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