Mercurial > hg > sv-dependency-builds

annotate src/fftw-3.3.5/doc/html/Thread-safety.html @ 84:08ae793730bd

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Add null config files
author Chris Cannam
date Mon, 02 Mar 2020 14:03:47 +0000
parents 2cd0e3b3e1fd
children
rev   line source
Chris@42 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Chris@42 2 <html>
Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
Chris@42 5
Chris@42 6 Copyright (C) 2003 Matteo Frigo.
Chris@42 7
Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
Chris@42 9
Chris@42 10 Permission is granted to make and distribute verbatim copies of this
Chris@42 11 manual provided the copyright notice and this permission notice are
Chris@42 12 preserved on all copies.
Chris@42 13
Chris@42 14 Permission is granted to copy and distribute modified versions of this
Chris@42 15 manual under the conditions for verbatim copying, provided that the
Chris@42 16 entire resulting derived work is distributed under the terms of a
Chris@42 17 permission notice identical to this one.
Chris@42 18
Chris@42 19 Permission is granted to copy and distribute translations of this manual
Chris@42 20 into another language, under the above conditions for modified versions,
Chris@42 21 except that this permission notice may be stated in a translation
Chris@42 22 approved by the Free Software Foundation. -->
Chris@42 23 <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Thread safety</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: Thread safety">
Chris@42 28 <meta name="keywords" content="FFTW 3.3.5: Thread safety">
Chris@42 29 <meta name="resource-type" content="document">
Chris@42 30 <meta name="distribution" content="global">
Chris@42 31 <meta name="Generator" content="makeinfo">
Chris@42 32 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
Chris@42 33 <link href="index.html#Top" rel="start" title="Top">
Chris@42 34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
Chris@42 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@42 36 <link href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" rel="up" title="Multi-threaded FFTW">
Chris@42 37 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="next" title="Distributed-memory FFTW with MPI">
Chris@42 38 <link href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" rel="prev" title="How Many Threads to Use?">
Chris@42 39 <style type="text/css">
Chris@42 40 <!--
Chris@42 41 a.summary-letter {text-decoration: none}
Chris@42 42 blockquote.smallquotation {font-size: smaller}
Chris@42 43 div.display {margin-left: 3.2em}
Chris@42 44 div.example {margin-left: 3.2em}
Chris@42 45 div.indentedblock {margin-left: 3.2em}
Chris@42 46 div.lisp {margin-left: 3.2em}
Chris@42 47 div.smalldisplay {margin-left: 3.2em}
Chris@42 48 div.smallexample {margin-left: 3.2em}
Chris@42 49 div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
Chris@42 50 div.smalllisp {margin-left: 3.2em}
Chris@42 51 kbd {font-style:oblique}
Chris@42 52 pre.display {font-family: inherit}
Chris@42 53 pre.format {font-family: inherit}
Chris@42 54 pre.menu-comment {font-family: serif}
Chris@42 55 pre.menu-preformatted {font-family: serif}
Chris@42 56 pre.smalldisplay {font-family: inherit; font-size: smaller}
Chris@42 57 pre.smallexample {font-size: smaller}
Chris@42 58 pre.smallformat {font-family: inherit; font-size: smaller}
Chris@42 59 pre.smalllisp {font-size: smaller}
Chris@42 60 span.nocodebreak {white-space:nowrap}
Chris@42 61 span.nolinebreak {white-space:nowrap}
Chris@42 62 span.roman {font-family:serif; font-weight:normal}
Chris@42 63 span.sansserif {font-family:sans-serif; font-weight:normal}
Chris@42 64 ul.no-bullet {list-style: none}
Chris@42 65 -->
Chris@42 66 </style>
Chris@42 67
Chris@42 68
Chris@42 69 </head>
Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Thread-safety"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Previous: <a href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" accesskey="p" rel="prev">How Many Threads to Use?</a>, Up: <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" accesskey="u" rel="up">Multi-threaded FFTW</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Thread-safety-1"></a>
Chris@42 79 <h3 class="section">5.4 Thread safety</h3>
Chris@42 80
Chris@42 81 <a name="index-threads-1"></a>
Chris@42 82 <a name="index-OpenMP-3"></a>
Chris@42 83 <a name="index-thread-safety-1"></a>
Chris@42 84 <p>Users writing multi-threaded programs (including OpenMP) must concern
Chris@42 85 themselves with the <em>thread safety</em> of the libraries they
Chris@42 86 use&mdash;that is, whether it is safe to call routines in parallel from
Chris@42 87 multiple threads. FFTW can be used in such an environment, but some
Chris@42 88 care must be taken because the planner routines share data
Chris@42 89 (e.g. wisdom and trigonometric tables) between calls and plans.
