[heap] Parallel Scavenge

Bug: chromium:738865
Change-Id: Ie18574bb067438816238e2cf930e6d2a7bc5ecef
Reviewed-on: https://chromium-review.googlesource.com/570579
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46798}

src/heap/heap.cc

@@ -1804,10 +1804,11 @@ class ScavengingItem : public ItemParallelJob::Item {
 
 class ScavengingTask final : public ItemParallelJob::Task {
  public:
-  ScavengingTask(Heap* heap, Scavenger* scavenger)
+  ScavengingTask(Heap* heap, Scavenger* scavenger, Scavenger::Barrier* barrier)
       : ItemParallelJob::Task(heap->isolate()),
         heap_(heap),
-        scavenger_(scavenger) {}
+        scavenger_(scavenger),
+        barrier_(barrier) {}
 
   void RunInParallel() final {
     double scavenging_time = 0.0;
@@ -1818,6 +1819,10 @@ class ScavengingTask final : public ItemParallelJob::Task {
         item->Process(scavenger_);
         item->MarkFinished();
       }
+      while (!barrier_->Done()) {
+        scavenger_->Process(barrier_);
+        barrier_->Wait();
+      }
       scavenger_->Process();
     }
     if (FLAG_trace_parallel_scavenge) {
@@ -1831,6 +1836,7 @@ class ScavengingTask final : public ItemParallelJob::Task {
  private:
   Heap* const heap_;
   Scavenger* const scavenger_;
+  Scavenger::Barrier* const barrier_;
 };
 
 class PageScavengingItem final : public ScavengingItem {
@@ -1868,8 +1874,14 @@ class PageScavengingItem final : public ScavengingItem {
 };
 
 int Heap::NumberOfScavengeTasks() {
-  CHECK(!FLAG_parallel_scavenge);
-  return 1;
+  if (!FLAG_parallel_scavenge) return 1;
+  const int num_scavenge_tasks =
+      static_cast<int>(new_space()->TotalCapacity()) / MB;
+  return Max(
+      1,
+      Min(Min(num_scavenge_tasks, kMaxScavengerTasks),
+          static_cast<int>(
+              V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads())));
 }
 
 void Heap::Scavenge() {
@@ -1910,12 +1922,13 @@ void Heap::Scavenge() {
   const bool is_logging = IsLogging(isolate());
   const bool is_incremental_marking = incremental_marking()->IsMarking();
   const int num_scavenge_tasks = NumberOfScavengeTasks();
+  Scavenger::Barrier barrier(num_scavenge_tasks);
   CopiedList copied_list(num_scavenge_tasks);
   PromotionList promotion_list(num_scavenge_tasks);
   for (int i = 0; i < num_scavenge_tasks; i++) {
     scavengers[i] = new Scavenger(this, is_logging, is_incremental_marking,
                                   &copied_list, &promotion_list, i);
-    job.AddTask(new ScavengingTask(this, scavengers[i]));
+    job.AddTask(new ScavengingTask(this, scavengers[i], &barrier));
   }
 
   RememberedSet<OLD_TO_NEW>::IterateMemoryChunks(

src/heap/heap.h

@@ -1631,7 +1631,11 @@ class Heap {
 
   static const int kInitialFeedbackCapacity = 256;
 
-  static const int kMaxScavengerTasks = 1;
+#ifdef V8_TARGET_ARCH_ARM
+  static const int kMaxScavengerTasks = 2;
+#else
+  static const int kMaxScavengerTasks = 8;
+#endif
 
   Heap();
 

src/heap/local-allocator.h

@@ -55,6 +55,21 @@ class LocalAllocator {
     }
   }
 
+  void FreeLast(AllocationSpace space, HeapObject* object, int object_size) {
+    switch (space) {
+      case NEW_SPACE:
+        FreeLastInNewSpace(object, object_size);
+        return;
+      case OLD_SPACE:
+        FreeLastInOldSpace(object, object_size);
+        return;
+      default:
+        // Only new and old space supported.
+        UNREACHABLE();
+        break;
+    }
+  }
+
  private:
   AllocationResult AllocateInNewSpace(int object_size,
                                       AllocationAlignment alignment) {
@@ -97,6 +112,22 @@ class LocalAllocator {
     return allocation;
   }
 
+  void FreeLastInNewSpace(HeapObject* object, int object_size) {
+    if (!new_space_lab_.TryFreeLast(object, object_size)) {
+      // We couldn't free the last object so we have to write a proper filler.
+      heap_->CreateFillerObjectAt(object->address(), object_size,
+                                  ClearRecordedSlots::kNo);
+    }
+  }
+
+  void FreeLastInOldSpace(HeapObject* object, int object_size) {
+    if (!compaction_spaces_.Get(OLD_SPACE)->TryFreeLast(object, object_size)) {
+      // We couldn't free the last object so we have to write a proper filler.
+      heap_->CreateFillerObjectAt(object->address(), object_size,
+                                  ClearRecordedSlots::kNo);
+    }
+  }
+
   Heap* const heap_;
   NewSpace* const new_space_;
   CompactionSpaceCollection compaction_spaces_;

src/heap/scavenger-inl.h

@@ -34,13 +34,32 @@ bool ContainsOnlyData(VisitorId visitor_id) {
 
 }  // namespace
 
-void Scavenger::MigrateObject(Map* map, HeapObject* source, HeapObject* target,
+void Scavenger::PageMemoryFence(Object* object) {
+#ifdef THREAD_SANITIZER
+  // Perform a dummy acquire load to tell TSAN that there is no data race
+  // with  page initialization.
+  if (object->IsHeapObject()) {
+    MemoryChunk* chunk =
+        MemoryChunk::FromAddress(HeapObject::cast(object)->address());
+    CHECK_NOT_NULL(chunk->synchronized_heap());
+  }
+#endif
+}
+
+bool Scavenger::MigrateObject(Map* map, HeapObject* source, HeapObject* target,
                               int size) {
   // Copy the content of source to target.
-  heap()->CopyBlock(target->address(), source->address(), size);
-
-  // Set the forwarding address.
-  source->set_map_word(MapWord::FromForwardingAddress(target));
+  target->set_map_word(MapWord::FromMap(map));
+  heap()->CopyBlock(target->address() + kPointerSize,
+                    source->address() + kPointerSize, size - kPointerSize);
+
+  HeapObject* old = base::AsAtomicWord::Release_CompareAndSwap(
+      reinterpret_cast<HeapObject**>(source->address()), map,
+      MapWord::FromForwardingAddress(target).ToMap());
+  if (old != map) {
+    // Other task migrated the object.
+    return false;
+  }
 
   if (V8_UNLIKELY(is_logging_)) {
     // Update NewSpace stats if necessary.
@@ -49,10 +68,12 @@ void Scavenger::MigrateObject(Map* map, HeapObject* source, HeapObject* target,
   }
 
   if (is_incremental_marking_) {
-    heap()->incremental_marking()->TransferColor(source, target);
+    heap()->incremental_marking()->TransferColor<AccessMode::ATOMIC>(source,
+                                                                     target);
   }
   heap()->UpdateAllocationSite<Heap::kCached>(map, source,
                                               &local_pretenuring_feedback_);
+  return true;
 }
 
 bool Scavenger::SemiSpaceCopyObject(Map* map, HeapObject** slot,
@@ -66,10 +87,16 @@ bool Scavenger::SemiSpaceCopyObject(Map* map, HeapObject** slot,
   if (allocation.To(&target)) {
     DCHECK(ObjectMarking::IsWhite(
         target, heap()->mark_compact_collector()->marking_state(target)));
-    MigrateObject(map, object, target, object_size);
+    const bool self_success = MigrateObject(map, object, target, object_size);
+    if (!self_success) {
+      allocator_.FreeLast(NEW_SPACE, target, object_size);
+      MapWord map_word = object->map_word();
+      *slot = map_word.ToForwardingAddress();
+      return true;
+    }
     *slot = target;
 
-    copied_list_.Insert(target, object_size);
+    copied_list_.Push(ObjectAndSize(target, object_size));
     copied_size_ += object_size;
     return true;
   }
@@ -86,7 +113,13 @@ bool Scavenger::PromoteObject(Map* map, HeapObject** slot, HeapObject* object,
   if (allocation.To(&target)) {
     DCHECK(ObjectMarking::IsWhite(
         target, heap()->mark_compact_collector()->marking_state(target)));
-    MigrateObject(map, object, target, object_size);
+    const bool self_success = MigrateObject(map, object, target, object_size);
+    if (!self_success) {
+      allocator_.FreeLast(OLD_SPACE, target, object_size);
+      MapWord map_word = object->map_word();
+      *slot = map_word.ToForwardingAddress();
+      return true;
+    }
     *slot = target;
 
     if (!ContainsOnlyData(static_cast<VisitorId>(map->visitor_id()))) {
@@ -106,14 +139,10 @@ void Scavenger::EvacuateObjectDefault(Map* map, HeapObject** slot,
   if (!heap()->ShouldBePromoted(object->address())) {
     // A semi-space copy may fail due to fragmentation. In that case, we
     // try to promote the object.
-    if (SemiSpaceCopyObject(map, slot, object, object_size)) {
-      return;
-    }
+    if (SemiSpaceCopyObject(map, slot, object, object_size)) return;
   }
 
