PartitionAlloc: Make in-place realloc() less likely to happen

Source/wtf/PartitionAlloc.cpp

 /*
  * Copyright (C) 2013 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 792 matching lines @@
         return false;
 
     // bucket->slotSize is the current size of the allocation.
     size_t currentSize = page->bucket->slotSize;
     if (newSize == currentSize)
         return true;
 
     char* charPtr = static_cast<char*>(partitionPageToPointer(page));
 
     if (newSize < currentSize) {
+        size_t mapSize = partitionPageToDirectMapExtent(page)->mapSize;
+
+        // Don't reallocate in-place if new size is less than 80 % of the full
+        // map size, to avoid holding on to too much unused address space.
+        if ((newSize / kSystemPageSize) * 5 < (mapSize / kSystemPageSize) * 4)
+            return false;
+
         // Shrink by decommitting unneeded pages and making them inaccessible.
         size_t decommitSize = currentSize - newSize;
         decommitSystemPages(charPtr + newSize, decommitSize);
         setSystemPagesInaccessible(charPtr + newSize, decommitSize);
     } else if (newSize <= partitionPageToDirectMapExtent(page)->mapSize) {
         // Grow within the actually allocated memory. Just need to make the
         // pages accessible again.
         size_t recommitSize = newSize - currentSize;
         setSystemPagesAccessible(charPtr + currentSize, recommitSize);
 
@@ skipping 117 matching lines @@
     printf("total live: %zu bytes\n", totalLive);
     printf("total resident: %zu bytes\n", totalResident);
     printf("total freeable: %zu bytes\n", totalFreeable);
     fflush(stdout);
 }
 
 #endif // !NDEBUG
 
 } // namespace WTF