base/memory/discardable_memory.h

   1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_MEMORY_DISCARDABLE_MEMORY_H_
   6 #define BASE_MEMORY_DISCARDABLE_MEMORY_H_
   7
   8 #include <string>
   9 #include <vector>
  10
  11 #include "base/base_export.h"
  12 #include "base/basictypes.h"
  13 #include "base/compiler_specific.h"
  14 #include "base/memory/scoped_ptr.h"
  15
  16 namespace base {
  17
  18 enum DiscardableMemoryType {
  19   DISCARDABLE_MEMORY_TYPE_NONE,
  20   DISCARDABLE_MEMORY_TYPE_ASHMEM,
  21   DISCARDABLE_MEMORY_TYPE_MAC,
  22   DISCARDABLE_MEMORY_TYPE_EMULATED,
  23   DISCARDABLE_MEMORY_TYPE_MALLOC
  24 };
  25
  26 enum DiscardableMemoryLockStatus {
  27   DISCARDABLE_MEMORY_LOCK_STATUS_FAILED,
  28   DISCARDABLE_MEMORY_LOCK_STATUS_PURGED,
  29   DISCARDABLE_MEMORY_LOCK_STATUS_SUCCESS
  30 };
  31
  32 // Platform abstraction for discardable memory. DiscardableMemory is used to
  33 // cache large objects without worrying about blowing out memory, both on mobile
  34 // devices where there is no swap, and desktop devices where unused free memory
  35 // should be used to help the user experience. This is preferable to releasing
  36 // memory in response to an OOM signal because it is simpler, though it has less
  37 // flexibility as to which objects get discarded.
  38 //
  39 // Discardable memory has two states: locked and unlocked. While the memory is
  40 // locked, it will not be discarded. Unlocking the memory allows the OS to
  41 // reclaim it if needed. Locks do not nest.
  42 //
  43 // Notes:
  44 //   - The paging behavior of memory while it is locked is not specified. While
  45 //     mobile platforms will not swap it out, it may qualify for swapping
  46 //     on desktop platforms. It is not expected that this will matter, as the
  47 //     preferred pattern of usage for DiscardableMemory is to lock down the
  48 //     memory, use it as quickly as possible, and then unlock it.
  49 //   - Because of memory alignment, the amount of memory allocated can be
  50 //     larger than the requested memory size. It is not very efficient for
  51 //     small allocations.
  52 //   - A discardable memory instance is not thread safe. It is the
  53 //     responsibility of users of discardable memory to ensure there are no
  54 //     races.
  55 //
  56 // References:
  57 //   - Linux: http://lwn.net/Articles/452035/
  58 //   - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/PurgeableBufferMac.cpp
  59 //          the comment starting with "vm_object_purgable_control" at
  60 //            http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/vm_object.c
  61 //
  62 // Thread-safety: DiscardableMemory instances are not thread-safe.
  63 class BASE_EXPORT DiscardableMemory {
  64  public:
  65   virtual ~DiscardableMemory() {}
  66
  67   // Gets the discardable memory type with a given name.
  68   static DiscardableMemoryType GetNamedType(const std::string& name);
  69
  70   // Gets the name of a discardable memory type.
  71   static const char* GetTypeName(DiscardableMemoryType type);
  72
  73   // Gets system supported discardable memory types. Default preferred type
  74   // at the front of vector.
  75   static void GetSupportedTypes(std::vector<DiscardableMemoryType>* types);
  76
  77   // Sets the preferred discardable memory type. This overrides the default
  78   // preferred type. Can only be called once prior to GetPreferredType()
  79   // or CreateLockedMemory(). Caller is responsible for correct ordering.
  80   static void SetPreferredType(DiscardableMemoryType type);
  81
  82   // Gets the preferred discardable memory type.
  83   static DiscardableMemoryType GetPreferredType();
  84
  85   // Create a DiscardableMemory instance with specified |type| and |size|.
  86   static scoped_ptr<DiscardableMemory> CreateLockedMemoryWithType(
  87       DiscardableMemoryType type, size_t size);
  88
  89   // Create a DiscardableMemory instance with preferred type and |size|.
  90   static scoped_ptr<DiscardableMemory> CreateLockedMemory(size_t size);
  91
  92   // Discardable memory implementations might allow an elevated usage level
  93   // while in frequent use. Call this to have the usage reduced to the base
  94   // level. Returns true if there's no need to call this again until
  95   // memory instances have been used. This indicates that all discardable
  96   // memory implementations have reduced usage to the base level or below.
  97   // Note: calling this too often or while discardable memory is in frequent
  98   // use can hurt performance, whereas calling it too infrequently can result
  99   // in memory bloat.
 100   static bool ReduceMemoryUsage();
 101
 102   // Locks the memory so that it will not be purged by the system. Returns
 103   // DISCARDABLE_MEMORY_LOCK_STATUS_SUCCESS on success. If the return value is
 104   // DISCARDABLE_MEMORY_LOCK_STATUS_FAILED then this object should be
 105   // discarded and a new one should be created. If the return value is
 106   // DISCARDABLE_MEMORY_LOCK_STATUS_PURGED then the memory is present but any
 107   // data that was in it is gone.
 108   virtual DiscardableMemoryLockStatus Lock() WARN_UNUSED_RESULT = 0;
 109
 110   // Unlocks the memory so that it can be purged by the system. Must be called
 111   // after every successful lock call.
 112   virtual void Unlock() = 0;
 113
 114   // Returns the memory address held by this object. The object must be locked
 115   // before calling this. Otherwise, this will cause a DCHECK error.
 116   virtual void* Memory() const = 0;
 117
 118   // Testing utility calls.
 119
 120   // Purge all discardable memory in the system. This call has global effects
 121   // across all running processes, so it should only be used for testing!
 122   static void PurgeForTesting();
 123 };
 124
 125 }  // namespace base
 126
 127 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_H_