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79 lines
3.1 KiB
C++
79 lines
3.1 KiB
C++
// Copyright (c) 2013 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_MEMORY_DISCARDABLE_MEMORY_H_
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#define BASE_MEMORY_DISCARDABLE_MEMORY_H_
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#include "base/base_export.h"
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#include "base/compiler_specific.h"
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namespace base {
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namespace trace_event {
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class MemoryAllocatorDump;
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class ProcessMemoryDump;
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}
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// Discardable memory is used to cache large objects without worrying about
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// blowing out memory, both on mobile devices where there is no swap, and
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// desktop devices where unused free memory should be used to help the user
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// experience. This is preferable to releasing memory in response to an OOM
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// signal because it is simpler and provides system-wide management of
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// purgable memory, though it has less flexibility as to which objects get
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// discarded.
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//
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// Discardable memory has two states: locked and unlocked. While the memory is
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// locked, it will not be discarded. Unlocking the memory allows the
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// discardable memory system and the OS to reclaim it if needed. Locks do not
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// nest.
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//
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// Notes:
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// - The paging behavior of memory while it is locked is not specified. While
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// mobile platforms will not swap it out, it may qualify for swapping
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// on desktop platforms. It is not expected that this will matter, as the
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// preferred pattern of usage for DiscardableMemory is to lock down the
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// memory, use it as quickly as possible, and then unlock it.
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// - Because of memory alignment, the amount of memory allocated can be
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// larger than the requested memory size. It is not very efficient for
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// small allocations.
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// - A discardable memory instance is not thread safe. It is the
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// responsibility of users of discardable memory to ensure there are no
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// races.
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//
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class BASE_EXPORT DiscardableMemory {
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public:
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DiscardableMemory();
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virtual ~DiscardableMemory();
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// Locks the memory so that it will not be purged by the system. Returns
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// true on success. If the return value is false then this object should be
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// discarded and a new one should be created.
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virtual bool Lock() WARN_UNUSED_RESULT = 0;
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// Unlocks the memory so that it can be purged by the system. Must be called
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// after every successful lock call.
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virtual void Unlock() = 0;
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// Returns the memory address held by this object. The object must be locked
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// before calling this.
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virtual void* data() const = 0;
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// Handy method to simplify calling data() with a reinterpret_cast.
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template<typename T> T* data_as() const {
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return reinterpret_cast<T*>(data());
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}
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// Used for dumping the statistics of discardable memory allocated in tracing.
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// Returns a new MemoryAllocatorDump in the |pmd| with the size of the
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// discardable memory. The MemoryAllocatorDump created is owned by |pmd|. See
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// ProcessMemoryDump::CreateAllocatorDump.
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virtual trace_event::MemoryAllocatorDump* CreateMemoryAllocatorDump(
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const char* name,
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trace_event::ProcessMemoryDump* pmd) const = 0;
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};
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} // namespace base
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#endif // BASE_MEMORY_DISCARDABLE_MEMORY_H_
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