190 lines
9.2 KiB
HTML
190 lines
9.2 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Futures (Guile Reference Manual)</title>
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<meta name="description" content="Futures (Guile Reference Manual)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Scheduling.html" rel="up" title="Scheduling">
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<link href="Parallel-Forms.html" rel="next" title="Parallel Forms">
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<link href="Blocking.html" rel="prev" title="Blocking">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Futures">
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<div class="nav-panel">
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<p>
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Next: <a href="Parallel-Forms.html" accesskey="n" rel="next">Parallel forms</a>, Previous: <a href="Blocking.html" accesskey="p" rel="prev">Blocking in Guile Mode</a>, Up: <a href="Scheduling.html" accesskey="u" rel="up">Threads, Mutexes, Asyncs and Dynamic Roots</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsection" id="Futures-1"><span>6.22.7 Futures<a class="copiable-link" href="#Futures-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-futures"></a>
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<a class="index-entry-id" id="index-fine_002dgrain-parallelism"></a>
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<a class="index-entry-id" id="index-parallelism"></a>
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<p>The <code class="code">(ice-9 futures)</code> module provides <em class="dfn">futures</em>, a construct
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for fine-grain parallelism. A future is a wrapper around an expression
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whose computation may occur in parallel with the code of the calling
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thread, and possibly in parallel with other futures. Like promises,
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futures are essentially proxies that can be queried to obtain the value
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of the enclosed expression:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(touch (future (+ 2 3)))
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⇒ 5
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</pre></div>
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<p>However, unlike promises, the expression associated with a future may be
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evaluated on another CPU core, should one be available. This supports
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<em class="dfn">fine-grain parallelism</em>, because even relatively small computations
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can be embedded in futures. Consider this sequential code:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (find-prime lst1 lst2)
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(or (find prime? lst1)
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(find prime? lst2)))
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</pre></div>
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<p>The two arms of <code class="code">or</code> are potentially computation-intensive. They
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are independent of one another, yet, they are evaluated sequentially
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when the first one returns <code class="code">#f</code>. Using futures, one could rewrite
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it like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (find-prime lst1 lst2)
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(let ((f (future (find prime? lst2))))
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(or (find prime? lst1)
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(touch f))))
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</pre></div>
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<p>This preserves the semantics of <code class="code">find-prime</code>. On a multi-core
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machine, though, the computation of <code class="code">(find prime? lst2)</code> may be
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done in parallel with that of the other <code class="code">find</code> call, which can
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reduce the execution time of <code class="code">find-prime</code>.
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</p>
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<p>Futures may be nested: a future can itself spawn and then <code class="code">touch</code>
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other futures, leading to a directed acyclic graph of futures. Using
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this facility, a parallel <code class="code">map</code> procedure can be defined along
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these lines:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(use-modules (ice-9 futures) (ice-9 match))
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(define (par-map proc lst)
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(match lst
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(()
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'())
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((head tail ...)
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(let ((tail (future (par-map proc tail)))
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(head (proc head)))
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(cons head (touch tail))))))
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</pre></div>
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<p>Note that futures are intended for the evaluation of purely functional
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expressions. Expressions that have side-effects or rely on I/O may
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require additional care, such as explicit synchronization
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(see <a class="pxref" href="Mutexes-and-Condition-Variables.html">Mutexes and Condition Variables</a>).
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</p>
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<p>Guile’s futures are implemented on top of POSIX threads
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(see <a class="pxref" href="Threads.html">Threads</a>). Internally, a fixed-size pool of threads is used to
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evaluate futures, such that offloading the evaluation of an expression
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to another thread doesn’t incur thread creation costs. By default, the
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pool contains one thread per available CPU core, minus one, to account
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for the main thread. The number of available CPU cores is determined
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using <code class="code">current-processor-count</code> (see <a class="pxref" href="Processes.html">Processes</a>).
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</p>
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<p>When a thread touches a future that has not completed yet, it processes
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any pending future while waiting for it to complete, or just waits if
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there are no pending futures. When <code class="code">touch</code> is called from within a
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future, the execution of the calling future is suspended, allowing its
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host thread to process other futures, and resumed when the touched
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future has completed. This suspend/resume is achieved by capturing the
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calling future’s continuation, and later reinstating it (see <a class="pxref" href="Prompts.html">delimited continuations</a>).
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-future"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">future</strong> <var class="def-var-arguments">exp</var><a class="copiable-link" href="#index-future"> ¶</a></span></dt>
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<dd><p>Return a future for expression <var class="var">exp</var>. This is equivalent to:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(make-future (lambda () exp))
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</pre></div>
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</dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dfuture"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-future</strong> <var class="def-var-arguments">thunk</var><a class="copiable-link" href="#index-make_002dfuture"> ¶</a></span></dt>
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<dd><p>Return a future for <var class="var">thunk</var>, a zero-argument procedure.
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</p>
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<p>This procedure returns immediately. Execution of <var class="var">thunk</var> may begin
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in parallel with the calling thread’s computations, if idle CPU cores
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are available, or it may start when <code class="code">touch</code> is invoked on the
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returned future.
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</p>
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<p>If the execution of <var class="var">thunk</var> throws an exception, that exception will
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be re-thrown when <code class="code">touch</code> is invoked on the returned future.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-future_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">future?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-future_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">obj</var> is a future.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-touch"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">touch</strong> <var class="def-var-arguments">f</var><a class="copiable-link" href="#index-touch"> ¶</a></span></dt>
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<dd><p>Return the result of the expression embedded in future <var class="var">f</var>.
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</p>
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<p>If the result was already computed in parallel, <code class="code">touch</code> returns
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instantaneously. Otherwise, it waits for the computation to complete,
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if it already started, or initiates it. In the former case, the calling
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thread may process other futures in the meantime.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Parallel-Forms.html">Parallel forms</a>, Previous: <a href="Blocking.html">Blocking in Guile Mode</a>, Up: <a href="Scheduling.html">Threads, Mutexes, Asyncs and Dynamic Roots</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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