Every Chrome process has
Most threads have a loop that gets tasks from a queue and runs them (the queue may be shared between multiple threads).
A task is a base::OnceClosure added to a queue for asynchronous execution.
A base::OnceClosure stores a function pointer and arguments. It has a Run() method that invokes the function pointer using the bound arguments. It is created using base::BindOnce. (ref. Callback<> and Bind() documentation).
void TaskA() {}
void TaskB(int v) {}
auto task_a = base::BindOnce(&TaskA);
auto task_b = base::BindOnce(&TaskB, 42);
A group of tasks can be executed in one of the following ways:
The discussion below covers all of these ways to execute tasks.
A task that can run on any thread and doesn’t have ordering or mutual exclusion requirements with other tasks should be posted using one of the base::PostTask*() functions defined in base/task_scheduler/post_task.h.
base::PostTask(FROM_HERE, base::BindOnce(&Task));
This posts tasks with default traits.
The base::PostTask*WithTraits() functions allow the caller to provide additional details about the task via TaskTraits (ref. Annotating Tasks with TaskTraits).
base::PostTaskWithTraits( FROM_HERE, {base::TaskPriority::BACKGROUND, MayBlock()}, base::BindOnce(&Task));
A parallel TaskRunner is an alternative to calling base::PostTask*() directly. This is mainly useful when it isn’t known in advance whether tasks will be posted in parallel, in sequence, or to a single-thread (ref. Posting a Sequenced Task, Posting Multiple Tasks to the Same Thread). Since TaskRunner is the base class of SequencedTaskRunner and SingleThreadTaskRunner, a scoped_refptr<TaskRunner> member can hold a TaskRunner, a SequencedTaskRunner or a SingleThreadTaskRunner.
class A { public: A() = default; void set_task_runner_for_testing( scoped_refptr<base::TaskRunner> task_runner) { task_runner_ = std::move(task_runner); } void DoSomething() { // In production, A is always posted in parallel. In test, it is posted to // the TaskRunner provided via set_task_runner_for_testing(). task_runner_->PostTask(FROM_HERE, base::BindOnce(&A)); } private: scoped_refptr<base::TaskRunner> task_runner_ = base::CreateTaskRunnerWithTraits({base::TaskPriority::USER_VISIBLE}); };
Unless a test needs to control precisely how tasks are executed, it is preferred to call base::PostTask*() directly (ref. Testing for less invasive ways of controlling tasks in tests).
A sequence is a set of tasks that run one at a time in posting order (not necessarily on the same thread). To post tasks as part of a sequence, use a SequencedTaskRunner.
A SequencedTaskRunner can be created by base::CreateSequencedTaskRunnerWithTraits().
scoped_refptr<SequencedTaskRunner> sequenced_task_runner = base::CreateSequencedTaskRunnerWithTraits(...); // TaskB runs after TaskA completes. sequenced_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskA)); sequenced_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskB));
The SequencedTaskRunner to which the current task was posted can be obtained via SequencedTaskRunnerHandle::Get().
SequencedTaskRunnerHandle::Get() from a parallel task, but it is valid from a single-threaded task (a SingleThreadTaskRunner is a SequencedTaskRunner).// The task will run after any task that has already been posted // to the SequencedTaskRunner to which the current task was posted // (in particular, it will run after the current task completes). // It is also guaranteed that it won’t run concurrently with any // task posted to that SequencedTaskRunner. base::SequencedTaskRunnerHandle::Get()-> PostTask(FROM_HERE, base::BindOnce(&Task));
Usage of locks is discouraged in Chrome. Sequences inherently provide thread-safety. Prefer classes that are always accessed from the same sequence to managing your own thread-safety with locks.
class A { public: A() { // Do not require accesses to be on the creation sequence. sequence_checker_.DetachFromSequence(); } void AddValue(int v) { // Check that all accesses are on the same sequence. DCHECK(sequence_checker_.CalledOnValidSequence()); values_.push_back(v); } private: base::SequenceChecker sequence_checker_; // No lock required, because all accesses are on the // same sequence. std::vector<int> values_; }; A a; scoped_refptr<SequencedTaskRunner> task_runner_for_a = ...; task_runner->PostTask(FROM_HERE, base::BindOnce(&A::AddValue, base::Unretained(&a))); task_runner->PostTask(FROM_HERE, base::BindOnce(&A::AddValue, base::Unretained(&a))); // Access from a different sequence causes a DCHECK failure. scoped_refptr<SequencedTaskRunner> other_task_runner = ...; other_task_runner->PostTask(FROM_HERE, base::BindOnce(&A::AddValue, base::Unretained(&a)));
If multiple tasks need to run on the same thread, post them to a SingleThreadTaskRunner. All tasks posted to the same SingleThreadTaskRunner run on the same thread in posting order.
