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// Copyright 2013 The Chromium Authors
// 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 "build/build_config.h"
namespace base {
namespace trace_event {
class MemoryAllocatorDump;
class ProcessMemoryDump;
} // namespace trace_event
// Discardable memory 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 and provides system-wide management of
// purgable memory, 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
// discardable memory system and 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.
// - A discardable memory instance is not thread safe. It is the
// responsibility of users of discardable memory to ensure there are no
// races.
//
class BASE_EXPORT DiscardableMemory {
public:
DiscardableMemory();
virtual ~DiscardableMemory();
// Locks the memory so that it will not be purged by the system. Returns
// true on success. If the return value is false then this object should be
// destroyed and a new one should be created.
[[nodiscard]] virtual bool Lock() = 0;
// Unlocks the memory so that it can be purged by the system. Must be called
// after every successful lock call.
virtual void Unlock() = 0;
// Returns the memory address held by this object. The object must be locked
// before calling this.
virtual void* data() const = 0;
// Forces the memory to be purged, such that any following Lock() will fail.
// The object must be unlocked before calling this.
virtual void DiscardForTesting() = 0;
// Handy method to simplify calling data() with a reinterpret_cast.
template <typename T>
T* data_as() const {
return reinterpret_cast<T*>(data());
}
// Used for dumping the statistics of discardable memory allocated in tracing.
// Returns a new MemoryAllocatorDump in the |pmd| with the size of the
// discardable memory. The MemoryAllocatorDump created is owned by |pmd|. See
// ProcessMemoryDump::CreateAllocatorDump.
virtual trace_event::MemoryAllocatorDump* CreateMemoryAllocatorDump(
const char* name,
trace_event::ProcessMemoryDump* pmd) const = 0;
};
enum class DiscardableMemoryBacking { kSharedMemory, kMadvFree };
BASE_EXPORT DiscardableMemoryBacking GetDiscardableMemoryBacking();
} // namespace base
#endif // BASE_MEMORY_DISCARDABLE_MEMORY_H_
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