Prevent redundant DoWorks due to canceled delayed tasks (v2)

third_party/WebKit/Source/platform/scheduler/base/time_domain.h

 // Copyright 2015 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 THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_TIME_DOMAIN_H_
 #define THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_TIME_DOMAIN_H_
 
 #include <map>
 
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/time/time.h"
 #include "platform/scheduler/base/lazy_now.h"
 #include "platform/scheduler/base/task_queue_impl.h"
 
 namespace blink {
 namespace scheduler {
 namespace internal {
 class TaskQueueImpl;
 }  // internal
 class TaskQueueManager;
 class TaskQueueManagerDelegate;
 
-// The TimeDomain's job is to keep track of moments when delayed tasks have been
-// scheduled to fire and to notify their TaskQueues via UpdateDelayedWorkQueue.
+// The TimeDomain's job is to wake task queues up when their next delayed tasks
+// are due to fire. TaskQueues request a wake up via ScheduleDelayedWork, when
+// the wake up is due the TimeDomain calls TaskQueue::WakeUpForDelayedWork.
+// The TimeDomain communicates with the TaskQueueManager to actually schedule
+// the wake-ups on the underlying base::MessageLoop. Various levels of de-duping
+// are employed to prevent unnecessary posting of TaskQueueManager::DoWork.
 //
-// The time domain keeps track of the next wakeup required to pump delayed tasks
-// and issues |RequestWakeup| calls to the subclass as needed.  Where possible
-// it tried to de-dupe these wakeups. Ideally it would be possible to cancel
-// them, but that's not currently supported by the base message loop.
-//
-// The clock itself is provided by subclasses of the TimeDomain and it may be
-// the real wall clock or a synthetic (virtual) time base.
+// Note the TimeDomain only knows about the first wakeup per queue, it's the
+// responsibility of TaskQueueImpl to keep the time domain up to date if this
+// changes.
 class BLINK_PLATFORM_EXPORT TimeDomain {
  public:
   class BLINK_PLATFORM_EXPORT Observer {
    public:
     virtual ~Observer() {}
 
     // Called when an empty TaskQueue registered with this TimeDomain has a task
     // enqueued.
     virtual void OnTimeDomainHasImmediateWork() = 0;
 
@@ skipping 35 matching lines @@
                     TimeDomain* destination_time_domain);
 
   // If there is a scheduled delayed task, |out_task_queue| is set to the queue
   // the next task was posted to and it returns true.  Returns false otherwise.
   bool NextScheduledTaskQueue(TaskQueue** out_task_queue) const;
 
   // Adds |queue| to the set of task queues that UpdateWorkQueues calls
   // UpdateWorkQueue on.
   void RegisterAsUpdatableTaskQueue(internal::TaskQueueImpl* queue);
 
-  // Schedules a call to TaskQueueImpl::MoveReadyDelayedTasksToDelayedWorkQueue
-  // when this TimeDomain reaches |delayed_run_time|.
+  // Schedules a call to TaskQueueImpl::WakeUpForDelayedWork when this
+  // TimeDomain reaches |delayed_run_time|.  This supersedes any previously
+  // registered wakeup for |queue|.
   void ScheduleDelayedWork(internal::TaskQueueImpl* queue,
                            base::TimeTicks delayed_run_time,
                            base::TimeTicks now);
 
-  // Cancels a call to TaskQueueImpl::MoveReadyDelayedTasksToDelayedWorkQueue
-  // previously requested with ScheduleDelayedWork.  Note this only works if
-  // delayed_run_time is _not_ the next scheduled run time.
-  void CancelDelayedWork(internal::TaskQueueImpl* queue,
-                         base::TimeTicks delayed_run_time);
-
   // Registers the |queue|.
   void RegisterQueue(internal::TaskQueueImpl* queue);
 
   // Removes |queue| from the set of task queues that UpdateWorkQueues calls
   // UpdateWorkQueue on. Returns true if |queue| was updatable.
   bool UnregisterAsUpdatableTaskQueue(internal::TaskQueueImpl* queue);
 
   // Removes |queue| from all internal data structures.
   void UnregisterQueue(internal::TaskQueueImpl* queue);
 
@@ skipping 12 matching lines @@
   virtual void RequestWakeup(base::TimeTicks now, base::TimeDelta delay) = 0;
 
   // For implementation specific tracing.
   virtual void AsValueIntoInternal(
       base::trace_event::TracedValue* state) const = 0;
 
   // Call TaskQueueImpl::UpdateDelayedWorkQueue for each queue where the delay
   // has elapsed.
   void WakeupReadyDelayedQueues(LazyNow* lazy_now);
 
- protected:
-  // Clears expired entries from |delayed_wakeup_multimap_|. Caution needs to be
-  // taken to ensure TaskQueueImpl::UpdateDelayedWorkQueue is called on the
-  // affected queues.
-  void ClearExpiredWakeups();
-
  private:
   void MoveNewlyUpdatableQueuesIntoUpdatableQueueSet();
 
-  typedef std::multimap<base::TimeTicks, internal::TaskQueueImpl*>
-      DelayedWakeupMultimap;
+  using DelayedWakeupMultimap =
+      std::multimap<base::TimeTicks, internal::TaskQueueImpl*>;
 
   DelayedWakeupMultimap delayed_wakeup_multimap_;
 
+  // This map makes it easy to remove a queue from |delayed_wakeup_multimap_|.
+  // NOTE inserting or removing elements from a std::map does not invalidate any
+  // iterators.
+  using QueueToDelayedWakeupMultimapIteratorMap =
+      std::map<internal::TaskQueueImpl*, DelayedWakeupMultimap::iterator>;

[altimin, 2016/09/23 10:04:15]

nit: unordered_map?

[alex clarke (OOO till 29th), 2016/09/23 11:08:04]

Yeah probably a small win, I expect boost::flat_map would work better here once
that's allowed.

+
+  QueueToDelayedWakeupMultimapIteratorMap
+      queue_to_delayed_wakeup_multimap_iterator_map_;
+
   // This lock guards only |newly_updatable_|.  It's not expected to be heavily
   // contended.
   base::Lock newly_updatable_lock_;
   std::vector<internal::TaskQueueImpl*> newly_updatable_;
 
   // Set of task queues with avaliable work on the incoming queue.  This should
   // only be accessed from the main thread.
   std::set<internal::TaskQueueImpl*> updatable_queue_set_;
 
   Observer* observer_;  // NOT OWNED.
 
   base::ThreadChecker main_thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(TimeDomain);
 };
 
 }  // namespace scheduler
 }  // namespace blink
 
 #endif  // THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_TIME_DOMAIN_H_