To post tasks to the main thread or to the IO thread, get the appropriate SingleThreadTaskRunner using content::BrowserThread::GetTaskRunnerForThread.
content::BrowserThread::GetTaskRunnerForThread(content::BrowserThread::UI) ->PostTask(FROM_HERE, ...); content::BrowserThread::GetTaskRunnerForThread(content::BrowserThread::IO) ->PostTask(FROM_HERE, ...);
The main thread and the IO thread are already super busy. Therefore, prefer posting to a general purpose thread when possible (ref. Posting a Parallel Task, Posting a Sequenced task). Good reasons to post to the main thread are to update the UI or access objects that are bound to it (e.g. Profile). A good reason to post to the IO thread is to access the internals of components that are bound to it (e.g. IPCs, network). Note: It is not necessary to have an explicit post task to the IO thread to send/receive an IPC or send/receive data on the network.
TODO
If multiple tasks need to run on the same thread and that thread doesn’t have to be the main thread or the IO thread, post them to a SingleThreadTaskRunner created by base::CreateSingleThreadTaskRunnerWithTraits.
scoped_refptr<SequencedTaskRunner> single_thread_task_runner = base::CreateSingleThreadTaskRunnerWithTraits(...); // TaskB runs after TaskA completes. Both tasks run on the same thread. single_thread_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskA)); single_thread_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskB));
base::ThreadChecker when it’s merely thread-unsafe and should use base::SequenceChecker), please consider fixing it instead of making things worse by also making your API thread-affine.SequencedTaskRunnerHandle::Get() instead of ThreadTaskRunnerHandle::Get() (ref. Posting to the Current Sequence). That will better document the requirements of the posted task. In a single-thread task, SequencedTaskRunnerHandle::Get() is equivalent to ThreadTaskRunnerHandle::Get().To post a task to the current thread, use ThreadTaskRunnerHandle.
// The task will run on the current thread in the future. base::ThreadTaskRunnerHandle::Get()->PostTask( FROM_HERE, base::BindOnce(&Task));
ThreadTaskRunnerHandle::Get() from a parallel or a sequenced task.Tasks that need to run on a COM Single-Thread Apartment (STA) thread must be posted to a SingleThreadTaskRunner returned by CreateCOMSTATaskRunnerWithTraits(). As mentioned in Posting Multiple Tasks to the Same Thread, all tasks posted to the same SingleThreadTaskRunner run on the same thread in posting order.
// Task(A|B|C)UsingCOMSTA will run on the same COM STA thread. void TaskAUsingCOMSTA() { // [ This runs on a COM STA thread. ] // Make COM STA calls. // ... // Post another task to the current COM STA thread. base::ThreadTaskRunnerHandle::Get()->PostTask( FROM_HERE, base::BindOnce(&TaskCUsingCOMSTA)); } void TaskBUsingCOMSTA() { } void TaskCUsingCOMSTA() { } auto com_sta_task_runner = base::CreateCOMSTATaskRunnerWithTraits(...); com_sta_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskAUsingCOMSTA)); com_sta_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskBUsingCOMSTA));
TaskTraits encapsulate information about a task that helps the task scheduler make better scheduling decisions.
All PostTask*() functions in base/task_scheduler/post_task.h have an overload that takes TaskTraits as argument and one that doesn’t. The overload that doesn’t take TaskTraits as argument is appropriate for tasks that:
base/task_scheduler/task_traits.h provides exhaustive documentation of available traits. Below are some examples of how to specify TaskTraits.
// This task has no explicit TaskTraits. It cannot block. Its priority // is inherited from the calling context (e.g. if it is posted from // a BACKGROUND task, it will have a BACKGROUND priority). It will either // block shutdown or be skipped on shutdown. base::PostTask(FROM_HERE, base::BindOnce(...)); // This task has the highest priority. The task scheduler will try to // run it before USER_VISIBLE and BACKGROUND tasks. base::PostTaskWithTraits( FROM_HERE, {base::TaskPriority::USER_BLOCKING}, base::BindOnce(...)); // This task has the lowest priority and is allowed to block (e.g. it // can read a file from disk). base::PostTaskWithTraits( FROM_HERE, {base::TaskPriority::BACKGROUND, base::MayBlock()}, base::BindOnce(...)); // This task blocks shutdown. The process won't exit before its // execution is complete. base::PostTaskWithTraits( FROM_HERE, {base::TaskShutdownBehavior::BLOCK_SHUTDOWN}, base::BindOnce(...));
Do not perform expensive work on the main thread, the IO thread or any sequence that is expected to run tasks with a low latency. Instead, perform expensive work asynchronously using base::PostTaskAndReply*() or SequencedTaskRunner::PostTaskAndReply().
