Uncommit and shrink semi-spaces only on low allocation rate.

src/heap/spaces.h

 // Copyright 2011 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_SPACES_H_
 #define V8_HEAP_SPACES_H_
 
 #include "src/allocation.h"
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
@@ skipping 2589 matching lines @@
   bool CommitFromSpaceIfNeeded() {
     if (from_space_.is_committed()) return true;
     return from_space_.Commit();
   }
 
   bool UncommitFromSpace() {
     if (!from_space_.is_committed()) return true;
     return from_space_.Uncommit();
   }
 
+  bool IsFromSpaceCommitted() { return from_space_.is_committed(); }
+
   inline intptr_t inline_allocation_limit_step() {
     return inline_allocation_limit_step_;
   }
 
   SemiSpace* active_space() { return &to_space_; }
 
  private:
   // Update allocation info to match the current to-space page.
   void UpdateAllocationInfo();
 
@@ skipping 268 matching lines @@
     count = 0;
   }
   // Must be small, since an iteration is used for lookup.
   static const int kMaxComments = 64;
 };
 #endif
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_SPACES_H_