Add DiscardableMemoryAllocator to work around FD limit issue.

base/memory/discardable_memory.h

Index: base/memory/discardable_memory.h
diff --git a/base/memory/discardable_memory.h b/base/memory/discardable_memory.h
index c6a8d82da72363d3dc2cecbb01547eeb64c226eb..e87476194aa892e0cb5039698cd4a3c17abd0733 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -44,6 +44,8 @@ enum LockDiscardableMemoryStatus {
 //   - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/PurgeableBufferMac.cpp
 //          the comment starting with "vm_object_purgable_control" at
 //            http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/vm_object.c
+//
+// Thread-safety: DiscardableMemory instances are not thread-safe.
 class BASE_EXPORT DiscardableMemory {
  public:
   virtual ~DiscardableMemory() {}
@@ -53,6 +55,10 @@ class BASE_EXPORT DiscardableMemory {
 
   static scoped_ptr<DiscardableMemory> CreateLockedMemory(size_t size);
 
+  // Returns the memory's actual size that is greater or equal than the

[willchan no longer on Chromium, 2013/10/28 19:54:05]

Can you explain why this is an important API to add? I'd equate this to malloc()
implementations where due to page alignment boundaries and other implementation
reasons, there may be extra usable memory, but the malloc() implementation
doesn't expose that, and it's considered illegal to use that extra memory. I
think it's a simpler not to support this API. If there is a good reason to
support this (performance?), I can be convinced otherwise.

[willchan no longer on Chromium, 2013/11/18 14:53:13]

Can you answer this question?

[Philippe, 2013/11/18 16:34:51]

Yes, sorry. I added this mainly to simplify testing TBH. However I expect this
could be useful to let the client minimize fragmentation (and avoid unnecessary
resize if this is used to implement a vector for instance). The changes I'm
making currently in Blink to use discardable memory (e.g. in WTF::Vector)
already make sure that the allocated sizes are page-aligned so I don't strictly
need this. I'm happy to remove this now and possibly reconsider it later if I
have data that shows this is useful.

+  // size requested at construction.
+  virtual size_t Size() const = 0;
+
   // Locks 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