Re-land "Concurrently unmap free pages."

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 1107 matching lines @@
   ExternalStringTable* external_string_table() {
     return &external_string_table_;
   }
 
   bool concurrent_sweeping_enabled() { return concurrent_sweeping_enabled_; }
 
   inline bool OldGenerationAllocationLimitReached();
 
   void QueueMemoryChunkForFree(MemoryChunk* chunk);
   void FilterStoreBufferEntriesOnAboutToBeFreedPages();
+  void FreeQueuedChunks(MemoryChunk* list_head);
   void FreeQueuedChunks();
+  void WaitUntilUnmappingOfFreeChunksCompleted();
 
   bool RecentIdleNotificationHappened();
 
   // Completely clear the Instanceof cache (to stop it keeping objects alive
   // around a GC).
   inline void CompletelyClearInstanceofCache();
 
   inline uint32_t HashSeed();
 
   inline Smi* NextScriptId();
@@ skipping 510 matching lines @@
 
   void Print();
   void PrintHandles();
 
   // Report heap statistics.
   void ReportHeapStatistics(const char* title);
   void ReportCodeStatistics(const char* title);
 #endif
 
  private:
+  class UnmapFreeMemoryTask;
+
   struct StrongRootsList;
 
   struct StringTypeTable {
     InstanceType type;
     int size;
     RootListIndex index;
   };
 
   struct ConstantStringTable {
     const char* contents;
@@ skipping 661 matching lines @@
 
   // Currently set GC flags that are respected by all GC components.
   int current_gc_flags_;
 
   ExternalStringTable external_string_table_;
 
   VisitorDispatchTable<ScavengingCallback> scavenging_visitors_table_;
 
   MemoryChunk* chunks_queued_for_free_;
 
+  base::Semaphore pending_unmap_job_semaphore_;
+
   base::Mutex relocation_mutex_;
 
   int gc_callbacks_depth_;
 
   bool deserialization_complete_;
 
   bool concurrent_sweeping_enabled_;
 
   // |live_array_buffers_| maps externally allocated memory used as backing
   // store for ArrayBuffers to the length of the respective memory blocks.
@@ skipping 396 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_