Add DiscardableMemoryAllocator to work around FD limit issue.

base/memory/discardable_memory.h

 // Copyright (c) 2013 The Chromium 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 BASE_MEMORY_DISCARDABLE_MEMORY_H_
 #define BASE_MEMORY_DISCARDABLE_MEMORY_H_
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
@@ skipping 12 matching lines @@
 // devices where there is no swap, and desktop devices where unused free memory
 // should be used to help the user experience. This is preferable to releasing
 // memory in response to an OOM signal because it is simpler, though it has less
 // flexibility as to which objects get discarded.
 //
 // Discardable memory has two states: locked and unlocked. While the memory is
 // locked, it will not be discarded. Unlocking the memory allows the OS to
 // reclaim it if needed. Locks do not nest.
 //
 // Notes:
+//   - If you need more than one instance of DiscardableMemory please use
+//     DiscardableMemoryAllocator to avoid using too many file descriptors on
+//     platforms where discardable memory is backed by a file.
 //   - The paging behavior of memory while it is locked is not specified. While
 //     mobile platforms will not swap it out, it may qualify for swapping
 //     on desktop platforms. It is not expected that this will matter, as the
 //     preferred pattern of usage for DiscardableMemory is to lock down the
 //     memory, use it as quickly as possible, and then unlock it.
 //   - Because of memory alignment, the amount of memory allocated can be
 //     larger than the requested memory size. It is not very efficient for
 //     small allocations.
 //
 // References:
 //   - Linux: http://lwn.net/Articles/452035/
 //   - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/PurgeableBufferMac.cpp
 //          the comment starting with "vm_object_purgable_control" at
 //            http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/vm_object.c
+//
+// Thread-safety: DiscardableMemory instances are not thread-safe.
 class BASE_EXPORT DiscardableMemory {
  public:
   virtual ~DiscardableMemory() {}
 
   // Returns whether the system supports discardable memory.
   static bool Supported();
 
   static scoped_ptr<DiscardableMemory> CreateLockedMemory(size_t size);
 
+  // Returns the memory's actual size that is greater or equal than the
+  // size requested at construction.
+  virtual size_t Size() const = 0;
+
   // Locks the memory so that it will not be purged by the system. Returns
   // DISCARDABLE_MEMORY_SUCCESS on success. If the return value is
   // DISCARDABLE_MEMORY_FAILED then this object should be discarded and
   // a new one should be created. If the return value is
   // DISCARDABLE_MEMORY_PURGED then the memory is present but any data that
   // was in it is gone.
   virtual LockDiscardableMemoryStatus Lock() WARN_UNUSED_RESULT = 0;
 
   // Unlocks the memory so that it can be purged by the system. Must be called
   // after every successful lock call.
@@ skipping 10 matching lines @@
   static bool PurgeForTestingSupported();
 
   // Purge all discardable memory in the system. This call has global effects
   // across all running processes, so it should only be used for testing!
   static void PurgeForTesting();
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_H_