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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="API documentation for the Rust `join` fn in crate `rayon`."><meta name="keywords" content="rust, rustlang, rust-lang, join"><title>rayon::join - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../rayon/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>rayon</a></p><script>window.sidebarCurrent = {name: 'join', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/rayon_core/join/mod.rs.html#94-102' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='index.html'>rayon</a>::<wbr><a class="fn" href=''>join</a></span></h1><pre class='rust fn'>pub fn join<A, B, RA, RB>(oper_a: A, oper_b: B) -> <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">(</a>RA, RB<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">)</a> <span class="where fmt-newline">where<br> A: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> RA + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a>,<br> B: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> RB + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a>,<br> RA: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a>,<br> RB: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a>, </span></pre><div class='docblock'><p>Takes two closures and <em>potentially</em> runs them in parallel. It
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returns a pair of the results from those closures.</p>
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<p>Conceptually, calling <code>join()</code> is similar to spawning two threads,
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one executing each of the two closures. However, the
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implementation is quite different and incurs very low
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overhead. The underlying technique is called "work stealing": the
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Rayon runtime uses a fixed pool of worker threads and attempts to
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only execute code in parallel when there are idle CPUs to handle
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it.</p>
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<p>When <code>join</code> is called from outside the thread pool, the calling
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thread will block while the closures execute in the pool. When
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<code>join</code> is called within the pool, the calling thread still actively
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participates in the thread pool. It will begin by executing closure
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A (on the current thread). While it is doing that, it will advertise
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closure B as being available for other threads to execute. Once closure A
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has completed, the current thread will try to execute closure B;
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if however closure B has been stolen, then it will look for other work
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while waiting for the thief to fully execute closure B. (This is the
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typical work-stealing strategy).</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>This example uses join to perform a quick-sort (note this is not a
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particularly optimized implementation: if you <strong>actually</strong> want to
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sort for real, you should prefer <a href="../rayon/slice/trait.ParallelSliceMut.html#method.par_sort">the <code>par_sort</code> method</a> offered
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by Rayon).</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">5</span>, <span class="number">1</span>, <span class="number">8</span>, <span class="number">22</span>, <span class="number">0</span>, <span class="number">44</span>];
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<span class="ident">quick_sort</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">v</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">v</span>, <span class="macro">vec</span><span class="macro">!</span>[<span class="number">0</span>, <span class="number">1</span>, <span class="number">5</span>, <span class="number">8</span>, <span class="number">22</span>, <span class="number">44</span>]);
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<span class="kw">fn</span> <span class="ident">quick_sort</span><span class="op"><</span><span class="ident">T</span>:<span class="ident">PartialOrd</span><span class="op">+</span><span class="ident">Send</span><span class="op">></span>(<span class="ident">v</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>]) {
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<span class="kw">if</span> <span class="ident">v</span>.<span class="ident">len</span>() <span class="op">></span> <span class="number">1</span> {
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<span class="kw">let</span> <span class="ident">mid</span> <span class="op">=</span> <span class="ident">partition</span>(<span class="ident">v</span>);
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<span class="kw">let</span> (<span class="ident">lo</span>, <span class="ident">hi</span>) <span class="op">=</span> <span class="ident">v</span>.<span class="ident">split_at_mut</span>(<span class="ident">mid</span>);
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<span class="ident">rayon</span>::<span class="ident">join</span>(<span class="op">|</span><span class="op">|</span> <span class="ident">quick_sort</span>(<span class="ident">lo</span>),
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<span class="op">|</span><span class="op">|</span> <span class="ident">quick_sort</span>(<span class="ident">hi</span>));
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}
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}
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<span class="comment">// Partition rearranges all items `<=` to the pivot</span>
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<span class="comment">// item (arbitrary selected to be the last item in the slice)</span>
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<span class="comment">// to the first half of the slice. It then returns the</span>
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<span class="comment">// "dividing point" where the pivot is placed.</span>
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<span class="kw">fn</span> <span class="ident">partition</span><span class="op"><</span><span class="ident">T</span>:<span class="ident">PartialOrd</span><span class="op">+</span><span class="ident">Send</span><span class="op">></span>(<span class="ident">v</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>]) <span class="op">-</span><span class="op">></span> <span class="ident">usize</span> {
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<span class="kw">let</span> <span class="ident">pivot</span> <span class="op">=</span> <span class="ident">v</span>.<span class="ident">len</span>() <span class="op">-</span> <span class="number">1</span>;
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">i</span> <span class="op">=</span> <span class="number">0</span>;
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<span class="kw">for</span> <span class="ident">j</span> <span class="kw">in</span> <span class="number">0</span>..<span class="ident">pivot</span> {
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<span class="kw">if</span> <span class="ident">v</span>[<span class="ident">j</span>] <span class="op"><</span><span class="op">=</span> <span class="ident">v</span>[<span class="ident">pivot</span>] {
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<span class="ident">v</span>.<span class="ident">swap</span>(<span class="ident">i</span>, <span class="ident">j</span>);
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<span class="ident">i</span> <span class="op">+</span><span class="op">=</span> <span class="number">1</span>;
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}
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}
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<span class="ident">v</span>.<span class="ident">swap</span>(<span class="ident">i</span>, <span class="ident">pivot</span>);
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<span class="ident">i</span>
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}</pre></div>
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<h1 id="warning-about-blocking-io" class="section-header"><a href="#warning-about-blocking-io">Warning about blocking I/O</a></h1>
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<p>The assumption is that the closures given to <code>join()</code> are
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CPU-bound tasks that do not perform I/O or other blocking
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operations. If you do perform I/O, and that I/O should block
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(e.g., waiting for a network request), the overall performance may
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be poor. Moreover, if you cause one closure to be blocked waiting
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on another (for example, using a channel), that could lead to a
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deadlock.</p>
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<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
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<p>No matter what happens, both closures will always be executed. If
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a single closure panics, whether it be the first or second
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closure, that panic will be propagated and hence <code>join()</code> will
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panic with the same panic value. If both closures panic, <code>join()</code>
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will panic with the panic value from the first closure.</p>
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