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Add null config files
author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 02 Mar 2020 14:03:47 +0000
parents 7867fa7e1b6b
children
rev   line source
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cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
cannam@127 7
cannam@127 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: Thread safety</title>
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cannam@127 27 <meta name="description" content="FFTW 3.3.5: Thread safety">
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cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="Thread-safety"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="Thread-safety-1"></a>
cannam@127 79 <h3 class="section">5.4 Thread safety</h3>
cannam@127 80
cannam@127 81 <a name="index-threads-1"></a>
cannam@127 82 <a name="index-OpenMP-3"></a>
cannam@127 83 <a name="index-thread-safety-1"></a>
cannam@127 84 <p>Users writing multi-threaded programs (including OpenMP) must concern
cannam@127 85 themselves with the <em>thread safety</em> of the libraries they
cannam@127 86 use&mdash;that is, whether it is safe to call routines in parallel from
cannam@127 87 multiple threads. FFTW can be used in such an environment, but some
cannam@127 88 care must be taken because the planner routines share data
cannam@127 89 (e.g. wisdom and trigonometric tables) between calls and plans.
cannam@127 90 </p>
cannam@127 91 <p>The upshot is that the only thread-safe (re-entrant) routine in FFTW is
cannam@127 92 <code>fftw_execute</code> (and the new-array variants thereof). All other routines
cannam@127 93 (e.g. the planner) should only be called from one thread at a time. So,
cannam@127 94 for example, you can wrap a semaphore lock around any calls to the
cannam@127 95 planner; even more simply, you can just create all of your plans from
cannam@127 96 one thread. We do not think this should be an important restriction
cannam@127 97 (FFTW is designed for the situation where the only performance-sensitive
cannam@127 98 code is the actual execution of the transform), and the benefits of
cannam@127 99 shared data between plans are great.
cannam@127 100 </p>
cannam@127 101 <p>Note also that, since the plan is not modified by <code>fftw_execute</code>,
cannam@127 102 it is safe to execute the <em>same plan</em> in parallel by multiple
cannam@127 103 threads. However, since a given plan operates by default on a fixed
cannam@127 104 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.
cannam@127 105 </p>
cannam@127 106 <p>(Users should note that these comments only apply to programs using
cannam@127 107 shared-memory threads or OpenMP. Parallelism using MPI or forked processes
cannam@127 108 involves a separate address-space and global variables for each process,
cannam@127 109 and is not susceptible to problems of this sort.)
cannam@127 110 </p>
cannam@127 111 <p>If you are configured FFTW with the <code>--enable-debug</code> or
cannam@127 112 <code>--enable-debug-malloc</code> flags (see <a href="Installation-on-Unix.html#Installation-on-Unix">Installation on Unix</a>),
cannam@127 113 then <code>fftw_execute</code> is not thread-safe. These flags are not
cannam@127 114 documented because they are intended only for developing
cannam@127 115 and debugging FFTW, but if you must use <code>--enable-debug</code> then you
cannam@127 116 should also specifically pass <code>--disable-debug-malloc</code> for
cannam@127 117 <code>fftw_execute</code> to be thread-safe.
cannam@127 118 </p>
cannam@127 119 <p>Starting from FFTW-3.3.5, FFTW supports a new API to make the
cannam@127 120 planner thread-safe:
cannam@127 121 </p><div class="example">
cannam@127 122 <pre class="example">void fftw_make_planner_thread_safe(void);
cannam@127 123 </pre></div>
cannam@127 124 <a name="index-fftw_005fmake_005fplanner_005fthread_005fsafe"></a>
cannam@127 125
cannam@127 126 <p>This call installs a hook that wraps a lock around all planner calls.
cannam@127 127 This API is meant for &ldquo;applications&rdquo; that use &ldquo;plugins&rdquo; in multiple
cannam@127 128 threads, where each &ldquo;plugin&rdquo; calls single-threaded FFTW but is unaware
cannam@127 129 of the other &ldquo;plugins&rdquo; doing the same thing at the same time. In this
cannam@127 130 case, the &ldquo;application&rdquo; calls <code>fftw_make_planner_thread_safe()</code>
cannam@127 131 at the beginning to protect &ldquo;plugins&rdquo; from each other.
cannam@127 132 </p><hr>
cannam@127 133 <div class="header">
cannam@127 134 <p>
cannam@127 135 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>
cannam@127 136 </div>
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cannam@127 138
cannam@127 139
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cannam@127 141 </html>