-  if (PromoteObject(map, slot, object, object_size)) {
-    return;
-  }
+  if (PromoteObject(map, slot, object, object_size)) return;
 
   // If promotion failed, we try to copy the object to the other semi-space
   if (SemiSpaceCopyObject(map, slot, object, object_size)) return;
@@ -124,12 +153,15 @@ void Scavenger::EvacuateObjectDefault(Map* map, HeapObject** slot,
 void Scavenger::EvacuateThinString(Map* map, HeapObject** slot,
                                    ThinString* object, int object_size) {
   if (!is_incremental_marking_) {
+    // Loading actual is fine in a parallel setting is there is no write.
     HeapObject* actual = object->actual();
     *slot = actual;
     // ThinStrings always refer to internalized strings, which are
     // always in old space.
     DCHECK(!heap()->InNewSpace(actual));
-    object->set_map_word(MapWord::FromForwardingAddress(actual));
+    base::AsAtomicWord::Relaxed_Store(
+        reinterpret_cast<Map**>(object->address()),
+        MapWord::FromForwardingAddress(actual).ToMap());
     return;
   }
 
@@ -146,7 +178,9 @@ void Scavenger::EvacuateShortcutCandidate(Map* map, HeapObject** slot,
     *slot = first;
 
     if (!heap()->InNewSpace(first)) {
-      object->set_map_word(MapWord::FromForwardingAddress(first));
+      base::AsAtomicWord::Relaxed_Store(
+          reinterpret_cast<Map**>(object->address()),
+          MapWord::FromForwardingAddress(first).ToMap());
       return;
     }
 
@@ -155,12 +189,16 @@ void Scavenger::EvacuateShortcutCandidate(Map* map, HeapObject** slot,
       HeapObject* target = first_word.ToForwardingAddress();
 
       *slot = target;
-      object->set_map_word(MapWord::FromForwardingAddress(target));
+      base::AsAtomicWord::Relaxed_Store(
+          reinterpret_cast<Map**>(object->address()),
+          MapWord::FromForwardingAddress(target).ToMap());
       return;
     }
-
-    EvacuateObject(slot, first_word.ToMap(), first);
-    object->set_map_word(MapWord::FromForwardingAddress(*slot));
+    Map* map = first_word.ToMap();
+    EvacuateObjectDefault(map, slot, first, first->SizeFromMap(map));
+    base::AsAtomicWord::Relaxed_Store(
+        reinterpret_cast<Map**>(object->address()),
+        MapWord::FromForwardingAddress(*slot).ToMap());
     return;
   }
 
@@ -172,12 +210,16 @@ void Scavenger::EvacuateObject(HeapObject** slot, Map* map,
   SLOW_DCHECK(heap_->InFromSpace(source));
   SLOW_DCHECK(!MapWord::FromMap(map).IsForwardingAddress());
   int size = source->SizeFromMap(map);
+  // Cannot use ::cast() below because that would add checks in debug mode
+  // that require re-reading the map.
   switch (static_cast<VisitorId>(map->visitor_id())) {
     case kVisitThinString:
-      EvacuateThinString(map, slot, ThinString::cast(source), size);
+      EvacuateThinString(map, slot, reinterpret_cast<ThinString*>(source),
+                         size);
       break;
     case kVisitShortcutCandidate:
-      EvacuateShortcutCandidate(map, slot, ConsString::cast(source), size);
+      EvacuateShortcutCandidate(map, slot,
+                                reinterpret_cast<ConsString*>(source), size);
       break;
     default:
       EvacuateObjectDefault(map, slot, source, size);
@@ -188,10 +230,7 @@ void Scavenger::EvacuateObject(HeapObject** slot, Map* map,
 void Scavenger::ScavengeObject(HeapObject** p, HeapObject* object) {
   DCHECK(heap()->InFromSpace(object));
 
-  // We use the first word (where the map pointer usually is) of a heap
-  // object to record the forwarding pointer.  A forwarding pointer can
-  // point to an old space, the code space, or the to space of the new
-  // generation.
+  // Relaxed load here. We either load a forwarding pointer or the map.
   MapWord first_word = object->map_word();
 
