| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Asynchronous green-threads."><meta name="keywords" content="rust, rustlang, rust-lang, task"><title>tokio::task - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc mod"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Module task</a></h2><div class="sidebar-elems"><section><ul class="block"><li><a href="#modules">Modules</a></li><li><a href="#structs">Structs</a></li><li><a href="#functions">Functions</a></li></ul></section></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Module <a href="../index.html">tokio</a>::<wbr><a class="mod" href="#">task</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../../src/tokio/task/mod.rs.html#1-333">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Asynchronous green-threads.</p> |
| <h3 id="what-are-tasks"><a href="#what-are-tasks">What are Tasks?</a></h3> |
| <p>A <em>task</em> is a light weight, non-blocking unit of execution. A task is similar |
| to an OS thread, but rather than being managed by the OS scheduler, they are |
| managed by the <a href="../runtime/index.html">Tokio runtime</a>. Another name for this general pattern is |
| <a href="https://en.wikipedia.org/wiki/Green_threads">green threads</a>. If you are familiar with <a href="https://tour.golang.org/concurrency/1">Go’s goroutines</a>, <a href="https://kotlinlang.org/docs/reference/coroutines-overview.html">Kotlin’s |
| coroutines</a>, or <a href="http://erlang.org/doc/getting_started/conc_prog.html#processes">Erlang’s processes</a>, you can think of Tokio’s tasks as |
| something similar.</p> |
| <p>Key points about tasks include:</p> |
| <ul> |
| <li> |
| <p>Tasks are <strong>light weight</strong>. Because tasks are scheduled by the Tokio |
| runtime rather than the operating system, creating new tasks or switching |
| between tasks does not require a context switch and has fairly low |
| overhead. Creating, running, and destroying large numbers of tasks is |
| quite cheap, especially compared to OS threads.</p> |
| </li> |
| <li> |
| <p>Tasks are scheduled <strong>cooperatively</strong>. Most operating systems implement |
| <em>preemptive multitasking</em>. This is a scheduling technique where the |
| operating system allows each thread to run for a period of time, and then |
| <em>preempts</em> it, temporarily pausing that thread and switching to another. |
| Tasks, on the other hand, implement <em>cooperative multitasking</em>. In |
| cooperative multitasking, a task is allowed to run until it <em>yields</em>, |
| indicating to the Tokio runtime’s scheduler that it cannot currently |
| continue executing. When a task yields, the Tokio runtime switches to |
| executing the next task.</p> |
| </li> |
| <li> |
| <p>Tasks are <strong>non-blocking</strong>. Typically, when an OS thread performs I/O or |
| must synchronize with another thread, it <em>blocks</em>, allowing the OS to |
| schedule another thread. When a task cannot continue executing, it must |
| yield instead, allowing the Tokio runtime to schedule another task. Tasks |
| should generally not perform system calls or other operations that could |
| block a thread, as this would prevent other tasks running on the same |
| thread from executing as well. Instead, this module provides APIs for |
| running blocking operations in an asynchronous context.</p> |
| </li> |
| </ul> |
| <h3 id="working-with-tasks"><a href="#working-with-tasks">Working with Tasks</a></h3> |
| <p>This module provides the following APIs for working with tasks:</p> |
| <h4 id="spawning"><a href="#spawning">Spawning</a></h4> |
| <p>Perhaps the most important function in this module is <a href="fn.spawn.html"><code>task::spawn</code></a>. This |
| function can be thought of as an async equivalent to the standard library’s |
| <a href="std::thread::spawn"><code>thread::spawn</code></a>. It takes an <code>async</code> block or other |
| <a href="std::future::Future">future</a>, and creates a new task to run that work concurrently:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::task; |
| |
| task::spawn(<span class="kw">async </span>{ |
| <span class="comment">// perform some work here... |
| </span>});</code></pre></div> |
| <p>Like <a href="std::thread::spawn"><code>std::thread::spawn</code></a>, <code>task::spawn</code> returns a <a href="struct.JoinHandle.html"><code>JoinHandle</code></a> struct. |
| A <code>JoinHandle</code> is itself a future which may be used to await the output of |
| the spawned task. For example:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::task; |
| |
| <span class="kw">let </span>join = task::spawn(<span class="kw">async </span>{ |
| <span class="comment">// ... |
| </span><span class="string">"hello world!" |
| </span>}); |
| |
| <span class="comment">// ... |
| |
| // Await the result of the spawned task. |
| </span><span class="kw">let </span>result = join.<span class="kw">await</span><span class="question-mark">?</span>; |
| <span class="macro">assert_eq!</span>(result, <span class="string">"hello world!"</span>);</code></pre></div> |
| <p>Again, like <code>std::thread</code>’s <a href="std::thread::JoinHandle"><code>JoinHandle</code> type</a>, if the spawned |
| task panics, awaiting its <code>JoinHandle</code> will return a <a href="struct.JoinError.html"><code>JoinError</code></a>. For |
| example:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::task; |
| |
| <span class="kw">let </span>join = task::spawn(<span class="kw">async </span>{ |
| <span class="macro">panic!</span>(<span class="string">"something bad happened!"</span>) |
| }); |
| |
