OLD | NEW |
1 // Copyright (c) 2013 The Chromium Authors. All rights reserved. | 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 | 2 // Use of this source code is governed by a BSD-style license that can be |
3 // found in the LICENSE file. | 3 // found in the LICENSE file. |
4 | 4 |
5 #ifndef BASE_MEMORY_DISCARDABLE_MEMORY_H_ | 5 #ifndef BASE_MEMORY_DISCARDABLE_MEMORY_H_ |
6 #define BASE_MEMORY_DISCARDABLE_MEMORY_H_ | 6 #define BASE_MEMORY_DISCARDABLE_MEMORY_H_ |
7 | 7 |
8 #include <string> | |
9 #include <vector> | |
10 | |
11 #include "base/base_export.h" | 8 #include "base/base_export.h" |
12 #include "base/basictypes.h" | 9 #include "base/basictypes.h" |
13 #include "base/compiler_specific.h" | 10 #include "base/compiler_specific.h" |
14 #include "base/memory/scoped_ptr.h" | |
15 | 11 |
16 namespace base { | 12 namespace base { |
17 | 13 |
18 // Platform abstraction for discardable memory. DiscardableMemory is used to | 14 // Discardable memory is used to cache large objects without worrying about |
19 // cache large objects without worrying about blowing out memory, both on mobile | 15 // blowing out memory, both on mobile devices where there is no swap, and |
20 // devices where there is no swap, and desktop devices where unused free memory | 16 // desktop devices where unused free memory should be used to help the user |
21 // should be used to help the user experience. This is preferable to releasing | 17 // experience. This is preferable to releasing memory in response to an OOM |
22 // memory in response to an OOM signal because it is simpler, though it has less | 18 // signal because it is simpler and provides system-wide management of |
23 // flexibility as to which objects get discarded. | 19 // purgable memory, though it has less flexibility as to which objects get |
| 20 // discarded. |
24 // | 21 // |
25 // Discardable memory has two states: locked and unlocked. While the memory is | 22 // Discardable memory has two states: locked and unlocked. While the memory is |
26 // locked, it will not be discarded. Unlocking the memory allows the OS to | 23 // locked, it will not be discarded. Unlocking the memory allows the |
27 // reclaim it if needed. Locks do not nest. | 24 // discardable memory system and the OS to reclaim it if needed. Locks do not |
| 25 // nest. |
28 // | 26 // |
29 // Notes: | 27 // Notes: |
30 // - The paging behavior of memory while it is locked is not specified. While | 28 // - The paging behavior of memory while it is locked is not specified. While |
31 // mobile platforms will not swap it out, it may qualify for swapping | 29 // mobile platforms will not swap it out, it may qualify for swapping |
32 // on desktop platforms. It is not expected that this will matter, as the | 30 // on desktop platforms. It is not expected that this will matter, as the |
33 // preferred pattern of usage for DiscardableMemory is to lock down the | 31 // preferred pattern of usage for DiscardableMemory is to lock down the |
34 // memory, use it as quickly as possible, and then unlock it. | 32 // memory, use it as quickly as possible, and then unlock it. |
35 // - Because of memory alignment, the amount of memory allocated can be | 33 // - Because of memory alignment, the amount of memory allocated can be |
36 // larger than the requested memory size. It is not very efficient for | 34 // larger than the requested memory size. It is not very efficient for |
37 // small allocations. | 35 // small allocations. |
38 // - A discardable memory instance is not thread safe. It is the | 36 // - A discardable memory instance is not thread safe. It is the |
39 // responsibility of users of discardable memory to ensure there are no | 37 // responsibility of users of discardable memory to ensure there are no |
40 // races. | 38 // races. |
41 // | 39 // |
42 // References: | |
43 // - Linux: http://lwn.net/Articles/452035/ | |
44 // - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/Pur
geableBufferMac.cpp | |
45 // the comment starting with "vm_object_purgable_control" at | |
46 // http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/v
m_object.c | |
47 // | |
48 // Thread-safety: DiscardableMemory instances are not thread-safe. | |
49 class BASE_EXPORT DiscardableMemory { | 40 class BASE_EXPORT DiscardableMemory { |
50 public: | 41 public: |
51 virtual ~DiscardableMemory() {} | 42 DiscardableMemory(); |
52 | 43 virtual ~DiscardableMemory(); |
53 // Create a DiscardableMemory instance with |size|. | |
54 static scoped_ptr<DiscardableMemory> CreateLockedMemory(size_t size); | |
55 | 44 |
56 // Locks the memory so that it will not be purged by the system. Returns | 45 // Locks the memory so that it will not be purged by the system. Returns |
57 // true on success. If the return value is false then this object should be | 46 // true on success. If the return value is false then this object should be |
58 // discarded and a new one should be created. | 47 // discarded and a new one should be created. |
59 virtual bool Lock() WARN_UNUSED_RESULT = 0; | 48 virtual bool Lock() WARN_UNUSED_RESULT = 0; |
60 | 49 |
61 // Unlocks the memory so that it can be purged by the system. Must be called | 50 // Unlocks the memory so that it can be purged by the system. Must be called |
62 // after every successful lock call. | 51 // after every successful lock call. |
63 virtual void Unlock() = 0; | 52 virtual void Unlock() = 0; |
64 | 53 |
65 // Returns the memory address held by this object. The object must be locked | 54 // Returns the memory address held by this object. The object must be locked |
66 // before calling this. Otherwise, this will cause a DCHECK error. | 55 // before calling this. Otherwise, this will cause a DCHECK error. |
67 virtual void* Memory() const = 0; | 56 virtual void* Memory() const = 0; |
68 }; | 57 }; |
69 | 58 |
70 } // namespace base | 59 } // namespace base |
71 | 60 |
72 #endif // BASE_MEMORY_DISCARDABLE_MEMORY_H_ | 61 #endif // BASE_MEMORY_DISCARDABLE_MEMORY_H_ |
OLD | NEW |