   // If the first word is a forwarding address, the object has already been
@@ -225,9 +264,14 @@ SlotCallbackResult Scavenger::CheckAndScavengeObject(Heap* heap,
     // callback in to space, the object is still live.
     // Unfortunately, we do not know about the slot. It could be in a
     // just freed free space object.
+    PageMemoryFence(object);
     if (heap->InToSpace(object)) {
       return KEEP_SLOT;
     }
+  } else if (heap->InToSpace(object)) {
+    // Already updated slot. This can happen when processing of the work list
+    // is interleaved with processing roots.
+    return KEEP_SLOT;
   }
   // Slots can point to "to" space if the slot has been recorded multiple
   // times in the remembered set. We remove the redundant slot now.

src/heap/scavenger.cc

@@ -26,12 +26,15 @@ class IterateAndScavengePromotedObjectsVisitor final : public ObjectVisitor {
          slot_address += kPointerSize) {
       Object** slot = reinterpret_cast<Object**>(slot_address);
       Object* target = *slot;
+      scavenger_->PageMemoryFence(target);
 
       if (target->IsHeapObject()) {
         if (heap_->InFromSpace(target)) {
           scavenger_->ScavengeObject(reinterpret_cast<HeapObject**>(slot),
                                      HeapObject::cast(target));
           target = *slot;
+          scavenger_->PageMemoryFence(target);
+
           if (heap_->InNewSpace(target)) {
             SLOW_DCHECK(target->IsHeapObject());
             SLOW_DCHECK(heap_->InToSpace(target));
@@ -73,9 +76,9 @@ void Scavenger::IterateAndScavengePromotedObject(HeapObject* target, int size) {
   // objects. Grey object's slots would be rescanned.
   // White object might not survive until the end of collection
   // it would be a violation of the invariant to record it's slots.
-  const bool record_slots =
-      heap()->incremental_marking()->IsCompacting() &&
-      ObjectMarking::IsBlack(target, MarkingState::Internal(target));
+  const bool record_slots = heap()->incremental_marking()->IsCompacting() &&
+                            ObjectMarking::IsBlack<AccessMode::ATOMIC>(
+                                target, MarkingState::Internal(target));
   IterateAndScavengePromotedObjectsVisitor visitor(heap(), this, record_slots);
   if (target->IsJSFunction()) {
     // JSFunctions reachable through kNextFunctionLinkOffset are weak. Slots for
@@ -86,34 +89,41 @@ void Scavenger::IterateAndScavengePromotedObject(HeapObject* target, int size) {
   }
 }
 
-void Scavenger::Process() {
+void Scavenger::Process(Barrier* barrier) {
   // Threshold when to switch processing the promotion list to avoid
   // allocating too much backing store in the worklist.
   const int kProcessPromotionListThreshold = kPromotionListSegmentSize / 2;
   ScavengeVisitor scavenge_visitor(heap(), this);
 
+  const bool have_barrier = barrier != nullptr;
   bool done;
+  size_t objects = 0;
   do {
     done = true;
-    AddressRange range;
+    ObjectAndSize object_and_size;
     while ((promotion_list_.LocalPushSegmentSize() <
             kProcessPromotionListThreshold) &&
-           copied_list_.Pop(&range)) {
-      for (Address current = range.first; current < range.second;) {
-        HeapObject* object = HeapObject::FromAddress(current);
-        int size = object->Size();
-        scavenge_visitor.Visit(object);
-        current += size;
-      }
+           copied_list_.Pop(&object_and_size)) {
+      scavenge_visitor.Visit(object_and_size.first);
       done = false;
+      if (have_barrier && ((++objects % kInterruptThreshold) == 0)) {
+        if (!copied_list_.IsGlobalPoolEmpty()) {
+          barrier->NotifyAll();
+        }
+      }
     }
-    ObjectAndSize object_and_size;
+
     while (promotion_list_.Pop(&object_and_size)) {
       HeapObject* target = object_and_size.first;
       int size = object_and_size.second;
       DCHECK(!target->IsMap());
       IterateAndScavengePromotedObject(target, size);
       done = false;
+      if (have_barrier && ((++objects % kInterruptThreshold) == 0)) {
+        if (!promotion_list_.IsGlobalPoolEmpty()) {
+          barrier->NotifyAll();
+        }
+      }
     }
   } while (!done);
 }

src/heap/scavenger.h

@@ -5,6 +5,7 @@
 #ifndef V8_HEAP_SCAVENGER_H_
 #define V8_HEAP_SCAVENGER_H_
 
+#include "src/base/platform/condition-variable.h"
 #include "src/heap/local-allocator.h"
 #include "src/heap/objects-visiting.h"
 #include "src/heap/slot-set.h"
@@ -13,62 +14,50 @@
 namespace v8 {
 namespace internal {
 
