Threading and Tasks in Chrome

Note: See Threading and Tasks FAQ for more examples.


Chrome has a multi-process architecture and each process is heavily multi-threaded. In this document we will go over the basic threading system shared by each process. The main goal is to keep the main thread (a.k.a. “UI” thread in the browser process) and IO thread (each process' thread for handling IPC) responsive. This means offloading any blocking I/O or other expensive operations to other threads. Our approach is to use message passing as the way of communicating between threads. We discourage locking and thread-safe objects. Instead, objects live on only one (often virtual -- we'll get to that later!) thread and we pass messages between those threads for communication.

This documentation assumes familiarity with computer science threading concepts.


Core Concepts

  • Task: A unit of work to be processed. Effectively a function pointer with optionally associated state. In Chrome this is base::Callback created via base::Bind (documentation).
  • Task queue: A queue of tasks to be processed.
  • Physical thread: An operating system provided thread (e.g. pthread on POSIX or CreateThread() on Windows). The Chrome cross-platform abstraction is base::PlatformThread. You should pretty much never use this directly.
  • base::Thread: A physical thread forever processing messages from a dedicated task queue until Quit(). You should pretty much never be creating your own base::Thread's.
  • Thread pool: A pool of physical threads with a shared task queue. In Chrome, this is base::ThreadPoolInstance. There's exactly one instance per Chrome process, it serves tasks posted through base/task/post_task.h and as such you should rarely need to use the base::ThreadPoolInstance API directly (more on posting tasks later).
  • Sequence or Virtual thread: A chrome-managed thread of execution. Like a physical thread, only one task can run on a given sequence / virtual thread at any given moment and each task sees the side-effects of the preceding tasks. Tasks are executed sequentially but may hop physical threads between each one.
  • Task runner: An interface through which tasks can be posted. In Chrome this is base::TaskRunner.
  • Sequenced task runner: A task runner which guarantees that tasks posted to it will run sequentially, in posted order. Each such task is guaranteed to see the side-effects of the task preceding it. Tasks posted to a sequenced task runner are typically processed by a single thread (virtual or physical). In Chrome this is base::SequencedTaskRunner which is-a base::TaskRunner.
  • Single-thread task runner: A sequenced task runner which guarantees that all tasks will be processed by the same physical thread. In Chrome this is base::SingleThreadTaskRunner which is-a base::SequencedTaskRunner. We prefer sequences to threads whenever possible.

Threading Lexicon

Note to the reader: the following terms are an attempt to bridge the gap between common threading nomenclature and the way we use them in Chrome. It might be a bit heavy if you're just getting started. Should this be hard to parse, consider skipping to the more detailed sections below and referring back to this as necessary.

  • Thread-unsafe: The vast majority of types in Chrome are thread-unsafe (by design). Access to such types/methods must be externally synchronized. Typically thread-unsafe types require that all tasks accessing their state be posted to the same base::SequencedTaskRunner and they verify this in debug builds with a SEQUENCE_CHECKER member. Locks are also an option to synchronize access but in Chrome we strongly prefer sequences to locks.
  • Thread-affine: Such types/methods need to be always accessed from the same physical thread (i.e. from the same base::SingleThreadTaskRunner) and typically have a THREAD_CHECKER member to verify that they are. Short of using a third-party API or having a leaf dependency which is thread-affine: there's pretty much no reason for a type to be thread-affine in Chrome. Note that base::SingleThreadTaskRunner is-a base::SequencedTaskRunner so thread-affine is a subset of thread-unsafe. Thread-affine is also sometimes referred to as thread-hostile.
  • Thread-safe: Such types/methods can be safely accessed concurrently.
  • Thread-compatible: Such types provide safe concurrent access to const methods but require synchronization for non-const (or mixed const/non-const access). Chrome doesn't expose reader-writer locks; as such, the only use case for this is objects (typically globals) which are initialized once in a thread-safe manner (either in the single-threaded phase of startup or lazily through a thread-safe static-local-initialization paradigm a la base::NoDestructor) and forever after immutable.
  • Immutable: A subset of thread-compatible types which cannot be modified after construction.
  • Sequence-friendly: Such types/methods are thread-unsafe types which support being invoked from a base::SequencedTaskRunner. Ideally this would be the case for all thread-unsafe types but legacy code sometimes has overzealous checks that enforce thread-affinity in mere thread-unsafe scenarios. See Prefer Sequences to Threads below for more details.


