Add DiscardableMemoryAllocator to work around FD limit issue.

base/memory/discardable_memory_android.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.
+
+// Please use discardable_memory.h since this is just an internal file providing
+// utility functions used both by discardable_memory_android.cc and
+// discardable_memory_allocator_android.cc.

[pasko, 2013/10/25 15:21:13]

Since people will just look into this header as the main starting point to
gather knowledge about DiscardableMemory implementation on Android, I'd like to
ask to put some overview of how  DiscardableMemory::CreateLockedMemory() works,
basically:
1. Creates real ashmem region per each instance of returned DiscardableMemory
2. when approaching the file descriptor limit for ashmem (90% used), start
allocating large regions (like 32MiB), return them to the user in chunks.
See DiscardableMemory::CreateLockedMemory() in discardable_memory_android.cc and
discardable_memory_allocator_android.h.

[Philippe, 2013/10/28 09:44:43]

I'm a little concerned about duplicating this information in several places.
This is also an internal header so I would prefer making it painless to maintain
(or not subject to containing stale information) rather than an entertaining
read for public clients :) They can easily open discardable_memory.h and
discardable_memory_android.cc to see what is going on if they really care (which
they should not have to do). What do you think?

[pasko, 2013/10/28 14:40:06]

I understand your concern about duplication and about the header being internal.
You are right, my proposal is probably not providing any value by being in an
internal header. I was just thinking an outline of the algorithm with 3 short
sentences put together can greatly simplify understanding of what is happening
while the implementation is scattered around 2 files (600 lines total):
discardable_memory_android.cc
discardable_memory_allocator_android.cc

One other way I could think of is to put the description of Android specifics to
discardable_memory.h, but I won't insist on that, up to you.

+
+#ifndef BASE_MEMORY_DISCARDABLE_MEMORY_ANDROID_H_
+#define BASE_MEMORY_DISCARDABLE_MEMORY_ANDROID_H_
+
+#include "base/basictypes.h"
+#include "base/memory/discardable_memory.h"
+
+namespace base {
+namespace internal {
+
+size_t AlignToNextPage(size_t size);
+
+bool CreateAshmemRegion(const char* name, size_t size, int* fd, void** address);
+
+bool DeleteAshmemRegion(int fd, size_t size, void* address);
+
+LockDiscardableMemoryStatus LockAshmemRegion(int fd,
+                                             size_t off,
+                                             size_t size,
+                                             const void* address);
+
+bool UnlockAshmemRegion(int fd, size_t off, size_t size, const void* address);
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_MEMORY_DISCARDABLE_MEMORY_ANDROID_H_