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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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3 <!-- This manual is for FFTW
4 (version 3.3.5, 30 July 2016).
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6 Copyright (C) 2003 Matteo Frigo.
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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25 <title>FFTW 3.3.5: Thread safety</title>
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72 <a name="Thread-safety"></a>
73 <div class="header">
74 <p>
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>
76 </div>
77 <hr>
78 <a name="Thread-safety-1"></a>
79 <h3 class="section">5.4 Thread safety</h3>
80
81 <a name="index-threads-1"></a>
82 <a name="index-OpenMP-3"></a>
83 <a name="index-thread-safety-1"></a>
84 <p>Users writing multi-threaded programs (including OpenMP) must concern
85 themselves with the <em>thread safety</em> of the libraries they
86 use&mdash;that is, whether it is safe to call routines in parallel from
87 multiple threads. FFTW can be used in such an environment, but some
88 care must be taken because the planner routines share data
89 (e.g. wisdom and trigonometric tables) between calls and plans.
90 </p>
91 <p>The upshot is that the only thread-safe (re-entrant) routine in FFTW is
92 <code>fftw_execute</code> (and the new-array variants thereof). All other routines
93 (e.g. the planner) should only be called from one thread at a time. So,
94 for example, you can wrap a semaphore lock around any calls to the
95 planner; even more simply, you can just create all of your plans from
96 one thread. We do not think this should be an important restriction
97 (FFTW is designed for the situation where the only performance-sensitive
98 code is the actual execution of the transform), and the benefits of
99 shared data between plans are great.
100 </p>
101 <p>Note also that, since the plan is not modified by <code>fftw_execute</code>,
102 it is safe to execute the <em>same plan</em> in parallel by multiple
103 threads. However, since a given plan operates by default on a fixed
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.
105 </p>
106 <p>(Users should note that these comments only apply to programs using
107 shared-memory threads or OpenMP. Parallelism using MPI or forked processes
108 involves a separate address-space and global variables for each process,
109 and is not susceptible to problems of this sort.)
110 </p>
111 <p>If you are configured FFTW with the <code>--enable-debug</code> or
112 <code>--enable-debug-malloc</code> flags (see <a href="Installation-on-Unix.html#Installation-on-Unix">Installation on Unix</a>),
113 then <code>fftw_execute</code> is not thread-safe. These flags are not
114 documented because they are intended only for developing
115 and debugging FFTW, but if you must use <code>--enable-debug</code> then you
116 should also specifically pass <code>--disable-debug-malloc</code> for
117 <code>fftw_execute</code> to be thread-safe.
118 </p>
119 <p>Starting from FFTW-3.3.5, FFTW supports a new API to make the
120 planner thread-safe:
121 </p><div class="example">
122 <pre class="example">void fftw_make_planner_thread_safe(void);
123 </pre></div>
124 <a name="index-fftw_005fmake_005fplanner_005fthread_005fsafe"></a>
125
126 <p>This call installs a hook that wraps a lock around all planner calls.
127 This API is meant for &ldquo;applications&rdquo; that use &ldquo;plugins&rdquo; in multiple
128 threads, where each &ldquo;plugin&rdquo; calls single-threaded FFTW but is unaware
129 of the other &ldquo;plugins&rdquo; doing the same thing at the same time. In this
130 case, the &ldquo;application&rdquo; calls <code>fftw_make_planner_thread_safe()</code>
131 at the beginning to protect &ldquo;plugins&rdquo; from each other.
132 </p><hr>
133 <div class="header">
134 <p>
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>
136 </div>
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