-static const int kCopiedListSegmentSize = 64;
-static const int kPromotionListSegmentSize = 64;
+static const int kCopiedListSegmentSize = 256;
+static const int kPromotionListSegmentSize = 256;
 
 using AddressRange = std::pair<Address, Address>;
-using CopiedList = Worklist<AddressRange, kCopiedListSegmentSize>;
 using ObjectAndSize = std::pair<HeapObject*, int>;
+using CopiedList = Worklist<ObjectAndSize, kCopiedListSegmentSize>;
 using PromotionList = Worklist<ObjectAndSize, kPromotionListSegmentSize>;
 
-// A list of copied ranges. Keeps the last consecutive range local and announces
-// all other ranges to a global work list.
-class CopiedRangesList {
+class Scavenger {
  public:
-  CopiedRangesList(CopiedList* copied_list, int task_id)
-      : current_start_(nullptr),
-        current_end_(nullptr),
-        copied_list_(copied_list, task_id) {}
-
-  ~CopiedRangesList() {
-    CHECK_NULL(current_start_);
-    CHECK_NULL(current_end_);
-  }
-
-  void Insert(HeapObject* object, int size) {
-    const Address object_address = object->address();
-    if (current_end_ != object_address) {
-      if (current_start_ != nullptr) {
-        copied_list_.Push(AddressRange(current_start_, current_end_));
-      }
-      current_start_ = object_address;
-      current_end_ = current_start_ + size;
-      return;
+  class Barrier {
+   public:
+    explicit Barrier(int tasks) : tasks_(tasks), waiting_(0), done_(false) {}
+
+    void NotifyAll() {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      if (waiting_ > 0) condition_.NotifyAll();
     }
-    DCHECK_EQ(current_end_, object_address);
-    current_end_ += size;
-    return;
-  }
-
-  bool Pop(AddressRange* entry) {
-    if (copied_list_.Pop(entry)) {
-      return true;
-    } else if (current_start_ != nullptr) {
-      *entry = AddressRange(current_start_, current_end_);
-      current_start_ = current_end_ = nullptr;
-      return true;
+
+    void Wait() {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      waiting_++;
+      if (waiting_ == tasks_) {
+        done_ = true;
+        condition_.NotifyAll();
+      } else {
+        // Spurious wakeup is ok here.
+        condition_.Wait(&mutex_);
+      }
+      waiting_--;
     }
-    return false;
-  }
 
- private:
-  Address current_start_;
-  Address current_end_;
-  CopiedList::View copied_list_;
-};
+    void Reset() { done_ = false; }
+
+    bool Done() { return done_; }
+
+   private:
+    base::ConditionVariable condition_;
+    base::Mutex mutex_;
+    int tasks_;
+    int waiting_;
+    bool done_;
+  };
 
-class Scavenger {
- public:
   Scavenger(Heap* heap, bool is_logging, bool is_incremental_marking,
             CopiedList* copied_list, PromotionList* promotion_list, int task_id)
       : heap_(heap),
@@ -92,7 +81,7 @@ class Scavenger {
 
   // Processes remaining work (=objects) after single objects have been
   // manually scavenged using ScavengeObject or CheckAndScavengeObject.
-  void Process();
+  void Process(Barrier* barrier = nullptr);
 
   // Finalize the Scavenger. Needs to be called from the main thread.
   void Finalize();
@@ -101,12 +90,17 @@ class Scavenger {
   size_t bytes_promoted() const { return promoted_size_; }
 
  private:
+  // Number of objects to process before interrupting for potentially waking
+  // up other tasks.
+  static const int kInterruptThreshold = 128;
   static const int kInitialLocalPretenuringFeedbackCapacity = 256;
 
   inline Heap* heap() { return heap_; }
 
+  inline void PageMemoryFence(Object* object);
+
   // Copies |source| to |target| and sets the forwarding pointer in |source|.
-  V8_INLINE void MigrateObject(Map* map, HeapObject* source, HeapObject* target,
+  V8_INLINE bool MigrateObject(Map* map, HeapObject* source, HeapObject* target,
                                int size);
 
