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<!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="Utilities for improved cooperative scheduling."><title>tokio::task::coop - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"href="../../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="tokio" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../../../static.files/storage-e2aeef58.js"></script><script defer src="../sidebar-items.js"></script><script defer src="../../../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-263c88ec.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-eab170b8.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-044be391.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]--><rustdoc-topbar><h2><a href="#">Module coop</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../../tokio/index.html">tokio</a><span class="version">1.49.0</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Module coop</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#cooperative-scheduling" title="Cooperative scheduling">Cooperative scheduling</a><ul><li><a href="#unconstrained" title="unconstrained">unconstrained</a></li></ul></li></ul><h3><a href="#structs">Module Items</a></h3><ul class="block"><li><a href="#structs" title="Structs">Structs</a></li><li><a href="#functions" title="Functions">Functions</a></li></ul></section><div id="rustdoc-modnav"><h2><a href="../index.html">In tokio::<wbr>task</a></h2></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><div class="rustdoc-breadcrumbs"><a href="../../index.html">tokio</a>::<wbr><a href="../index.html">task</a></div><h1>Module <span>coop</span> <button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../../src/tokio/task/coop/mod.rs.html#1-573">Source</a> </span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Utilities for improved cooperative scheduling.</p>
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<h4 id="cooperative-scheduling"><a class="doc-anchor" href="#cooperative-scheduling">§</a>Cooperative scheduling</h4>
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<p>A single call to <a href="https://doc.rust-lang.org/1.93.1/core/future/future/trait.Future.html#tymethod.poll" title="method core::future::future::Future::poll"><code>poll</code></a> on a top-level task may potentially do a lot of
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work before it returns <code>Poll::Pending</code>. If a task runs for a long period of
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time without yielding back to the executor, it can starve other tasks
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waiting on that executor to execute them, or drive underlying resources.
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Since Rust does not have a runtime, it is difficult to forcibly preempt a
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long-running task. Instead, this module provides an opt-in mechanism for
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futures to collaborate with the executor to avoid starvation.</p>
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<p>Consider a future like this one:</p>
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<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) {
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<span class="kw">while let </span><span class="prelude-val">Some</span>(<span class="kw">_</span>) = input.next().<span class="kw">await </span>{}
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}</code></pre></div>
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<p>It may look harmless, but consider what happens under heavy load if the
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input stream is <em>always</em> ready. If we spawn <code>drop_all</code>, the task will never
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yield, and will starve other tasks and resources on the same executor.</p>
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<p>To account for this, Tokio has explicit yield points in a number of library
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functions, which force tasks to return to the executor periodically.</p>
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<h5 id="unconstrained"><a class="doc-anchor" href="#unconstrained">§</a>unconstrained</h5>
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<p>If necessary, <a href="fn.unconstrained.html" title="fn tokio::task::coop::unconstrained"><code>task::unconstrained</code></a> lets you opt a future out of Tokio’s cooperative
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scheduling. When a future is wrapped with <code>unconstrained</code>, it will never be forced to yield to
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Tokio. For example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::{task, sync::mpsc};
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<span class="kw">let </span>fut = <span class="kw">async </span>{
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<span class="kw">let </span>(tx, <span class="kw-2">mut </span>rx) = mpsc::unbounded_channel();
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<span class="kw">for </span>i <span class="kw">in </span><span class="number">0</span>..<span class="number">1000 </span>{
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<span class="kw">let _ </span>= tx.send(());
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<span class="comment">// This will always be ready. If coop was in effect, this code would be forced to yield
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// periodically. However, if left unconstrained, then this code will never yield.
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</span>rx.recv().<span class="kw">await</span>;
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}
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};
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task::coop::unconstrained(fut).<span class="kw">await</span>;</code></pre></div></div></details><h2 id="structs" class="section-header">Structs<a href="#structs" class="anchor">§</a></h2><dl class="item-table"><dt><a class="struct" href="struct.Coop.html" title="struct tokio::task::coop::Coop">Coop</a></dt><dd>Future wrapper to ensure cooperative scheduling created by <a href="fn.cooperative.html" title="fn tokio::task::coop::cooperative"><code>cooperative</code></a>.</dd><dt><a class="struct" href="struct.RestoreOnPending.html" title="struct tokio::task::coop::RestoreOnPending">Restore<wbr>OnPending</a></dt><dd>Value returned by the <a href="fn.poll_proceed.html" title="fn tokio::task::coop::poll_proceed"><code>poll_proceed</code></a> method.</dd><dt><a class="struct" href="struct.Unconstrained.html" title="struct tokio::task::coop::Unconstrained">Unconstrained</a></dt><dd>Future for the <a href="fn.unconstrained.html" title="fn tokio::task::coop::unconstrained"><code>unconstrained</code></a> method.</dd></dl><h2 id="functions" class="section-header">Functions<a href="#functions" class="anchor">§</a></h2><dl class="item-table"><dt><a class="fn" href="fn.consume_budget.html" title="fn tokio::task::coop::consume_budget">consume_<wbr>budget</a></dt><dd>Consumes a unit of budget and returns the execution back to the Tokio
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runtime <em>if</em> the task’s coop budget was exhausted.</dd><dt><a class="fn" href="fn.cooperative.html" title="fn tokio::task::coop::cooperative">cooperative</a></dt><dd>Creates a wrapper future that makes the inner future cooperate with the Tokio scheduler.</dd><dt><a class="fn" href="fn.has_budget_remaining.html" title="fn tokio::task::coop::has_budget_remaining">has_<wbr>budget_<wbr>remaining</a></dt><dd>Returns <code>true</code> if there is still budget left on the task.</dd><dt><a class="fn" href="fn.poll_proceed.html" title="fn tokio::task::coop::poll_proceed">poll_<wbr>proceed</a></dt><dd>Decrements the task budget and returns <a href="https://doc.rust-lang.org/1.93.1/core/task/poll/enum.Poll.html#variant.Pending" title="variant core::task::poll::Poll::Pending"><code>Poll::Pending</code></a> if the budget is depleted.
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This indicates that the task should yield to the scheduler. Otherwise, returns
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<a href="struct.RestoreOnPending.html" title="struct tokio::task::coop::RestoreOnPending"><code>RestoreOnPending</code></a> which can be used to commit the budget consumption.</dd><dt><a class="fn" href="fn.unconstrained.html" title="fn tokio::task::coop::unconstrained">unconstrained</a></dt><dd>Turn off cooperative scheduling for a future. The future will never be forced to yield by
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Tokio. Using this exposes your service to starvation if the unconstrained future never yields
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otherwise.</dd></dl></section></div></main></body></html> |