >> kWordSizeLog2;
	usage_.external_in_words += size_in_words;
	}
	void FreedExternal(intptr_t size) {
	ASSERT(size >= 0);
	intptr_t size_in_words = size >> kWordSizeLog2;
	usage_.external_in_words -= size_in_words;
	}

	// Bulk data allocation.
	FreeList* DataFreeList(intptr_t i = 0) {
	return &freelists_[OldPage::kData + i];
	}
	void AcquireLock(FreeList* freelist);
	void ReleaseLock(FreeList* freelist);

	uword TryAllocateDataLocked(FreeList* freelist,
	intptr_t size,
	GrowthPolicy growth_policy) {
	bool is_protected = false;
	bool is_locked = true;
	return TryAllocateInternal(size, freelist, OldPage::kData, growth_policy,
	is_protected, is_locked);
	}

	Monitor* tasks_lock() const { return &tasks_lock_; }
	intptr_t tasks() const { return tasks_; }
	void set_tasks(intptr_t val) {
	ASSERT(val >= 0);
	tasks_ = val;
	}
	intptr_t concurrent_marker_tasks() const { return concurrent_marker_tasks_; }
	void set_concurrent_marker_tasks(intptr_t val) {
	ASSERT(val >= 0);
	concurrent_marker_tasks_ = val;
	}
	Phase phase() const { return phase_; }
	void set_phase(Phase val) { phase_ = val; }

	// Attempt to allocate from bump block rather than normal freelist.
	uword TryAllocateDataBumpLocked(intptr_t size) {
	return TryAllocateDataBumpLocked(&freelists_[OldPage::kData], size);
	}
	uword TryAllocateDataBumpLocked(FreeList* freelist, intptr_t size);
	DART_FORCE_INLINE
	uword TryAllocatePromoLocked(FreeList* freelist, intptr_t size) {
	uword result = freelist->TryAllocateBumpLocked(size);
	if (result != 0) {
	return result;
	}
	return TryAllocatePromoLockedSlow(freelist, size);
	}
	uword TryAllocatePromoLockedSlow(FreeList* freelist, intptr_t size);
	ObjectPtr AllocateSnapshot(intptr_t size);

	void SetupImagePage(void* pointer, uword size, bool is_executable);

	// Return any bump allocation block to the freelist.
	void AbandonBumpAllocation();
	// Have threads release marking stack blocks, etc.
	void AbandonMarkingForShutdown();

	bool enable_concurrent_mark() const { return enable_concurrent_mark_; }
	void set_enable_concurrent_mark(bool enable_concurrent_mark) {
	enable_concurrent_mark_ = enable_concurrent_mark;
	}

	bool IsObjectFromImagePages(ObjectPtr object);

	private:
	// Ids for time and data records in Heap::GCStats.
	enum {
	// Time
	kConcurrentSweep = 0,
	kSafePoint = 1,
	kMarkObjects = 2,
	kResetFreeLists = 3,
	kSweepPages = 4,
	kSweepLargePages = 5,
	// Data
	kGarbageRatio = 0,
	kGCTimeFraction = 1,
	kPageGrowth = 2,
	kAllowedGrowth = 3
	};

	uword TryAllocateInternal(intptr_t size,
	FreeList* freelist,
	OldPage::PageType type,
	GrowthPolicy growth_policy,
	bool is_protected,
	bool is_locked);
	uword TryAllocateInFreshPage(intptr_t size,
	FreeList* freelist,
	OldPage::PageType type,
	GrowthPolicy growth_policy,
	bool is_locked);
	uword TryAllocateInFreshLargePage(intptr_t size,
	OldPage::PageType type,
	GrowthPolicy growth_policy);

	void EvaluateConcurrentMarking(GrowthPolicy growth_policy);