Example: Running the code below on the main thread will prevent the browser from responding to user input for a long time.
// GetHistoryItemsFromDisk() may block for a long time. // AddHistoryItemsToOmniboxDropDown() updates the UI and therefore must // be called on the main thread. AddHistoryItemsToOmniboxDropdown(GetHistoryItemsFromDisk("keyword"));
The code below solves the problem by scheduling a call to GetHistoryItemsFromDisk() in a thread pool followed by a call to AddHistoryItemsToOmniboxDropdown() on the origin sequence (the main thread in this case). The return value of the first call is automatically provided as argument to the second call.
base::PostTaskWithTraitsAndReplyWithResult( FROM_HERE, {base::MayBlock()}, base::BindOnce(&GetHistoryItemsFromDisk, "keyword"), base::BindOnce(&AddHistoryItemsToOmniboxDropdown));
To post a task that must run once after a delay expires, use base::PostDelayedTask*() or TaskRunner::PostDelayedTask().
base::PostDelayedTaskWithTraits( FROM_HERE, {base::TaskPriority::BACKGROUND}, base::BindOnce(&Task), base::TimeDelta::FromHours(1)); scoped_refptr<base::SequencedTaskRunner> task_runner = base::CreateSequencedTaskRunnerWithTraits({base::TaskPriority::BACKGROUND}); task_runner->PostDelayedTask( FROM_HERE, base::BindOnce(&Task), base::TimeDelta::FromHours(1));
base::TaskPriority::BACKGROUND to prevent it from slowing down the browser when its delay expires.To post a task that must run at regular intervals, use base::RepeatingTimer.
class A { public: ~A() { // The timer is stopped automatically when it is deleted. } void StartDoingStuff() { timer_.Start(FROM_HERE, TimeDelta::FromSeconds(1), this, &MyClass::DoStuff); } void StopDoingStuff() { timer_.Stop(); } private: void DoStuff() { // This method is called every second on the sequence that invoked // StartDoingStuff(). } base::RepeatingTimer timer_; };
base::WeakPtr can be used to ensure that any callback bound to an object is canceled when that object is destroyed.
int Compute() { … } class A { public: A() : weak_ptr_factory_(this) {} void ComputeAndStore() { // Schedule a call to Compute() in a thread pool followed by // a call to A::Store() on the current sequence. The call to // A::Store() is canceled when |weak_ptr_factory_| is destroyed. // (guarantees that |this| will not be used-after-free). base::PostTaskAndReplyWithResult( FROM_HERE, base::BindOnce(&Compute), base::BindOnce(&A::Store, weak_ptr_factory_.GetWeakPtr())); } private: void Store(int value) { value_ = value; } int value_; base::WeakPtrFactory<A> weak_ptr_factory_; };
Note: WeakPtr is not thread-safe: GetWeakPtr(), ~WeakPtrFactory(), and Compute() (bound to a WeakPtr) must all run on the same sequence.
base::CancelableTaskTracker allows cancellation to happen on a different sequence than the one on which tasks run. Keep in mind that CancelableTaskTracker cannot cancel tasks that have already started to run.
auto task_runner = base::CreateTaskRunnerWithTraits(base::TaskTraits()); base::CancelableTaskTracker cancelable_task_tracker; cancelable_task_tracker.PostTask(task_runner.get(), FROM_HERE, base::Bind(&base::DoNothing)); // Cancels Task(), only if it hasn't already started running. cancelable_task_tracker.TryCancelAll();
To test code that uses base::ThreadTaskRunnerHandle, base::SequencedTaskRunnerHandle or a function in base/task_scheduler/post_task.h, instantiate a base::test::ScopedTaskEnvironment for the scope of the test.
