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// 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"
namespace base {
enum LockDiscardableMemoryStatus {
DISCARDABLE_MEMORY_FAILED = -1,
DISCARDABLE_MEMORY_PURGED = 0,
DISCARDABLE_MEMORY_SUCCESS = 1
};
// Platform abstraction for discardable memory. DiscardableMemory is used to
// cache large objects without worrying about blowing out memory, both on mobile
// 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:
// - 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
class BASE_EXPORT DiscardableMemory {
public:
DiscardableMemory();
// If the discardable memory is locked, the destructor will unlock it.
// The opened file will also be closed after this.
~DiscardableMemory();
// Check whether the system supports discardable memory.
static bool Supported();
// Initialize the DiscardableMemory object. On success, this function returns
// true and the memory is locked. This should only be called once.
// This call could fail because of platform-specific limitations and the user
// should stop using the DiscardableMemory afterwards.
bool InitializeAndLock(size_t size);
// Lock 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.
LockDiscardableMemoryStatus Lock() WARN_UNUSED_RESULT;
// Unlock the memory so that it can be purged by the system. Must be called
// after every successful lock call.
void Unlock();
// Return the memory address held by this object. The object must be locked
// before calling this. Otherwise, this will cause a DCHECK error.
void* Memory() const;
// Testing utility calls.
// Check whether a purge of all discardable memory in the system is supported.
// Use only for testing!
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();
private:
#if defined(OS_ANDROID)
// Maps the discardable memory into the caller's address space.
// Returns true on success, false otherwise.
bool Map();
// Unmaps the discardable memory from the caller's address space.
void Unmap();
// Reserve a file descriptor. When reaching the fd limit, this call returns
// false and initialization should fail.
bool ReserveFileDescriptor();
// Release a file descriptor so that others can reserve it.
void ReleaseFileDescriptor();
#endif // OS_ANDROID
void* memory_;
size_t size_;
bool is_locked_;
#if defined(OS_ANDROID)
int fd_;
#endif // OS_ANDROID
DISALLOW_COPY_AND_ASSIGN(DiscardableMemory);
};
} // namespace base
#endif // BASE_MEMORY_DISCARDABLE_MEMORY_H_
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