Lazily initialize MessageLoop for faster thread startup

base/message_loop/incoming_task_queue.cc

 // 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.
 
 #include "base/message_loop/incoming_task_queue.h"
 
 #include <limits>
 
 #include "base/location.h"
 #include "base/message_loop/message_loop.h"
@@ skipping 91 matching lines @@
   int high_res_tasks = high_res_task_count_;
   high_res_task_count_ = 0;
   return high_res_tasks;
 }
 
 void IncomingTaskQueue::WillDestroyCurrentMessageLoop() {
   AutoLock lock(incoming_queue_lock_);
   message_loop_ = NULL;
 }
 
+void IncomingTaskQueue::StartScheduling() {
+  AutoLock lock(incoming_queue_lock_);
+  DCHECK(!is_ready_for_scheduling_);

[Nico, 2015/04/24 21:31:15]

DCHECK(!message_loop_scheduled_) (…right?)

[kinuko, 2015/04/27 16:36:04]

Done.

+  is_ready_for_scheduling_ = true;

[danakj, 2015/04/24 20:54:39]

Can you DCHECK !message_loop_scheduled_ here?

[kinuko, 2015/04/27 16:36:04]

Done.

+  if (!incoming_queue_.empty()) {
+    message_loop_->ScheduleWork();
+    message_loop_scheduled_ = true;
+  }
+}
+
+bool IncomingTaskQueue::empty() {
+  AutoLock lock(incoming_queue_lock_);
+  return incoming_queue_.empty();
+}
+
 IncomingTaskQueue::~IncomingTaskQueue() {
   // Verify that WillDestroyCurrentMessageLoop() has been called.
   DCHECK(!message_loop_);
 }
 
 TimeTicks IncomingTaskQueue::CalculateDelayedRuntime(TimeDelta delay) {
   TimeTicks delayed_run_time;
   if (delay > TimeDelta())
     delayed_run_time = TimeTicks::Now() + delay;
   else
@@ skipping 19 matching lines @@
   // delayed_run_time value) and for identifying the task in about:tracing.
   pending_task->sequence_num = next_sequence_num_++;
 
   message_loop_->task_annotator()->DidQueueTask("MessageLoop::PostTask",
                                                 *pending_task);
 
   bool was_empty = incoming_queue_.empty();
   incoming_queue_.push(*pending_task);
   pending_task->task.Reset();
 
-  if (always_schedule_work_ || (!message_loop_scheduled_ && was_empty)) {
+  if (is_ready_for_scheduling_ &&
+      (always_schedule_work_ || (!message_loop_scheduled_ && was_empty))) {
     // Wake up the message loop.

[danakj, 2015/04/24 20:54:39]

Can you break this into a helper method so you can share the code (and more
especially the comments)?

[kinuko, 2015/04/27 16:36:04]

Done.

     message_loop_->ScheduleWork();
     // After we've scheduled the message loop, we do not need to do so again
     // until we know it has processed all of the work in our queue and is
     // waiting for more work again. The message loop will always attempt to
     // reload from the incoming queue before waiting again so we clear this flag
     // in ReloadWorkQueue().
     message_loop_scheduled_ = true;
   }
 
   return true;
 }
 
 }  // namespace internal
 }  // namespace base