Let QUIC streams write 16k before ceding. Behind FLAG_quic_batch_writes.

net/quic/quic_write_blocked_list.h

 // Copyright 2014 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.
 //
 #ifndef NET_QUIC_QUIC_WRITE_BLOCKED_LIST_H_
 #define NET_QUIC_QUIC_WRITE_BLOCKED_LIST_H_
 
 #include <set>
 
 #include "net/base/net_export.h"
+#include "net/quic/quic_flags.h"
 #include "net/quic/quic_protocol.h"
 #include "net/spdy/write_blocked_list.h"
 
 namespace net {
 
 // Keeps tracks of the QUIC streams that have data to write, sorted by
 // priority.  QUIC stream priority order is:
 // Crypto stream > Headers stream > Data streams by requested priority.
 class NET_EXPORT_PRIVATE QuicWriteBlockedList {
  private:
   typedef WriteBlockedList<QuicStreamId> QuicWriteBlockedListBase;
 
  public:
-  static const QuicPriority kHighestPriority;
-  static const QuicPriority kLowestPriority;
+   static const QuicPriority kHighestPriority =
+      static_cast<QuicPriority>(net::kHighestPriority);
+   static const QuicPriority kLowestPriority =
+      static_cast<QuicPriority>(net::kLowestPriority);
 
   QuicWriteBlockedList();
   ~QuicWriteBlockedList();
 
   bool HasWriteBlockedDataStreams() const {
     return base_write_blocked_list_.HasWriteBlockedStreams();
   }
 
   bool HasWriteBlockedCryptoOrHeadersStream() const {
     return crypto_stream_blocked_ || headers_stream_blocked_;
   }
 
   size_t NumBlockedStreams() const {
     size_t num_blocked = base_write_blocked_list_.NumBlockedStreams();
     if (crypto_stream_blocked_) {
       ++num_blocked;
     }
     if (headers_stream_blocked_) {
       ++num_blocked;
     }
 
     return num_blocked;
   }
 
+  // Pops the highest priorty stream, special casing crypto and headers streams.
+  // Latches the most recently popped data stream for batch writing purposes.
   QuicStreamId PopFront() {
     if (crypto_stream_blocked_) {
       crypto_stream_blocked_ = false;
       return kCryptoStreamId;
     }
 
     if (headers_stream_blocked_) {
       headers_stream_blocked_ = false;
       return kHeadersStreamId;
     }
 
     SpdyPriority priority =
         base_write_blocked_list_.GetHighestPriorityWriteBlockedList();
     QuicStreamId id = base_write_blocked_list_.PopFront(priority);
+
+    if (base_write_blocked_list_.NumBlockedStreams(priority) == 0) {
+      // If no streams are blocked, don't bother latching.  This stream will be
+      // the first popped for its priority anyway.
+      batch_write_stream_id_[priority] = 0;
+    } else if (batch_write_stream_id_[priority] != id) {
+      // If newly latching this batch write stream, let it write 16k.
+      batch_write_stream_id_[priority] = id;
+      bytes_left_for_batch_write_[priority] = 16000;
+      last_priority_popped_ = priority;
+    }
+
     return id;
   }
 
-  void PushBack(QuicStreamId stream_id, QuicPriority priority) {
+  void UpdateBytesForStream(QuicStreamId stream_id, size_t bytes) {
+    if (batch_write_stream_id_[last_priority_popped_] == stream_id) {
+      // If this was the last data stream popped by PopFront, update the
+      // bytes remaining in its batch write.
+      bytes_left_for_batch_write_[last_priority_popped_] -= bytes;
+    } else {
+      // If a batch write stream was set, it should only be preempted by the
+      // crypto or headers streams.  Any higher priority data stream would
+      // *become* the new batch write stream.
+      if (FLAGS_respect_send_alarm &&
+          FLAGS_quic_batch_writes) {
+        DCHECK(stream_id == kCryptoStreamId || stream_id == kHeadersStreamId ||
+               batch_write_stream_id_[last_priority_popped_] == 0 ||
+               bytes == 0);
+      }
+    }
+  }
+
+  // Pushes a stream to the back of the std::list for this priority level *unless* it
+  // is latched for doing batched writes in which case it goes to the front of
+  // the std::list for this priority level.
+  // Headers and crypto streams are special cased to always resume first.
+  void AddStream(QuicStreamId stream_id, QuicPriority priority) {
     if (stream_id == kCryptoStreamId) {
       DCHECK_EQ(kHighestPriority, priority);
       // TODO(avd) Add DCHECK(!crypto_stream_blocked_)
       crypto_stream_blocked_ = true;
       return;
     }
 
     if (stream_id == kHeadersStreamId) {
       DCHECK_EQ(kHighestPriority, priority);
       // TODO(avd) Add DCHECK(!headers_stream_blocked_);
       headers_stream_blocked_ = true;
       return;
     }
-
-    base_write_blocked_list_.PushBack(
-        stream_id, static_cast<SpdyPriority>(priority));
+    if (FLAGS_quic_batch_writes &&
+        stream_id == batch_write_stream_id_[last_priority_popped_] &&
+        bytes_left_for_batch_write_[last_priority_popped_] > 0) {
+      // If the batch write stream has more data to write, push it to the front
+      // for its priority level.
+      base_write_blocked_list_.PushFront(
+          stream_id, static_cast<SpdyPriority>(priority));
+    } else {
+      base_write_blocked_list_.PushBack(
+          stream_id, static_cast<SpdyPriority>(priority));
+    }
     return;
   }
 
   bool crypto_stream_blocked() const { return crypto_stream_blocked_; }
   bool headers_stream_blocked() const { return headers_stream_blocked_; }
 
  private:
   QuicWriteBlockedListBase base_write_blocked_list_;
+
+  // If performing batch writes, this will be the stream ID of the stream doing
+  // batch writes for this priority level.  We will allow this stream to write
+  // until it has written kBatchWriteSize bytes, it has no more data to write,
+  // or a higher priority stream preempts.
+  QuicStreamId batch_write_stream_id_[kLowestPriority + 1];
+  // Set to kBatchWriteSize when we set a new batch_write_stream_id_ for a given
+  // priority.  This is decremented with each write the stream does until it is
+  // done with its batch write.
+  int32 bytes_left_for_batch_write_[kLowestPriority + 1];
+  // Tracks the last priority popped for UpdateBytesForStream.
+  QuicPriority last_priority_popped_;
+
   bool crypto_stream_blocked_;
   bool headers_stream_blocked_;
 
   DISALLOW_COPY_AND_ASSIGN(QuicWriteBlockedList);
 };
 
 }  // namespace net
 
 
 #endif  // NET_QUIC_QUIC_WRITE_BLOCKED_LIST_H_