Modify AsyncResourceHandler to use a single shared memory buffer (in ring

content/browser/renderer_host/resource_buffer.cc

+// Copyright (c) 2012 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.
+
+#include "content/browser/renderer_host/resource_buffer.h"
+
+#include <math.h>

[brettw, 2012/09/17 23:53:10]

Blank line after this.

[darin (slow to review), 2012/09/18 05:00:50]

Done.

+#include "base/logging.h"
+
+namespace content {
+
+// A circular buffer allocator.
+//
+// We keep track of the starting offset (alloc_start_) and the ending offset
+// (alloc_end_).  There are two layouts to keep in mind:
+//
+// #1:
+//    ------------[XXXXXXXXXXXXXXXXXXXXXXX]----

[brettw, 2012/09/17 23:53:10]

In these diagrams it might be better to have end pointing to the "-" right after
the "]" to show it's pointing to the next free spot.

[darin (slow to review), 2012/09/18 05:00:50]

Done.

+//                ^                       ^
+//                start                   end
+//
+// #2:
+//    XXXXXXXXXX]---------------------[XXXXXXXX
+//              ^                     ^
+//              end                   start
+//
+// If end <= start, then we have the buffer wraparound case (depicted second).
+// If the buffer is empty, then start and end will be set to -1.
+//
+// Allocations are always contiguous.
+
+ResourceBuffer::ResourceBuffer()
+    : buf_size_(0),
+      min_alloc_size_(0),
+      max_alloc_size_(0),
+      alloc_start_(-1),
+      alloc_end_(-1) {
+}
+
+ResourceBuffer::~ResourceBuffer() {
+}
+
+bool ResourceBuffer::Initialize(int buffer_size,
+                                int min_allocation_size,
+                                int max_allocation_size) {
+  DCHECK(!IsInitialized());
+
+  // It would be wasteful if these are not multiples of min_allocation_size.
+  DCHECK_EQ(0, buffer_size % min_allocation_size);
+  DCHECK_EQ(0, max_allocation_size % min_allocation_size);
+
+  buf_size_ = buffer_size;
+  min_alloc_size_ = min_allocation_size;
+  max_alloc_size_ = max_allocation_size;
+
+  return shared_mem_.CreateAndMapAnonymous(buf_size_);
+}
+
+bool ResourceBuffer::IsInitialized() const {
+  return shared_mem_.memory() != NULL;
+}
+
+bool ResourceBuffer::ShareToProcess(
+    base::ProcessHandle process_handle,
+    base::SharedMemoryHandle* shared_memory_handle,
+    int* shared_memory_size) {
+  DCHECK(IsInitialized());
+
+  if (!shared_mem_.ShareToProcess(process_handle, shared_memory_handle))
+    return false;
+
+  *shared_memory_size = buf_size_;
+  return true;
+}
+
+bool ResourceBuffer::CanAllocate() const {
+  DCHECK(IsInitialized());
+
+  if (alloc_start_ == -1)
+    return true;
+
+  int diff = alloc_end_ - alloc_start_;
+  if (diff > 0)
+    return (buf_size_ - diff) >= min_alloc_size_;
+
+  return -diff >= min_alloc_size_;
+}
+
+char* ResourceBuffer::Allocate(int* size) {
+  DCHECK(CanAllocate());
+
+  int alloc_offset = 0;
+  int alloc_size;
+
+  if (alloc_start_ == -1) {

[brettw, 2012/09/17 23:53:10]

The different cases in this nested if statement could use quick comments to aid
skimming it. Even something like "Buffer wraparound case." would be helpful.

+    alloc_start_ = 0;
+    alloc_end_ = buf_size_;
+    alloc_size = buf_size_;
+  } else if (alloc_start_ < alloc_end_) {
+    if ((buf_size_ - alloc_end_) >= min_alloc_size_) {
+      alloc_offset = alloc_end_;
+      alloc_size = buf_size_ - alloc_end_;
+      alloc_end_ = buf_size_;
+    } else {
+      DCHECK(alloc_start_ > 0);
+      alloc_size = alloc_start_;
+      alloc_end_ = alloc_start_;
+    }
+  } else {
+    DCHECK(alloc_end_ < alloc_start_);
+    alloc_offset = alloc_end_;
+    alloc_size = alloc_start_ - alloc_end_;
+    alloc_end_ = alloc_start_;
+  }
+
+  alloc_sizes_.push(alloc_size);

[brettw, 2012/09/17 23:53:10]

It took me a minute to realize why this goes before the fixup for
max_alloc_size_ below (it looked like a bug). A comment about this would be
helpful.

[darin (slow to review), 2012/09/18 05:00:50]

Done.

+
+  if (alloc_size > max_alloc_size_) {
+    alloc_size = max_alloc_size_;
+    ShrinkLastAllocation(alloc_size);
+  }
+
+  *size = alloc_size;
+  return static_cast<char*>(shared_mem_.memory()) + alloc_offset;
+}
+
+int ResourceBuffer::GetLastAllocationOffset() const {
+  DCHECK(!alloc_sizes_.empty());
+  DCHECK(alloc_end_ >= alloc_sizes_.back());
+  return alloc_end_ - alloc_sizes_.back();
+}
+
+void ResourceBuffer::ShrinkLastAllocation(int new_size) {
+  DCHECK(!alloc_sizes_.empty());
+
+  int aligned_size = (new_size / min_alloc_size_) * min_alloc_size_;
+  if (aligned_size < new_size)
+    aligned_size += min_alloc_size_;
+
+  DCHECK_LE(new_size, aligned_size);
+  DCHECK_GE(alloc_sizes_.back(), aligned_size);
+
+  int* last_allocation_size = &alloc_sizes_.back();
+  alloc_end_ -= (*last_allocation_size - aligned_size);
+  *last_allocation_size = aligned_size;
+}
+
+void ResourceBuffer::RecycleLeastRecentlyAllocated() {
+  DCHECK(!alloc_sizes_.empty());
+  int allocation_size = alloc_sizes_.front();
+  alloc_sizes_.pop();
+
+  alloc_start_ += allocation_size;
+  DCHECK(alloc_start_ <= buf_size_);
+
+  if (alloc_start_ == alloc_end_) {
+    DCHECK(alloc_sizes_.empty());
+    alloc_start_ = -1;
+    alloc_end_ = -1;
+  } else if (alloc_start_ == buf_size_) {
+    DCHECK(!alloc_sizes_.empty());
+    alloc_start_ = 0;
+  }
+}
+
+}  // namespace content