Wait for sweeper threads only if we have to.

src/mark-compact.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_MARK_COMPACT_H_
 #define V8_MARK_COMPACT_H_
 
 #include "src/compiler-intrinsics.h"
 #include "src/spaces.h"
 
@@ skipping 652 matching lines @@
   void SweepInParallel(PagedSpace* space);
 
   void WaitUntilSweepingCompleted();
 
   bool IsSweepingCompleted();
 
   void RefillFreeList(PagedSpace* space);
 
   bool AreSweeperThreadsActivated();
 
-  bool IsConcurrentSweepingInProgress();
+  // If a paged space is passed in, this method checks if the given space is
+  // swept concurrently. Otherwise, this method checks if concurrent sweeping
+  // is in progress right now on any space.
+  bool IsConcurrentSweepingInProgress(PagedSpace* space = NULL);
 
   void set_sequential_sweeping(bool sequential_sweeping) {
     sequential_sweeping_ = sequential_sweeping;
   }
 
   bool sequential_sweeping() const {
     return sequential_sweeping_;
   }
 
   // Mark the global table which maps weak objects to dependent code without
@@ skipping 295 matching lines @@
  private:
   MarkCompactCollector* collector_;
 };
 
 
 const char* AllocationSpaceName(AllocationSpace space);
 
 } }  // namespace v8::internal
 
 #endif  // V8_MARK_COMPACT_H_