gecko-dev/xpcom/threads/PrioritizedEventQueue.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef mozilla_PrioritizedEventQueue_h
#define mozilla_PrioritizedEventQueue_h

#include "mozilla/AbstractEventQueue.h"
#include "mozilla/TimeStamp.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/UniquePtr.h"
#include "nsCOMPtr.h"
#include "nsIIdlePeriod.h"

class nsIRunnable;

namespace mozilla {

// This AbstractEventQueue implementation has one queue for each EventPriority.
// The type of queue used for each priority is determined by the template
// parameter.
//
// When an event is pushed, its priority is determined by QIing the runnable to
// nsIRunnablePriority, or by falling back to the aPriority parameter if the QI
// fails.
//
// When an event is popped, a queue is selected based on heuristics that
// optimize for performance. Roughly, events are selected from the highest
// priority queue that is non-empty. However, there are a few exceptions:
// - We try to avoid processing too many high-priority events in a row so
//   that the normal priority queue is not starved. When there are high-
//   and normal-priority events available, we interleave popping from the
//   normal and high queues.
// - We do not select events from the idle queue if the current idle period
//   is almost over.
template<class InnerQueueT>
class PrioritizedEventQueue final : public AbstractEventQueue
{
public:
  PrioritizedEventQueue(UniquePtr<InnerQueueT> aHighQueue,
                        UniquePtr<InnerQueueT> aInputQueue,
                        UniquePtr<InnerQueueT> aNormalQueue,
                        UniquePtr<InnerQueueT> aIdleQueue,
                        already_AddRefed<nsIIdlePeriod> aIdlePeriod);

  void PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
                EventPriority aPriority,
                const MutexAutoLock& aProofOfLock) final;
  already_AddRefed<nsIRunnable> GetEvent(EventPriority* aPriority,
                                         const MutexAutoLock& aProofOfLock) final;
  bool HasPendingEvent(const MutexAutoLock& aProofOfLock) final;
  size_t Count(const MutexAutoLock& aProofOfLock) const final;

  // When checking the idle deadline, we need to drop whatever mutex protects
  // this queue. This method allows that mutex to be stored so that we can drop
  // it and reacquire it when checking the idle deadline. The mutex must live at
  // least as long as the queue.
  void SetMutexRef(Mutex& aMutex) { mMutex = &aMutex; }

  // nsThread.cpp sends telemetry containing the most recently computed idle
  // deadline. We store a reference to a field in nsThread where this deadline
  // will be stored so that it can be fetched quickly for telemetry.
  void SetNextIdleDeadlineRef(TimeStamp& aDeadline) { mNextIdleDeadline = &aDeadline; }

  void EnableInputEventPrioritization(const MutexAutoLock& aProofOfLock) final;

private:
  class EnablePrioritizationRunnable;

  // Returns a null TimeStamp if we're not in the idle period.
  mozilla::TimeStamp GetIdleDeadline();

  UniquePtr<InnerQueueT> mHighQueue;
  UniquePtr<InnerQueueT> mInputQueue;
  UniquePtr<InnerQueueT> mNormalQueue;
  UniquePtr<InnerQueueT> mIdleQueue;

  // We need to drop the queue mutex when checking the idle deadline, so we keep
  // a pointer to it here.
  Mutex* mMutex = nullptr;

  // Pointer to a place where the most recently computed idle deadline is
  // stored.
  TimeStamp* mNextIdleDeadline = nullptr;

  // Try to process one high priority runnable after each normal
  // priority runnable. This gives the processing model HTML spec has for
  // 'Update the rendering' in the case only vsync messages are in the
  // secondary queue and prevents starving the normal queue.
  bool mProcessHighPriorityQueue = false;

  // mIdlePeriod keeps track of the current idle period. If at any
  // time the main event queue is empty, calling
  // mIdlePeriod->GetIdlePeriodHint() will give an estimate of when
  // the current idle period will end.
  nsCOMPtr<nsIIdlePeriod> mIdlePeriod;

  // Set to true if HasPendingEvents() has been called and returned true because
  // of a pending idle event.  This is used to remember to return that idle
  // event from GetIdleEvent() to ensure that HasPendingEvents() never lies.
  bool mHasPendingEventsPromisedIdleEvent = false;

  TimeStamp mInputHandlingStartTime;

  // When we enable input event prioritization, we immediately begin adding new
  // input events to the input event queue (and set
  // mWriteToInputQueue). However, we do not begin processing events from the
  // input queue until all events that were in the normal priority queue have
  // been processed. That ensures that input events will not jump ahead of
  // events that were in the queue before prioritization was enabled.
  bool mWriteToInputQueue = false;
  bool mReadFromInputQueue = false;
};

class EventQueue;
extern template class PrioritizedEventQueue<EventQueue>;

} // namespace mozilla

#endif // mozilla_PrioritizedEventQueue_h