Chris@42 90 </p>
Chris@42 91 <p>The upshot is that the only thread-safe (re-entrant) routine in FFTW is
Chris@42 92 <code>fftw_execute</code> (and the new-array variants thereof). All other routines
Chris@42 93 (e.g. the planner) should only be called from one thread at a time. So,
Chris@42 94 for example, you can wrap a semaphore lock around any calls to the
Chris@42 95 planner; even more simply, you can just create all of your plans from
Chris@42 96 one thread. We do not think this should be an important restriction
Chris@42 97 (FFTW is designed for the situation where the only performance-sensitive
Chris@42 98 code is the actual execution of the transform), and the benefits of
Chris@42 99 shared data between plans are great.
Chris@42 100 </p>
Chris@42 101 <p>Note also that, since the plan is not modified by <code>fftw_execute</code>,
Chris@42 102 it is safe to execute the <em>same plan</em> in parallel by multiple
Chris@42 103 threads. However, since a given plan operates by default on a fixed
Chris@42 104 array, you need to use one of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>) so that different threads compute the transform of different data.
Chris@42 105 </p>
Chris@42 106 <p>(Users should note that these comments only apply to programs using
Chris@42 107 shared-memory threads or OpenMP. Parallelism using MPI or forked processes
Chris@42 108 involves a separate address-space and global variables for each process,
Chris@42 109 and is not susceptible to problems of this sort.)
Chris@42 110 </p>
Chris@42 111 <p>If you are configured FFTW with the <code>--enable-debug</code> or
Chris@42 112 <code>--enable-debug-malloc</code> flags (see <a href="Installation-on-Unix.html#Installation-on-Unix">Installation on Unix</a>),
Chris@42 113 then <code>fftw_execute</code> is not thread-safe. These flags are not
Chris@42 114 documented because they are intended only for developing
Chris@42 115 and debugging FFTW, but if you must use <code>--enable-debug</code> then you
Chris@42 116 should also specifically pass <code>--disable-debug-malloc</code> for
Chris@42 117 <code>fftw_execute</code> to be thread-safe.
Chris@42 118 </p>
Chris@42 119 <p>Starting from FFTW-3.3.5, FFTW supports a new API to make the
Chris@42 120 planner thread-safe:
Chris@42 121 </p><div class="example">
Chris@42 122 <pre class="example">void fftw_make_planner_thread_safe(void);
Chris@42 123 </pre></div>
Chris@42 124 <a name="index-fftw_005fmake_005fplanner_005fthread_005fsafe"></a>
Chris@42 125
Chris@42 126 <p>This call installs a hook that wraps a lock around all planner calls.
Chris@42 127 This API is meant for &ldquo;applications&rdquo; that use &ldquo;plugins&rdquo; in multiple
Chris@42 128 threads, where each &ldquo;plugin&rdquo; calls single-threaded FFTW but is unaware
Chris@42 129 of the other &ldquo;plugins&rdquo; doing the same thing at the same time. In this
Chris@42 130 case, the &ldquo;application&rdquo; calls <code>fftw_make_planner_thread_safe()</code>
Chris@42 131 at the beginning to protect &ldquo;plugins&rdquo; from each other.
Chris@42 132 </p><hr>
Chris@42 133 <div class="header">
Chris@42 134 <p>
Chris@42 135 Previous: <a href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" accesskey="p" rel="prev">How Many Threads to Use?</a>, Up: <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" accesskey="u" rel="up">Multi-threaded FFTW</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@42 136 </div>
Chris@42 137
Chris@42 138
Chris@42 139
Chris@42 140 </body>
Chris@42 141 </html>