Function tokio::task::spawn_blocking

source ·
pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R> where
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static,
Available on crate feature rt only.
Expand description

Runs the provided closure on a thread where blocking is acceptable.

In general, issuing a blocking call or performing a lot of compute in a future without yielding is problematic, as it may prevent the executor from driving other futures forward. This function runs the provided closure on a thread dedicated to blocking operations. See the CPU-bound tasks and blocking code section for more information.

Tokio will spawn more blocking threads when they are requested through this function until the upper limit configured on the Builder is reached. After reaching the upper limit, the tasks are put in a queue. The thread limit is very large by default, because spawn_blocking is often used for various kinds of IO operations that cannot be performed asynchronously. When you run CPU-bound code using spawn_blocking, you should keep this large upper limit in mind. When running many CPU-bound computations, a semaphore or some other synchronization primitive should be used to limit the number of computation executed in parallel. Specialized CPU-bound executors, such as rayon, may also be a good fit.

This function is intended for non-async operations that eventually finish on their own. If you want to spawn an ordinary thread, you should use thread::spawn instead.

Closures spawned using spawn_blocking cannot be cancelled abruptly; there is no standard low level API to cause a thread to stop running. However, a useful pattern is to pass some form of “cancellation token” into the thread. This could be an AtomicBool that the task checks periodically. Another approach is to have the thread primarily read or write from a channel, and to exit when the channel closes; assuming the other side of the channel is dropped when cancellation occurs, this will cause the blocking task thread to exit soon after as well.

When you shut down the executor, it will wait indefinitely for all blocking operations to finish. You can use shutdown_timeout to stop waiting for them after a certain timeout. Be aware that this will still not cancel the tasks — they are simply allowed to keep running after the method returns. It is possible for a blocking task to be cancelled if it has not yet started running, but this is not guaranteed.

Note that if you are using the single threaded runtime, this function will still spawn additional threads for blocking operations. The current-thread scheduler’s single thread is only used for asynchronous code.

In simple cases, it is sufficient to have the closure accept input parameters at creation time and return a single value (or struct/tuple, etc.).

For more complex situations in which it is desirable to stream data to or from the synchronous context, the mpsc channel has blocking_send and blocking_recv methods for use in non-async code such as the thread created by spawn_blocking.

Another option is SyncIoBridge for cases where the synchronous context is operating on byte streams. For example, you might use an asynchronous HTTP client such as hyper to fetch data, but perform complex parsing of the payload body using a library written for synchronous I/O.

Finally, see also Bridging with sync code for discussions around the opposite case of using Tokio as part of a larger synchronous codebase.


Pass an input value and receive result of computation:

use tokio::task;

// Initial input
let mut v = "Hello, ".to_string();
let res = task::spawn_blocking(move || {
    // Stand-in for compute-heavy work or using synchronous APIs
    // Pass ownership of the value back to the asynchronous context

// `res` is the value returned from the thread
assert_eq!(res.as_str(), "Hello, world");

Use a channel:

use tokio::task;
use tokio::sync::mpsc;

let (tx, mut rx) = mpsc::channel(2);
let start = 5;
let worker = task::spawn_blocking(move || {
    for x in 0..10 {
        // Stand in for complex computation
        tx.blocking_send(start + x).unwrap();

let mut acc = 0;
while let Some(v) = rx.recv().await {
    acc += v;
assert_eq!(acc, 95);