gecko-dev/xpcom/threads/TaskDispatcher.h
Nicholas Nethercote 779cd4aa7b Bug 1241656 - Lazify AutoTaskDispatcher::mDirectTasks. r=bholley.
This avoids large amounts of heap churn while watching YouTube videos on Mac
and Linux.

--HG--
extra : rebase_source : 8606947287574826c455c25336c1a42d77ce6dcc
2016-01-25 13:30:42 +11:00

276 lines
9.1 KiB
C++

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim:set ts=2 sw=2 sts=2 et cindent: */
/* 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/. */
#if !defined(TaskDispatcher_h_)
#define TaskDispatcher_h_
#include "mozilla/AbstractThread.h"
#include "mozilla/UniquePtr.h"
#include "mozilla/unused.h"
#include "nsISupportsImpl.h"
#include "nsTArray.h"
#include "nsThreadUtils.h"
#include <queue>
namespace mozilla {
/*
* A classic approach to cross-thread communication is to dispatch asynchronous
* runnables to perform updates on other threads. This generally works well, but
* there are sometimes reasons why we might want to delay the actual dispatch of
* these tasks until a specified moment. At present, this is primarily useful to
* ensure that mirrored state gets updated atomically - but there may be other
* applications as well.
*
* TaskDispatcher is a general abstract class that accepts tasks and dispatches
* them at some later point. These groups of tasks are per-target-thread, and
* contain separate queues for several kinds of tasks (see comments below). - "state change tasks" (which
* run first, and are intended to be used to update the value held by mirrors),
* and regular tasks, which are other arbitrary operations that the are gated
* to run after all the state changes have completed.
*/
class TaskDispatcher
{
public:
TaskDispatcher() {}
virtual ~TaskDispatcher() {}
// Direct tasks are run directly (rather than dispatched asynchronously) when
// the tail dispatcher fires. A direct task may cause other tasks to be added
// to the tail dispatcher.
virtual void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) = 0;
// State change tasks are dispatched asynchronously always run before regular
// tasks. They are intended to be used to update the value held by mirrors
// before any other dispatched tasks are run on the target thread.
virtual void AddStateChangeTask(AbstractThread* aThread,
already_AddRefed<nsIRunnable> aRunnable) = 0;
// Regular tasks are dispatched asynchronously, and run after state change
// tasks.
virtual void AddTask(AbstractThread* aThread,
already_AddRefed<nsIRunnable> aRunnable,
AbstractThread::DispatchFailureHandling aFailureHandling = AbstractThread::AssertDispatchSuccess) = 0;
virtual void DispatchTasksFor(AbstractThread* aThread) = 0;
virtual bool HasTasksFor(AbstractThread* aThread) = 0;
virtual void DrainDirectTasks() = 0;
};
/*
* AutoTaskDispatcher is a stack-scoped TaskDispatcher implementation that fires
* its queued tasks when it is popped off the stack.
*/
class AutoTaskDispatcher : public TaskDispatcher
{
public:
explicit AutoTaskDispatcher(bool aIsTailDispatcher = false)
: mIsTailDispatcher(aIsTailDispatcher)
{}
~AutoTaskDispatcher()
{
// Given that direct tasks may trigger other code that uses the tail
// dispatcher, it's better to avoid processing them in the tail dispatcher's
// destructor. So we require TailDispatchers to manually invoke
// DrainDirectTasks before the AutoTaskDispatcher gets destroyed. In truth,
// this is only necessary in the case where this AutoTaskDispatcher can be
// accessed by the direct tasks it dispatches (true for TailDispatchers, but
// potentially not true for other hypothetical AutoTaskDispatchers). Feel
// free to loosen this restriction to apply only to mIsTailDispatcher if a
// use-case requires it.
MOZ_ASSERT(!HaveDirectTasks());
for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
DispatchTaskGroup(Move(mTaskGroups[i]));
}
}
bool HaveDirectTasks() const
{
return mDirectTasks.isSome() && !mDirectTasks->empty();
}
void DrainDirectTasks() override
{
while (HaveDirectTasks()) {
nsCOMPtr<nsIRunnable> r = mDirectTasks->front();
mDirectTasks->pop();
r->Run();
}
}
void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) override
{
if (mDirectTasks.isNothing()) {
mDirectTasks.emplace();
}
mDirectTasks->push(Move(aRunnable));
}
void AddStateChangeTask(AbstractThread* aThread,