Every Chrome process has

  • a main thread
    • in the browser process (BrowserThread::UI): updates the UI
    • in renderer processes (Blink main thread): runs most of Blink
  • an IO thread
    • in the browser process (BrowserThread::IO): handles IPCs and network requests
    • in renderer processes: handles IPCs
  • a few more special-purpose threads
  • and a pool of general-purpose threads

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:

  • Parallel: No task execution ordering, possibly all at once on any thread
  • Sequenced: Tasks executed in posting order, one at a time on any thread.
  • Single Threaded: Tasks executed in posting order, one at a time on a single thread.

Prefer Sequences to Physical Threads

Sequenced execution (on virtual threads) is strongly preferred to single-threaded execution (on physical threads). Except for types/methods bound to the main thread (UI) or IO threads: thread-safety is better achieved via base::SequencedTaskRunner than through managing your own physical threads (ref. Posting a Sequenced Task below).

All APIs which are exposed for “current physical thread” have an equivalent for “current sequence” (mapping).

If you find yourself writing a sequence-friendly type and it fails thread-affinity checks (e.g., THREAD_CHECKER) in a leaf dependency: consider making that dependency sequence-friendly as well. Most core APIs in Chrome are sequence-friendly, but some legacy types may still over-zealously use ThreadChecker/ThreadTaskRunnerHandle/SingleThreadTaskRunner when they could instead rely on the “current sequence” and no longer be thread-affine.

Posting a Parallel Task

Direct Posting to the Thread Pool

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/post_task.h.

base::PostTask(FROM_HERE, base::BindOnce(&Task));

This posts tasks with default traits.

The base::PostTask*() functions allow the caller to provide additional details about the task via TaskTraits (ref. Annotating Tasks with TaskTraits).

    FROM_HERE, {base::TaskPriority::BEST_EFFORT, MayBlock()},

Posting via a TaskRunner

A parallel base::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 base::TaskRunner is the base class of base::SequencedTaskRunner and base::SingleThreadTaskRunner, a scoped_refptr<TaskRunner> member can hold a base::TaskRunner, a base::SequencedTaskRunner or a base::SingleThreadTaskRunner.

class A {
  A() = default;

  void DoSomething() {
    task_runner_->PostTask(FROM_HERE, base::BindOnce(&A));

  scoped_refptr<base::TaskRunner> task_runner_ =

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).

Posting a Sequenced Task

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 base::SequencedTaskRunner.

Posting to a New Sequence

A base::SequencedTaskRunner can be created by base::CreateSequencedTaskRunner().

scoped_refptr<SequencedTaskRunner> sequenced_task_runner =

// TaskB runs after TaskA completes.
sequenced_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskA));
sequenced_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskB));

Posting to the Current (Virtual) Thread

The preferred way of posting to the current (virtual) thread is via base::SequencedTaskRunnerHandle::Get().

// The task will run on the current (virtual) thread's default task queue.
    FROM_HERE, base::BindOnce(&Task);

Note that SequencedTaskRunnerHandle::Get() returns the default queue for the current virtual thread. On threads with multiple task queues (e.g. BrowserThread::UI) this can be a different queue than the one the current task belongs to. The “current” task runner is intentionally not exposed via a static getter. Either you know it already and can post to it directly or you don't and the only sensible destination is the default queue.

Using Sequences Instead of Locks

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.

Thread-safe but not thread-affine; how so? Tasks posted to the same sequence will run in sequential order. After a sequenced task completes, the next task may be picked up by a different worker thread, but that task is guaranteed to see any side-effects caused by the previous one(s) on its sequence.

class A {
  A() {
    // Do not require accesses to be on the creation sequence.

  void AddValue(int v) {
    // Check that all accesses are on the same sequence.


  // No lock required, because all accesses are on the
  // same sequence.
  std::vector<int> values_;

A a;
scoped_refptr<SequencedTaskRunner> task_runner_for_a = ...;
                      base::BindOnce(&A::AddValue, base::Unretained(&a), 42));
                      base::BindOnce(&A::AddValue, base::Unretained(&a), 27));

// Access from a different sequence causes a DCHECK failure.
scoped_refptr<SequencedTaskRunner> other_task_runner = ...;
                            base::BindOnce(&A::AddValue, base::Unretained(&a), 1));

Locks should only be used to swap in a shared data structure that can be accessed on multiple threads. If one thread updates it based on expensive computation or through disk access, then that slow work should be done without holding the lock. Only when the result is available should the lock be used to swap in the new data. An example of this is in PluginList::LoadPlugins (content/browser/ If you must use locks, here are some best practices and pitfalls to avoid.

