Add DiscardableMemoryAllocator to work around FD limit issue.

base/memory/discardable_memory_allocator.h

+// Copyright 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_ALLOCATOR_H_
+#define BASE_MEMORY_DISCARDABLE_MEMORY_ALLOCATOR_H_
+
+#include <string>
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+
+namespace base {
+
+class DiscardableMemory;
+
+// Allows multiple pieces of discardable memory to be allocated by efficiently
+// managing resources.
+// On certain platforms such as Android, discardable memory is backed by a file
+// (descriptor) thus is a limited resource. This allocator minimizes the problem
+// by operating on large pieces of memory and returning to the client chunks in
+// it.
+// Allocated chunks are systematically aligned on a page boundary therefore this
+// allocator is not recommended for clients performing (a lot of) small
+// allocations. This can be useful for instance for clients implementing large
+// resource in-memory caches.
+//
+// Thread-safety:
+// The allocator must be deleted on the thread it was constructed on although
+// its Allocate() method can be invoked on any thread. See discardable_memory.h
+// for DiscardableMemory's threading guarantees.
+class BASE_EXPORT DiscardableMemoryAllocator {
+ public:
+  virtual ~DiscardableMemoryAllocator() {}
+
+  // Note that |name| is only used for debugging/measurement purposes.
+  static scoped_ptr<DiscardableMemoryAllocator> Create(const std::string& name);
+
+  // Note that the allocator must outlive the returned DiscardableMemory
+  // instance.
+  virtual scoped_ptr<DiscardableMemory> Allocate(size_t size) = 0;
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_DISCARDABLE_MEMORY_ALLOCATOR_H_