[mojo-edk] Trim read buffer to keep its size down

mojo/edk/system/channel.cc

 // Copyright 2016 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 "mojo/edk/system/channel.h"
 
 #include <string.h>
 
 #include <algorithm>
 #include <limits>
@@ skipping 16 matching lines @@
     "Invalid Header size.");
 
 #if defined(OS_CHROMEOS) || defined(OS_ANDROID)
 static_assert(sizeof(Channel::Message::Header) == 8,
               "Header must be 8 bytes on ChromeOS and Android");
 #endif
 
 }  // namespace
 
 const size_t kReadBufferSize = 4096;
-const size_t kMaxUnusedReadBufferCapacity = 256 * 1024;
+const size_t kMaxUnusedReadBufferCapacity = 64 * 1024;
 const size_t kMaxChannelMessageSize = 256 * 1024 * 1024;
 const size_t kMaxAttachedHandles = 128;
 
 Channel::Message::Message(size_t payload_size,
                           size_t max_handles,
                           Header::MessageType message_type)
     : max_handles_(max_handles) {
   DCHECK_LE(max_handles_, kMaxAttachedHandles);
 
   size_t extra_header_size = 0;
@@ skipping 353 matching lines @@
       size_ = std::max(num_preserved_bytes, kReadBufferSize);
       char* new_data = static_cast<char*>(
           base::AlignedAlloc(size_, kChannelMessageAlignment));
       memcpy(new_data, data_ + num_discarded_bytes_, num_preserved_bytes);
       base::AlignedFree(data_);
       data_ = new_data;
       num_discarded_bytes_ = 0;
       num_occupied_bytes_ = num_preserved_bytes;
     }
 
-    // TODO: we should also adaptively shrink the buffer in case of the
-    // occasional abnormally large read.
+    if (num_occupied_bytes_ == 0 && size_ > kMaxUnusedReadBufferCapacity) {
+      // Opportunistically shrink the read buffer back down to a small size if
+      // it's grown very large. We only do this if there are no remaining
+      // unconsumed bytes in the buffer to avoid copies in most the common
+      // cases.
+      size_ = kMaxUnusedReadBufferCapacity;
+      base::AlignedFree(data_);
+      data_ = static_cast<char*>(
+          base::AlignedAlloc(size_, kChannelMessageAlignment));
+    }
   }
 
  private:
   char* data_ = nullptr;
 
   // The total size of the allocated buffer.
   size_t size_ = 0;
 
   // The number of discarded bytes at the beginning of the allocated buffer.
   size_t num_discarded_bytes_ = 0;
@@ skipping 92 matching lines @@
   return true;
 }
 
 void Channel::OnError() {
   if (delegate_)
     delegate_->OnChannelError();
 }
 
 }  // namespace edk
 }  // namespace mojo