base: Avoid purging more discardable memory than necessary.

base/memory/discardable_memory_provider.h

 // Copyright 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_PROVIDER_H_
 #define BASE_MEMORY_DISCARDABLE_MEMORY_PROVIDER_H_
 
 #include "base/base_export.h"
 #include "base/containers/hash_tables.h"
 #include "base/containers/mru_cache.h"
@@ skipping 37 matching lines @@
 
   static DiscardableMemoryProvider* GetInstance();
 
   // Sets the instance of DiscardableMemoryProvider to be returned by
   // GetInstance. This should only be used by tests and must be called
   // prior to GetInstance(). The ownership of the given provider is
   // retained by the caller.
   static void SetInstanceForTest(DiscardableMemoryProvider* provider);
 
   // The maximum number of bytes of discardable memory that may be allocated
-  // before we assume moderate memory pressure. If this amount is zero, it is
-  // interpreted as having no limit at all.
+  // before we force a purge. If this amount is zero, it is interpreted as
+  // having no limit at all.
   void SetDiscardableMemoryLimit(size_t bytes);
 
   // Sets the amount of memory to reclaim when we're under moderate pressure.
   void SetBytesToReclaimUnderModeratePressure(size_t bytes);
 
   // Adds the given discardable memory to the provider's collection.
   void Register(const DiscardableMemory* discardable, size_t bytes);
 
   // Removes the given discardable memory from the provider's collection.
   void Unregister(const DiscardableMemory* discardable);
@@ skipping 33 matching lines @@
 
     size_t bytes;
     uint8* memory;
   };
   typedef HashingMRUCache<const DiscardableMemory*, Allocation> AllocationMap;
 
   // This can be called as a hint that the system is under memory pressure.
   static void NotifyMemoryPressure(
       MemoryPressureListener::MemoryPressureLevel pressure_level);
 
-  // Purges least recently used memory based on the value of
-  // |bytes_to_reclaim_under_moderate_pressure_|.
+  // Purges |bytes_to_reclaim_under_moderate_pressure_| bytes of
+  // discardable memory.
   void Purge();
 
   // Purges least recently used memory until usage is less or equal to |limit|.
   // Caller must acquire |lock_| prior to calling this function.
   void PurgeLRUWithLockAcquiredUntilUsageIsWithin(size_t limit);
 
   // Ensures that we don't allocate beyond our memory limit.
   // Caller must acquire |lock_| prior to calling this function.
   void EnforcePolicyWithLockAcquired();
 
@@ skipping 17 matching lines @@
   // pressure.
   MemoryPressureListener memory_pressure_listener_;
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableMemoryProvider);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_PROVIDER_H_