| <span class="comment">// The returned result indicates that the task failed. |
| </span><span class="macro">assert!</span>(join.<span class="kw">await</span>.is_err());</code></pre></div> |
| <p><code>spawn</code>, <code>JoinHandle</code>, and <code>JoinError</code> are present when the “rt” |
| feature flag is enabled.</p> |
| <h4 id="blocking-and-yielding"><a href="#blocking-and-yielding">Blocking and Yielding</a></h4> |
| <p>As we discussed above, code running in asynchronous tasks should not perform |
| operations that can block. A blocking operation performed in a task running |
| on a thread that is also running other tasks would block the entire thread, |
| preventing other tasks from running.</p> |
| <p>Instead, Tokio provides two APIs for running blocking operations in an |
| asynchronous context: <a href="fn.spawn_blocking.html"><code>task::spawn_blocking</code></a> and <a href="fn.block_in_place.html"><code>task::block_in_place</code></a>.</p> |
| <p>Be aware that if you call a non-async method from async code, that non-async |
| method is still inside the asynchronous context, so you should also avoid |
| blocking operations there. This includes destructors of objects destroyed in |
| async code.</p> |
| <h5 id="spawn_blocking"><a href="#spawn_blocking">spawn_blocking</a></h5> |
| <p>The <code>task::spawn_blocking</code> function is similar to the <code>task::spawn</code> function |
| discussed in the previous section, but rather than spawning an |
| <em>non-blocking</em> future on the Tokio runtime, it instead spawns a |
| <em>blocking</em> function on a dedicated thread pool for blocking tasks. For |
| example:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::task; |
| |
| task::spawn_blocking(|| { |
| <span class="comment">// do some compute-heavy work or call synchronous code |
| </span>});</code></pre></div> |
| <p>Just like <code>task::spawn</code>, <code>task::spawn_blocking</code> returns a <code>JoinHandle</code> |
| which we can use to await the result of the blocking operation:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>join = task::spawn_blocking(|| { |
| <span class="comment">// do some compute-heavy work or call synchronous code |
| </span><span class="string">"blocking completed" |
| </span>}); |
| |
| <span class="kw">let </span>result = join.<span class="kw">await</span><span class="question-mark">?</span>; |
| <span class="macro">assert_eq!</span>(result, <span class="string">"blocking completed"</span>);</code></pre></div> |
| <h5 id="block_in_place"><a href="#block_in_place">block_in_place</a></h5> |
| <p>When using the <a href="../runtime/index.html#threaded-scheduler">multi-threaded runtime</a>, the <a href="fn.block_in_place.html"><code>task::block_in_place</code></a> |
| function is also available. Like <code>task::spawn_blocking</code>, this function |
| allows running a blocking operation from an asynchronous context. Unlike |
| <code>spawn_blocking</code>, however, <code>block_in_place</code> works by transitioning the |
| <em>current</em> worker thread to a blocking thread, moving other tasks running on |
| that thread to another worker thread. This can improve performance by avoiding |
| context switches.</p> |
| <p>For example:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::task; |
| |
| <span class="kw">let </span>result = task::block_in_place(|| { |
| <span class="comment">// do some compute-heavy work or call synchronous code |
| </span><span class="string">"blocking completed" |
| </span>}); |
| |
| <span class="macro">assert_eq!</span>(result, <span class="string">"blocking completed"</span>);</code></pre></div> |
| <h5 id="yield_now"><a href="#yield_now">yield_now</a></h5> |
| <p>In addition, this module provides a <a href="fn.yield_now.html"><code>task::yield_now</code></a> async function |
| that is analogous to the standard library’s <a href="std::thread::yield_now"><code>thread::yield_now</code></a>. Calling |
| and <code>await</code>ing this function will cause the current task to yield to the |
| Tokio runtime’s scheduler, allowing other tasks to be |
| scheduled. Eventually, the yielding task will be polled again, allowing it |
| to execute. For example:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::task; |
| |
| <span class="kw">async </span>{ |
| task::spawn(<span class="kw">async </span>{ |
| <span class="comment">// ... |
| </span><span class="macro">println!</span>(<span class="string">"spawned task done!"</span>) |
| }); |
| |
| <span class="comment">// Yield, allowing the newly-spawned task to execute first. |
| </span>task::yield_now().<span class="kw">await</span>; |
| <span class="macro">println!</span>(<span class="string">"main task done!"</span>); |
| }</code></pre></div> |
| <h4 id="cooperative-scheduling"><a href="#cooperative-scheduling">Cooperative scheduling</a></h4> |
| <p>A single call to <a href="method@std::future::Future::poll"><code>poll</code></a> on a top-level task may potentially do a lot of |
| work before it returns <code>Poll::Pending</code>. If a task runs for a long period of |
| time without yielding back to the executor, it can starve other tasks |
| waiting on that executor to execute them, or drive underlying resources. |
| Since Rust does not have a runtime, it is difficult to forcibly preempt a |
| long-running task. Instead, this module provides an opt-in mechanism for |
| futures to collaborate with the executor to avoid starvation.</p> |
| <p>Consider a future like this one:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">async fn </span>drop_all<I: Stream + Unpin>(<span class="kw-2">mut </span>input: I) { |