	// Makes bump block walkable; do not call concurrently with mutator.
	void MakeIterable() const;

	void AddPageLocked(OldPage* page);
	void AddLargePageLocked(OldPage* page);
	void AddExecPageLocked(OldPage* page);
	void RemovePageLocked(OldPage* page, OldPage* previous_page);
	void RemoveLargePageLocked(OldPage* page, OldPage* previous_page);
	void RemoveExecPageLocked(OldPage* page, OldPage* previous_page);

	OldPage* AllocatePage(OldPage::PageType type, bool link = true);
	OldPage* AllocateLargePage(intptr_t size, OldPage::PageType type);

	void TruncateLargePage(OldPage* page, intptr_t new_object_size_in_bytes);
	void FreePage(OldPage* page, OldPage* previous_page);
	void FreeLargePage(OldPage* page, OldPage* previous_page);
	void FreePages(OldPage* pages);

	void CollectGarbageHelper(bool compact,
	bool finalize,
	int64_t pre_wait_for_sweepers,
	int64_t pre_safe_point);
	void SweepLarge();
	void Sweep();
	void ConcurrentSweep(IsolateGroup* isolate_group);
	void Compact(Thread* thread);

	static intptr_t LargePageSizeInWordsFor(intptr_t size);

	bool CanIncreaseCapacityInWordsLocked(intptr_t increase_in_words) {
	if (max_capacity_in_words_ == 0) {
	// Unlimited.
	return true;
	}
	intptr_t free_capacity_in_words =
	(max_capacity_in_words_ - usage_.capacity_in_words);
	return ((free_capacity_in_words > 0) &&
	(increase_in_words <= free_capacity_in_words));
	}

	Heap* const heap_;

	// One list for executable pages at freelists_[OldPage::kExecutable].
	// FLAG_scavenger_tasks count of lists for data pages starting at
	// freelists_[OldPage::kData]. The sweeper inserts into the data page
	// freelists round-robin. The scavenger workers each use one of the data
	// page freelists without locking.
	const intptr_t num_freelists_;
	FreeList* freelists_;
	static constexpr intptr_t kOOMReservationSize = 32 * KB;
	FreeListElement* oom_reservation_ = nullptr;

	// Use ExclusivePageIterator for safe access to these.
	mutable Mutex pages_lock_;
	OldPage* pages_ = nullptr;
	OldPage* pages_tail_ = nullptr;
	OldPage* exec_pages_ = nullptr;
	OldPage* exec_pages_tail_ = nullptr;
	OldPage* large_pages_ = nullptr;
	OldPage* large_pages_tail_ = nullptr;
	OldPage* image_pages_ = nullptr;

	// Various sizes being tracked for this generation.
	intptr_t max_capacity_in_words_;

	// NOTE: The capacity component of usage_ is updated by the concurrent
	// sweeper. Use (Increase)CapacityInWords(Locked) for thread-safe access.
	SpaceUsage usage_;
	RelaxedAtomic<intptr_t> allocated_black_in_words_;

	// Keep track of running MarkSweep tasks.
	mutable Monitor tasks_lock_;
	intptr_t tasks_;
	intptr_t concurrent_marker_tasks_;
	Phase phase_;

	#if defined(DEBUG)
	Thread* iterating_thread_;
	#endif
	PageSpaceController page_space_controller_;
	GCMarker* marker_;

	int64_t gc_time_micros_;
	intptr_t collections_;
	intptr_t mark_words_per_micro_;

	bool enable_concurrent_mark_;

	friend class BasePageIterator;
	friend class ExclusivePageIterator;
	friend class ExclusiveCodePageIterator;
	friend class ExclusiveLargePageIterator;
	friend class HeapIterationScope;
	friend class HeapSnapshotWriter;
	friend class PageSpaceController;
	friend class ConcurrentSweeperTask;
	friend class GCCompactor;
	friend class CompactorTask;

	DISALLOW_IMPLICIT_CONSTRUCTORS(PageSpace);
	};

	} // namespace dart

	#endif // RUNTIME_VM_HEAP_PAGES_H_