PartitionAlloc: support in-place resize of directly mapped allocation

Source/wtf/PartitionAlloc.h

 /*
  * 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 111 matching lines @@
 static const size_t kMaxPartitionSize = 2046u * 1024u * 1024u;
 
 // Allocation granularity of sizeof(void*) bytes.
 static const size_t kAllocationGranularity = sizeof(void*);
 static const size_t kAllocationGranularityMask = kAllocationGranularity - 1;
 static const size_t kBucketShift = (kAllocationGranularity == 8) ? 3 : 2;
 
 // Underlying partition storage pages are a power-of-two size. It is typical
 // for a partition page to be based on multiple system pages. Most references to
 // "page" refer to partition pages.
-// We also have the concept of "super pages" -- these are the underlying system // allocations we make. Super pages contain multiple partition pages inside them
+// We also have the concept of "super pages" -- these are the underlying system
+// allocations we make. Super pages contain multiple partition pages inside them
 // and include space for a small amount of metadata per partition page.
 // Inside super pages, we store "slot spans". A slot span is a continguous range
 // of one or more partition pages that stores allocations of the same size.
 // Slot span sizes are adjusted depending on the allocation size, to make sure
 // the packing does not lead to unused (wasted) space at the end of the last
 // system page of the span. For our current max slot span size of 64k and other
 // constant values, we pack _all_ partitionAllocGeneric() sizes perfectly up
 // against the end of a system page.
 static const size_t kPartitionPageShift = 14; // 16KB
 static const size_t kPartitionPageSize = 1 << kPartitionPageShift;
@@ skipping 32 matching lines @@
 // In terms of allocation sizes, order 0 covers 0, order 1 covers 1, order 2
 // covers 2->3, order 3 covers 4->7, order 4 covers 8->15.
 static const size_t kGenericMinBucketedOrder = 4; // 8 bytes.
 static const size_t kGenericMaxBucketedOrder = 20; // Largest bucketed order is 1<<(20-1) (storing 512KB -> almost 1MB)
 static const size_t kGenericNumBucketedOrders = (kGenericMaxBucketedOrder - kGenericMinBucketedOrder) + 1;
 static const size_t kGenericNumBucketsPerOrderBits = 3; // Eight buckets per order (for the higher orders), e.g. order 8 is 128, 144, 160, ..., 240
 static const size_t kGenericNumBucketsPerOrder = 1 << kGenericNumBucketsPerOrderBits;
 static const size_t kGenericSmallestBucket = 1 << (kGenericMinBucketedOrder - 1);
 static const size_t kGenericMaxBucketSpacing = 1 << ((kGenericMaxBucketedOrder - 1) - kGenericNumBucketsPerOrderBits);
 static const size_t kGenericMaxBucketed = (1 << (kGenericMaxBucketedOrder - 1)) + ((kGenericNumBucketsPerOrder - 1) * kGenericMaxBucketSpacing);
+static const size_t kGenericMinDirectMapped = 10 * kPartitionPageSize; // Limit when downsizing a direct mapping using realloc(); allows 10 % allocation overhead.

[Chris Evans, 2014/03/06 05:55:13]

At the risk of giving away my OCD, can we make that 16 * blah? A nice power of
two? :P

Also, can we work the term "downsize" into the name to make it more apparent
when we use this? e.g. kGenericMinDirectMappedDownsize or some such?

 static const size_t kGenericMaxDirectMapped = INT_MAX - kSystemPageSize;
 static const size_t kBitsPerSizet = sizeof(void*) * CHAR_BIT;
 
 // Constants for the memory reclaim logic.
 static const size_t kMaxFreeableSpans = 16;
 
 #ifndef NDEBUG
 // These two byte values match tcmalloc.
 static const unsigned char kUninitializedByte = 0xAB;
 static const unsigned char kFreedByte = 0xCD;
@@ skipping 461 matching lines @@
 using WTF::partitionAlloc;
 using WTF::partitionFree;
 using WTF::partitionAllocGeneric;
 using WTF::partitionFreeGeneric;
 using WTF::partitionReallocGeneric;
 using WTF::partitionAllocActualSize;
 using WTF::partitionAllocSupportsGetSize;
 using WTF::partitionAllocGetSize;
 
 #endif // WTF_PartitionAlloc_h