| <span class="kw">while let </span><span class="prelude-val">Some</span>(<span class="kw">_</span>) = input.next().<span class="kw">await </span>{} |
| }</code></pre></div> |
| <p>It may look harmless, but consider what happens under heavy load if the |
| input stream is <em>always</em> ready. If we spawn <code>drop_all</code>, the task will never |
| yield, and will starve other tasks and resources on the same executor.</p> |
| <p>To account for this, Tokio has explicit yield points in a number of library |
| functions, which force tasks to return to the executor periodically.</p> |
| <h5 id="unconstrained"><a href="#unconstrained">unconstrained</a></h5> |
| <p>If necessary, <a href="fn.unconstrained.html"><code>task::unconstrained</code></a> lets you opt a future out of of Tokio’s cooperative |
| scheduling. When a future is wrapped with <code>unconstrained</code>, it will never be forced to yield to |
| Tokio. For example:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::{task, sync::mpsc}; |
| |
| <span class="kw">let </span>fut = <span class="kw">async </span>{ |
| <span class="kw">let </span>(tx, <span class="kw-2">mut </span>rx) = mpsc::unbounded_channel(); |
| |
| <span class="kw">for </span>i <span class="kw">in </span><span class="number">0</span>..<span class="number">1000 </span>{ |
| <span class="kw">let _ </span>= tx.send(()); |
| <span class="comment">// This will always be ready. If coop was in effect, this code would be forced to yield |
| // periodically. However, if left unconstrained, then this code will never yield. |
| </span>rx.recv().<span class="kw">await</span>; |
| } |
| }; |
| |
| task::unconstrained(fut).<span class="kw">await</span>;</code></pre></div> |
| </div></details><h2 id="modules" class="small-section-header"><a href="#modules">Modules</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="mod" href="futures/index.html" title="tokio::task::futures mod">futures</a></div><div class="item-right docblock-short">Task-related futures.</div></div></div><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.AbortHandle.html" title="tokio::task::AbortHandle struct">AbortHandle</a></div><div class="item-right docblock-short">An owned permission to abort a spawned task, without awaiting its completion.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.JoinError.html" title="tokio::task::JoinError struct">JoinError</a></div><div class="item-right docblock-short">Task failed to execute to completion.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.JoinHandle.html" title="tokio::task::JoinHandle struct">JoinHandle</a></div><div class="item-right docblock-short">An owned permission to join on a task (await its termination).</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.JoinSet.html" title="tokio::task::JoinSet struct">JoinSet</a></div><div class="item-right docblock-short">A collection of tasks spawned on a Tokio runtime.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.LocalEnterGuard.html" title="tokio::task::LocalEnterGuard struct">LocalEnterGuard</a></div><div class="item-right docblock-short">Context guard for LocalSet</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.LocalKey.html" title="tokio::task::LocalKey struct">LocalKey</a></div><div class="item-right docblock-short">A key for task-local data.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.LocalSet.html" title="tokio::task::LocalSet struct">LocalSet</a></div><div class="item-right docblock-short">A set of tasks which are executed on the same thread.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Unconstrained.html" title="tokio::task::Unconstrained struct">Unconstrained</a></div><div class="item-right docblock-short">Future for the <a href="fn.unconstrained.html"><code>unconstrained</code></a> method.</div></div></div><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.block_in_place.html" title="tokio::task::block_in_place fn">block_in_place</a></div><div class="item-right docblock-short">Runs the provided blocking function on the current thread without |
| blocking the executor.</div></div><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.spawn.html" title="tokio::task::spawn fn">spawn</a></div><div class="item-right docblock-short">Spawns a new asynchronous task, returning a |
| <a href="struct.JoinHandle.html"><code>JoinHandle</code></a> for it.</div></div><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.spawn_blocking.html" title="tokio::task::spawn_blocking fn">spawn_blocking</a></div><div class="item-right docblock-short">Runs the provided closure on a thread where blocking is acceptable.</div></div><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.spawn_local.html" title="tokio::task::spawn_local fn">spawn_local</a></div><div class="item-right docblock-short">Spawns a <code>!Send</code> future on the current <a href="struct.LocalSet.html"><code>LocalSet</code></a>.</div></div><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.unconstrained.html" title="tokio::task::unconstrained fn">unconstrained</a></div><div class="item-right docblock-short">Turn off cooperative scheduling for a future. The future will never be forced to yield by |
| Tokio. Using this exposes your service to starvation if the unconstrained future never yields |
| otherwise.</div></div><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.yield_now.html" title="tokio::task::yield_now fn">yield_now</a></div><div class="item-right docblock-short">Yields execution back to the Tokio runtime.</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