   V8_INLINE bool SemiSpaceCopyObject(Map* map, HeapObject** slot,
@@ -138,13 +132,15 @@ class Scavenger {
 
   Heap* const heap_;
   PromotionList::View promotion_list_;
-  CopiedRangesList copied_list_;
+  CopiedList::View copied_list_;
   base::HashMap local_pretenuring_feedback_;
   size_t copied_size_;
   size_t promoted_size_;
   LocalAllocator allocator_;
   bool is_logging_;
   bool is_incremental_marking_;
+
+  friend class IterateAndScavengePromotedObjectsVisitor;
 };
 
 // Helper class for turning the scavenger into an object visitor that is also

src/heap/spaces-inl.h

@@ -172,6 +172,17 @@ intptr_t PagedSpace::RelinkFreeListCategories(Page* page) {
   return added;
 }
 
+bool PagedSpace::TryFreeLast(HeapObject* object, int object_size) {
+  if (allocation_info_.top() != nullptr) {
+    const Address object_address = object->address();
+    if ((allocation_info_.top() - object_size) == object_address) {
+      allocation_info_.set_top(object_address);
+      return true;
+    }
+  }
+  return false;
+}
+
 MemoryChunk* MemoryChunk::FromAnyPointerAddress(Heap* heap, Address addr) {
   MemoryChunk* chunk = MemoryChunk::FromAddress(addr);
   uintptr_t offset = addr - chunk->address();
@@ -519,6 +530,17 @@ bool LocalAllocationBuffer::TryMerge(LocalAllocationBuffer* other) {
   return false;
 }
 
+bool LocalAllocationBuffer::TryFreeLast(HeapObject* object, int object_size) {
+  if (IsValid()) {
+    const Address object_address = object->address();
+    if ((allocation_info_.top() - object_size) == object_address) {
+      allocation_info_.set_top(object_address);
+      return true;
+    }
+  }
+  return false;
+}
+
 }  // namespace internal
 }  // namespace v8
 

src/heap/spaces.h

@@ -1967,6 +1967,8 @@ class LocalAllocationBuffer {
   // Returns true if the merge was successful, false otherwise.
   inline bool TryMerge(LocalAllocationBuffer* other);
 
+  inline bool TryFreeLast(HeapObject* object, int object_size);
+
   // Close a LAB, effectively invalidating it. Returns the unused area.
   AllocationInfo Close();
 
@@ -2099,6 +2101,8 @@ class V8_EXPORT_PRIVATE PagedSpace : NON_EXPORTED_BASE(public Space) {
     return size_in_bytes - wasted;
   }
 
+  inline bool TryFreeLast(HeapObject* object, int object_size);
+
   void ResetFreeList() { free_list_.Reset(); }
 
   // Set space allocation info.

src/objects/string-inl.h

@@ -514,7 +514,7 @@ String* ConsString::second() {
 }
 
 Object* ConsString::unchecked_second() {
-  return READ_FIELD(this, kSecondOffset);
+  return RELAXED_READ_FIELD(this, kSecondOffset);
 }
 
 void ConsString::set_second(String* value, WriteBarrierMode mode) {

test/cctest/heap/test-page-promotion.cc

@@ -26,6 +26,8 @@ v8::Isolate* NewIsolateForPagePromotion(int min_semi_space_size = 8,
   FLAG_parallel_compaction = false;
   FLAG_page_promotion = true;
   FLAG_page_promotion_threshold = 0;
+  // Parallel scavenge introduces too much fragmentation.
+  FLAG_parallel_scavenge = false;
   FLAG_min_semi_space_size = min_semi_space_size;
   // We cannot optimize for size as we require a new space with more than one
   // page.

test/cctest/test-api.cc

@@ -47,6 +47,7 @@
 #include "src/execution.h"
 #include "src/futex-emulation.h"
 #include "src/heap/incremental-marking.h"
+#include "src/heap/local-allocator.h"
 #include "src/lookup.h"
 #include "src/objects-inl.h"
 #include "src/parsing/preparse-data.h"
@@ -7577,6 +7578,8 @@ static void SetFlag(const v8::WeakCallbackInfo<FlagAndPersistent>& data) {
 
 static void IndependentWeakHandle(bool global_gc, bool interlinked) {
   i::FLAG_stress_incremental_marking = false;
+  // Parallel scavenge introduces too much fragmentation.
+  i::FLAG_parallel_scavenge = false;
   v8::Isolate* iso = CcTest::isolate();
   v8::HandleScope scope(iso);
   v8::Local<Context> context = Context::New(iso);