class MyTest : public testing::Test { public: // ... protected: base::test::ScopedTaskEnvironment scoped_task_environment_; }; TEST(MyTest, MyTest) { base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, base::BindOnce(&A)); base::SequencedTaskRunnerHandle::Get()->PostTask(FROM_HERE, base::BindOnce(&B)); base::ThreadTaskRunnerHandle::Get()->PostDelayedTask( FROM_HERE, base::BindOnce(&C), base::TimeDelta::Max()); // This runs the (Thread|Sequenced)TaskRunnerHandle queue until it is empty. // Delayed tasks are not added to the queue until they are ripe for execution. base::RunLoop().RunUntilIdle(); // A and B have been executed. C is not ripe for execution yet. base::RunLoop run_loop; base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, base::BindOnce(&D)); base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, run_loop.QuitClosure()); base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, base::BindOnce(&E)); // This runs the (Thread|Sequenced)TaskRunnerHandle queue until QuitClosure is // invoked. run_loop.Run(); // D and run_loop.QuitClosure() have been executed. E is still in the queue. // Tasks posted to task scheduler run asynchronously as they are posted. base::PostTaskWithTraits(FROM_HERE, base::TaskTraits(), base::BindOnce(&F)); auto task_runner = base::CreateSequencedTaskRunnerWithTraits(base::TaskTraits()); task_runner->PostTask(FROM_HERE, base::BindOnce(&G)); // To block until all tasks posted to task scheduler are done running: base::TaskScheduler::GetInstance()->FlushForTesting(); // F and G have been executed. base::PostTaskWithTraitsAndReplyWithResult( FROM_HERE, base::TaskTrait(), base::BindOnce(&H), base::BindOnce(&I)); // This runs the (Thread|Sequenced)TaskRunnerHandle queue until both the // (Thread|Sequenced)TaskRunnerHandle queue and the TaskSchedule queue are // empty: scoped_task_environment_.RunUntilIdle(); // E, H, I have been executed. }
The Chrome browser process has a few legacy named threads (aka “BrowserThreads”). Each of these threads runs a specific type of task (e.g. the FILE thread handles low priority file operations, the FILE_USER_BLOCKING thread handles high priority file operations, the CACHE thread handles cache operations…). Usage of these named threads is now discouraged. New code should post tasks to task scheduler via base/task_scheduler/post_task.h instead.
If for some reason you absolutely need to post a task to a legacy named thread (e.g. because it needs mutual exclusion with a task running on one of these threads), this is how you do it:
content::BrowserThread::GetTaskRunnerForThread(content::BrowserThread::[IDENTIFIER]) ->PostTask(FROM_HERE, base::BindOnce(&Task));
Where IDENTIFIER is one of: DB, FILE, FILE_USER_BLOCKING, PROCESS_LAUNCHER, CACHE.
The Chrome browser process has a “blocking pool” API:
content::BrowserThread::PostBlockingPoolSequencedTask content::BrowserThread::GetBlockingPool
All tasks posted through this API are redirected to base/task_scheduler/post_task.h. Therefore, there is no reason to add calls to this API.
TaskScheduler needs to be initialized in a process before the functions in base/task_scheduler/post_task.h can be used. Initialization of TaskScheduler in the Chrome browser process and child processes (renderer, GPU, utility) has already been taken care of. To use TaskScheduler in another process, initialize TaskScheduler early in the main function:
// This initializes and starts TaskScheduler with default params. base::TaskScheduler::CreateAndStartWithDefaultParams(“process_name”); // The base/task_scheduler/post_task.h API can now be used. Tasks will be // scheduled as they are posted. // This initializes TaskScheduler. base::TaskScheduler::Create(“process_name”); // The base/task_scheduler/post_task.h API can now be used. No threads // will be created and no tasks will be scheduled until after Start() is called. base::TaskScheduler::GetInstance()->Start(params); // TaskScheduler can now create threads and schedule tasks.
And shutdown TaskScheduler late in the main function:
base::TaskScheduler::GetInstance()->Shutdown(); // Tasks posted with TaskShutdownBehavior::BLOCK_SHUTDOWN and // tasks posted with TaskShutdownBehavior::SKIP_ON_SHUTDOWN that // have started to run before the Shutdown() call have now completed their // execution. Tasks posted with // TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN may still be // running.
TaskRunners shouldn't be passed through several components. Instead, the components that uses a TaskRunner should be the one that creates it.
See this example of a refactoring where a TaskRunner was passed through a lot of components only to be used in an eventual leaf. The leaf can and should now obtain its TaskRunner directly from base/task_scheduler/post_task.h.