[Not for review] Discardable memory emulation

base/memory/discardable_memory.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // 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 "base/basictypes.h"
 #include "base/compiler_specific.h"
 
 namespace base {
 
 enum LockDiscardableMemoryStatus {
   DISCARDABLE_MEMORY_FAILED = -1,
   DISCARDABLE_MEMORY_PURGED = 0,
   DISCARDABLE_MEMORY_SUCCESS = 1
 };
 
+#if !defined(OS_ANDROID) && !defined(OS_MACOSX)

[nduca, 2013/05/29 13:57:00]

so, on windows 8, we have discardable memory but pre-8 we dont. How does the
provider fit in in that case?

maybe we can put everything behind a provider interface somehow?

[Ian Vollick, 2013/06/06 00:21:47]

I *think* this is done, but please let me know if my approach isn't what you
were hoping for.

+// Used to emulate discardable memory on unsupported platforms.
+class DiscardableMemoryProvider;
+#endif
+
 // Platform abstraction for discardable memory. DiscardableMemory 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, 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 OS to
 // reclaim it if needed. Locks do not nest.
@@ skipping 17 matching lines @@
  public:
   DiscardableMemory();
 
   // If the discardable memory is locked, the destructor will unlock it.
   // The opened file will also be closed after this.
   ~DiscardableMemory();
 
   // Check whether the system supports discardable memory.
   static bool Supported();
 
+#if !defined(OS_ANDROID) && !defined(OS_MACOSX)
+  // Installs a provider to emulate discardable memory on unsupported platforms.
+  static void SetProvider(DiscardableMemoryProvider* provider);
+#endif
+
   // Initialize the DiscardableMemory object. On success, this function returns
   // true and the memory is locked. This should only be called once.
   // This call could fail because of platform-specific limitations and the user
   // should stop using the DiscardableMemory afterwards.
   bool InitializeAndLock(size_t size);
 
   // Lock the memory so that it will not be purged by the system. Returns
   // DISCARDABLE_MEMORY_SUCCESS on success. If the return value is
   // DISCARDABLE_MEMORY_FAILED then this object should be discarded and
   // a new one should be created. If the return value is
@@ skipping 29 matching lines @@
   void Unmap();
 
   // Reserve a file descriptor. When reaching the fd limit, this call returns
   // false and initialization should fail.
   bool ReserveFileDescriptor();
 
   // Release a file descriptor so that others can reserve it.
   void ReleaseFileDescriptor();
 #endif  // OS_ANDROID
 
+#if defined(OS_ANDROID) || defined(OS_MACOSX)

[jonathan.backer, 2013/05/29 14:02:54]

Maybe a #define DISCARDABLE_MEMORY_SUPPORTED defined(OS_ANDROID) ||
defined(OS_MACOSX)?

[Ian Vollick, 2013/06/06 00:21:47]

Done.

   void* memory_;
   size_t size_;
   bool is_locked_;
 #if defined(OS_ANDROID)
   int fd_;
 #endif  // OS_ANDROID
+#endif  // OS_ANDROID || OS_MACOSX
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableMemory);
 };
 
+#if !defined(OS_ANDROID) && !defined(OS_MACOSX)
+class BASE_EXPORT DiscardableMemoryProvider {
+ public:
+  virtual bool Register(const DiscardableMemory*, size_t size) = 0;
+  virtual void Unregister(const DiscardableMemory*) = 0;
+  virtual LockDiscardableMemoryStatus Lock(const DiscardableMemory*) = 0;
+  virtual void Unlock(const DiscardableMemory*) = 0;
+  virtual void* Memory(const DiscardableMemory*) const = 0;
+  virtual bool PurgeForTestingSupported() const = 0;
+  virtual void PurgeForTesting() = 0;
+};
+#endif
+
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_H_