already_AddRefed<nsIRunnable> aRunnable) override
{
EnsureTaskGroup(aThread).mStateChangeTasks.AppendElement(aRunnable);
}
void AddTask(AbstractThread* aThread,
already_AddRefed<nsIRunnable> aRunnable,
AbstractThread::DispatchFailureHandling aFailureHandling) override
{
PerThreadTaskGroup& group = EnsureTaskGroup(aThread);
group.mRegularTasks.AppendElement(aRunnable);
// The task group needs to assert dispatch success if any of the runnables
// it's dispatching want to assert it.
if (aFailureHandling == AbstractThread::AssertDispatchSuccess) {
group.mFailureHandling = AbstractThread::AssertDispatchSuccess;
}
}
bool HasTasksFor(AbstractThread* aThread) override
{
return !!GetTaskGroup(aThread) ||
(aThread == AbstractThread::GetCurrent() && HaveDirectTasks());
}
void DispatchTasksFor(AbstractThread* aThread) override
{
for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
if (mTaskGroups[i]->mThread == aThread) {
DispatchTaskGroup(Move(mTaskGroups[i]));
mTaskGroups.RemoveElementAt(i);
return;
}
}
}
private:
struct PerThreadTaskGroup
{
public:
explicit PerThreadTaskGroup(AbstractThread* aThread)
: mThread(aThread), mFailureHandling(AbstractThread::DontAssertDispatchSuccess)
{
MOZ_COUNT_CTOR(PerThreadTaskGroup);
}
~PerThreadTaskGroup() { MOZ_COUNT_DTOR(PerThreadTaskGroup); }
RefPtr<AbstractThread> mThread;
nsTArray<nsCOMPtr<nsIRunnable>> mStateChangeTasks;
nsTArray<nsCOMPtr<nsIRunnable>> mRegularTasks;
AbstractThread::DispatchFailureHandling mFailureHandling;
};
class TaskGroupRunnable : public nsRunnable
{
public:
explicit TaskGroupRunnable(UniquePtr<PerThreadTaskGroup>&& aTasks) : mTasks(Move(aTasks)) {}
NS_IMETHODIMP Run()
{
// State change tasks get run all together before any code is run, so
// that all state changes are made in an atomic unit.
for (size_t i = 0; i < mTasks->mStateChangeTasks.Length(); ++i) {
mTasks->mStateChangeTasks[i]->Run();
}
// Once the state changes have completed, drain any direct tasks
// generated by those state changes (i.e. watcher notification tasks).
// This needs to be outside the loop because we don't want to run code
// that might observe intermediate states.
MaybeDrainDirectTasks();
for (size_t i = 0; i < mTasks->mRegularTasks.Length(); ++i) {
mTasks->mRegularTasks[i]->Run();
// Scope direct tasks tightly to the task that generated them.
MaybeDrainDirectTasks();
}
return NS_OK;
}
private:
void MaybeDrainDirectTasks()
{
AbstractThread* currentThread = AbstractThread::GetCurrent();
if (currentThread) {
currentThread->TailDispatcher().DrainDirectTasks();
}
}
UniquePtr<PerThreadTaskGroup> mTasks;
};
PerThreadTaskGroup& EnsureTaskGroup(AbstractThread* aThread)
{
PerThreadTaskGroup* existing = GetTaskGroup(aThread);
if (existing) {
return *existing;
}
mTaskGroups.AppendElement(new PerThreadTaskGroup(aThread));
return *mTaskGroups.LastElement();
}
PerThreadTaskGroup* GetTaskGroup(AbstractThread* aThread)
{
for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
if (mTaskGroups[i]->mThread == aThread) {
return mTaskGroups[i].get();
}
}
// Not found.
return nullptr;
}
void DispatchTaskGroup(UniquePtr<PerThreadTaskGroup> aGroup)
{
RefPtr<AbstractThread> thread = aGroup->mThread;
AbstractThread::DispatchFailureHandling failureHandling = aGroup->mFailureHandling;
AbstractThread::DispatchReason reason = mIsTailDispatcher ? AbstractThread::TailDispatch
: AbstractThread::NormalDispatch;
nsCOMPtr<nsIRunnable> r = new TaskGroupRunnable(Move(aGroup));
thread->Dispatch(r.forget(), failureHandling, reason);
}
// Direct tasks. We use a Maybe<> because (a) this class is hot, (b)
// mDirectTasks often doesn't get anything put into it, and (c) the
// std::queue implementation in GNU libstdc++ does two largish heap
// allocations when creating a new std::queue.
mozilla::Maybe<std::queue<nsCOMPtr<nsIRunnable>>> mDirectTasks;
// Task groups, organized by thread.
nsTArray<UniquePtr<PerThreadTaskGroup>> mTaskGroups;
// True if this TaskDispatcher represents the tail dispatcher for the thread
// upon which it runs.
const bool mIsTailDispatcher;
};
// Little utility class to allow declaring AutoTaskDispatcher as a default
// parameter for methods that take a TaskDispatcher&.
template<typename T>
class PassByRef
{
public:
PassByRef() {}
operator T&() { return mVal; }
private:
T mVal;
};
} // namespace mozilla
#endif