In order to write non-blocking code, many APIs in Chrome are asynchronous. Usually this means that they either need to be executed on a particular thread/sequence and will return results via a custom delegate interface, or they take a base::Callback<> object that is called when the requested operation is completed. Executing work on a specific thread/sequence is covered in the PostTask sections above.

Posting Multiple Tasks to the Same Thread

If multiple tasks need to run on the same thread, post them to a base::SingleThreadTaskRunner. All tasks posted to the same base::SingleThreadTaskRunner run on the same thread in posting order.

Posting to the Main Thread or to the IO Thread in the Browser Process

To post tasks to the main thread or to the IO thread, use base::PostTask() or get the appropriate SingleThreadTaskRunner using base::CreateSingleThreadTaskRunner, supplying a BrowserThread::ID as trait. For this, you'll also need to include content/public/browser/browser_task_traits.h.

base::PostTask(FROM_HERE, {content::BrowserThread::UI}, ...);

    ->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.

Posting to the Main Thread in a Renderer Process


Posting to a Custom SingleThreadTaskRunner

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 base::SingleThreadTaskRunner created by base::CreateSingleThreadTaskRunner.

scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner =

// 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));

Remember that we prefer sequences to physical threads and that this thus should rarely be necessary.

Posting Tasks to a COM Single-Thread Apartment (STA) Thread (Windows)

Tasks that need to run on a COM Single-Thread Apartment (STA) thread must be posted to a base::SingleThreadTaskRunner returned by base::CreateCOMSTATaskRunner(). As mentioned in Posting Multiple Tasks to the Same Thread, all tasks posted to the same base::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.
      FROM_HERE, base::BindOnce(&TaskCUsingCOMSTA));
void TaskBUsingCOMSTA() { }
void TaskCUsingCOMSTA() { }

auto com_sta_task_runner = base::CreateCOMSTATaskRunner(...);
com_sta_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskAUsingCOMSTA));
com_sta_task_runner->PostTask(FROM_HERE, base::BindOnce(&TaskBUsingCOMSTA));

Annotating Tasks with TaskTraits

base::TaskTraits encapsulate information about a task that helps the thread pool make better scheduling decisions.

All base::PostTask*() functions in base/task/post_task.h have an overload that takes base::TaskTraits as argument and one that doesn’t. The overload that doesn’t take base::TaskTraits as argument is appropriate for tasks that:

  • Don’t block (ref. MayBlock and WithBaseSyncPrimitives).
  • Prefer inheriting the current priority to specifying their own.
  • Can either block shutdown or be skipped on shutdown (thread pool is free to choose a fitting default). Tasks that don’t match this description must be posted with explicit TaskTraits.

base/task/task_traits.h provides exhaustive documentation of available traits. The content layer also provides additional traits in content/public/browser/browser_task_traits.h to facilitate posting a task onto a BrowserThread.

Below are some examples of how to specify base::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 BEST_EFFORT task, it will have a BEST_EFFORT priority). It will either
// block shutdown or be skipped on shutdown.
base::PostTask(FROM_HERE, base::BindOnce(...));

// This task has the highest priority. The thread pool will try to
// run it before USER_VISIBLE and BEST_EFFORT tasks.
    FROM_HERE, {base::TaskPriority::USER_BLOCKING},

// This task has the lowest priority and is allowed to block (e.g. it
// can read a file from disk).
    FROM_HERE, {base::TaskPriority::BEST_EFFORT, base::MayBlock()},

// This task blocks shutdown. The process won't exit before its
// execution is complete.
    FROM_HERE, {base::TaskShutdownBehavior::BLOCK_SHUTDOWN},

// This task will run on the Browser UI thread.
    FROM_HERE, {content::BrowserThread::UI},

Keeping the Browser Responsive

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 base::SequencedTaskRunner::PostTaskAndReply(). Note that asynchronous/overlapped I/O on the IO thread are fine.

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.

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.

    FROM_HERE, {base::MayBlock()},
    base::BindOnce(&GetHistoryItemsFromDisk, "keyword"),

Posting a Task with a Delay

Posting a One-Off Task with a Delay

To post a task that must run once after a delay expires, use base::PostDelayedTask*() or base::TaskRunner::PostDelayedTask().

  FROM_HERE, {base::TaskPriority::BEST_EFFORT}, base::BindOnce(&Task),

scoped_refptr<base::SequencedTaskRunner> task_runner =
    FROM_HERE, base::BindOnce(&Task), base::TimeDelta::FromHours(1));
NOTE: A task that has a 1-hour delay probably doesn’t have to run right away when its delay expires. Specify base::TaskPriority::BEST_EFFORT to prevent it from slowing down the browser when its delay expires.

