base: Add free list implementation to browser-wide discardable memory system.

base/memory/discardable_memory_shmem.h

Index: base/memory/discardable_memory_shmem.h
diff --git a/base/memory/discardable_memory_shmem.h b/base/memory/discardable_memory_shmem.h
index d25f84cb46dbdb364d1a86dbf4c9286c8759d02e..46bc5887ec00b8e70eba42f5c380afba40d121bc 100644
--- a/base/memory/discardable_memory_shmem.h
+++ b/base/memory/discardable_memory_shmem.h
@@ -10,7 +10,7 @@
 #include "base/memory/discardable_memory_manager.h"
 
 namespace base {
-class DiscardableSharedMemory;
+class DiscardableMemoryShmemChunk;
 
 namespace internal {
 
@@ -40,7 +40,7 @@ class DiscardableMemoryShmem
 
  private:
   const size_t bytes_;
-  scoped_ptr<DiscardableSharedMemory> shared_memory_;
+  scoped_ptr<DiscardableMemoryShmemChunk> chunk_;

[danakj, 2014/12/18 21:40:50]

What is the point of wrapping DiscardableSharedMemory in this new interface?

[reveman, 2014/12/19 00:49:54]

Each DiscardableSharedMemory instance is used for multiple DiscardableMemory
instances. We have X number of DiscardableSharedMemory instances and Y number of
DiscardableMemoryShmemChunk instances for each DiscardableSharedMemory instance.

DiscardableMemoryShmemChunk will eventually be removed and we'll just have X
DiscardableSharedMemory and Y DiscardableMemory instances. See the comment in
discardable_memory_shmem_allocator.h. I just need to first enable this code and
remove the old emulated discardable memory system before I can do this.

[danakj, 2014/12/19 21:08:08]

Sorry this explanation confuses me. Specifically DiscardableSharedMemory is
created and put in a scoped_ptr, then Passed to and wrapped in a new scoped_ptr
inside a DiscardableMemoryShmemChunkImpl. So these seem to be 1:1. So what do
you mean by there are X DiscardableSharedMemory and Y
DiscardableMemoryShmemChunk?

[reveman, 2014/12/19 22:16:03]

It's wrapped in a linked_ptr (See
DiscardableSharedMemoryHeap::Span::shared_memory_), so it's ref counted. That
could be wrapped in a RefCounted type instead if linked_ptr is not efficient
enough. I'm not sure that's necessary though.

   bool is_locked_;
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableMemoryShmem);