Decommits when coallescing spans instead of committing.

third_party/tcmalloc/page_heap.h

 // Copyright (c) 2008, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 195 matching lines @@
     }
   }
 
   // Allocate a large span of length == n.  If successful, returns a
   // span of exactly the specified length.  Else, returns NULL.
   Span* AllocLarge(Length n);
 
   // Commit the span.
   void CommitSpan(Span* span);
 
+  // Decommits the span
+  void DecommitSpan(Span* span);
+
   // Incrementally release some memory to the system.
   // IncrementalScavenge(n) is called whenever n pages are freed.
   void IncrementalScavenge(Length n);
 
   // Number of pages to deallocate before doing more scavenging
   int64_t scavenge_counter_;
 
   // Index of last free list we scavenged
   int scavenge_index_;
 };
 
 }  // namespace tcmalloc
 
 #endif  // TCMALLOC_PAGE_HEAP_H_