Posting a Repeating Task with a Delay

To post a task that must run at regular intervals, use base::RepeatingTimer.

class A {
  ~A() {
    // The timer is stopped automatically when it is deleted.
  void StartDoingStuff() {
    timer_.Start(FROM_HERE, TimeDelta::FromSeconds(1),
                 this, &MyClass::DoStuff);
  void StopDoingStuff() {
  void DoStuff() {
    // This method is called every second on the sequence that invoked
    // StartDoingStuff().
  base::RepeatingTimer timer_;

Cancelling a Task

Using base::WeakPtr

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 {
  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).
        FROM_HERE, base::BindOnce(&Compute),
        base::BindOnce(&A::Store, weak_ptr_factory_.GetWeakPtr()));

  void Store(int value) { value_ = value; }

  int value_;
  base::WeakPtrFactory<A> weak_ptr_factory_{this};

Note: WeakPtr is not thread-safe: GetWeakPtr(), ~WeakPtrFactory(), and Compute() (bound to a WeakPtr) must all run on the same sequence.

Using base::CancelableTaskTracker

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::CreateTaskRunner({base::ThreadPool()});
base::CancelableTaskTracker cancelable_task_tracker;
cancelable_task_tracker.PostTask(task_runner.get(), FROM_HERE,
// Cancels Task(), only if it hasn't already started running.


For more details see Testing Components Which Post Tasks.

To test code that uses base::ThreadTaskRunnerHandle, base::SequencedTaskRunnerHandle or a function in base/task/post_task.h, instantiate a base::test::TaskEnvironment for the scope of the test. If you need BrowserThreads, use content::BrowserTaskEnvironment instead of base::test::TaskEnvironment.

Tests can run the base::test::TaskEnvironment's message pump using a base::RunLoop, which can be made to run until Quit() (explicitly or via RunLoop::QuitClosure()), or to RunUntilIdle() ready-to-run tasks and immediately return.

TaskEnvironment configures RunLoop::Run() to LOG(FATAL) if it hasn't been explicitly quit after TestTimeouts::action_timeout(). This is preferable to having the test hang if the code under test fails to trigger the RunLoop to quit. The timeout can be overridden with ScopedRunTimeoutForTest.

class MyTest : public testing::Test {
  // ...
   base::test::TaskEnvironment task_environment_;

TEST(MyTest, MyTest) {
  base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, base::BindOnce(&A));
      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.
  // 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.
  // D and run_loop.QuitClosure() have been executed. E is still in the queue.

  // Tasks posted to thread pool run asynchronously as they are posted.
  base::PostTask(FROM_HERE, {base::ThreadPool()}, base::BindOnce(&F));
  auto task_runner =
  task_runner->PostTask(FROM_HERE, base::BindOnce(&G));

  // To block until all tasks posted to thread pool are done running:
  // F and G have been executed.

      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:
  // E, H, I have been executed.

Using ThreadPool in a New Process

ThreadPoolInstance needs to be initialized in a process before the functions in base/task/post_task.h can be used. Initialization of ThreadPoolInstance in the Chrome browser process and child processes (renderer, GPU, utility) has already been taken care of. To use ThreadPoolInstance in another process, initialize ThreadPoolInstance early in the main function:

// This initializes and starts ThreadPoolInstance with default params.
// The base/task/post_task.h API can now be used with base::ThreadPool trait.
// Tasks will be // scheduled as they are posted.

// This initializes ThreadPoolInstance.
// The base/task/post_task.h API can now be used with base::ThreadPool trait. No
// threads will be created and no tasks will be scheduled until after Start() is
// called.
// ThreadPool can now create threads and schedule tasks.

And shutdown ThreadPoolInstance late in the main function:

// 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.

TaskRunner ownership (encourage no dependency injection)

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/post_task.h.

As mentioned above, base::test::TaskEnvironment allows unit tests to control tasks posted from underlying TaskRunners. In rare cases where a test needs to more precisely control task ordering: dependency injection of TaskRunners can be useful. For such cases the preferred approach is the following:

class Foo {

  // Overrides |background_task_runner_| in tests.
  void SetBackgroundTaskRunnerForTesting(
      scoped_refptr<base::SequencedTaskRunner> background_task_runner) {
    background_task_runner_ = std::move(background_task_runner);

  scoped_refptr<base::SequencedTaskRunner> background_task_runner_ =
          {base::MayBlock(), base::TaskPriority::BEST_EFFORT});

Note that this still allows removing all layers of plumbing between //chrome and that component since unit tests will use the leaf layer directly.


See Threading and Tasks FAQ for more examples.