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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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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.
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Chris@42 10 Permission is granted to make and distribute verbatim copies of this
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Usage of Multi-threaded FFTW</title>
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Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Usage-of-Multi_002dthreaded-FFTW"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" accesskey="n" rel="next">How Many Threads to Use?</a>, Previous: <a href="Installation-and-Supported-Hardware_002fSoftware.html#Installation-and-Supported-Hardware_002fSoftware" accesskey="p" rel="prev">Installation and Supported Hardware/Software</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="Usage-of-Multi_002dthreaded-FFTW-1"></a>
Chris@42 79 <h3 class="section">5.2 Usage of Multi-threaded FFTW</h3>
Chris@42 80
Chris@42 81 <p>Here, it is assumed that the reader is already familiar with the usage
Chris@42 82 of the uniprocessor FFTW routines, described elsewhere in this manual.
Chris@42 83 We only describe what one has to change in order to use the
Chris@42 84 multi-threaded routines.
Chris@42 85 </p>
Chris@42 86 <a name="index-OpenMP-1"></a>
Chris@42 87 <p>First, programs using the parallel complex transforms should be linked
Chris@42 88 with <code>-lfftw3_threads -lfftw3 -lm</code> on Unix, or <code>-lfftw3_omp
Chris@42 89 -lfftw3 -lm</code> if you compiled with OpenMP. You will also need to link
Chris@42 90 with whatever library is responsible for threads on your system
Chris@42 91 (e.g. <code>-lpthread</code> on GNU/Linux) or include whatever compiler flag
Chris@42 92 enables OpenMP (e.g. <code>-fopenmp</code> with gcc).
Chris@42 93 <a name="index-linking-on-Unix"></a>
Chris@42 94 </p>
Chris@42 95
Chris@42 96 <p>Second, before calling <em>any</em> FFTW routines, you should call the
Chris@42 97 function:
Chris@42 98 </p>
Chris@42 99 <div class="example">
Chris@42 100 <pre class="example">int fftw_init_threads(void);
Chris@42 101 </pre></div>
Chris@42 102 <a name="index-fftw_005finit_005fthreads"></a>
Chris@42 103
Chris@42 104 <p>This function, which need only be called once, performs any one-time
Chris@42 105 initialization required to use threads on your system. It returns zero
Chris@42 106 if there was some error (which should not happen under normal
Chris@42 107 circumstances) and a non-zero value otherwise.
Chris@42 108 </p>
Chris@42 109 <p>Third, before creating a plan that you want to parallelize, you should
Chris@42 110 call:
Chris@42 111 </p>
Chris@42 112 <div class="example">
Chris@42 113 <pre class="example">void fftw_plan_with_nthreads(int nthreads);
Chris@42 114 </pre></div>
Chris@42 115 <a name="index-fftw_005fplan_005fwith_005fnthreads"></a>
Chris@42 116
Chris@42 117 <p>The <code>nthreads</code> argument indicates the number of threads you want
Chris@42 118 FFTW to use (or actually, the maximum number). All plans subsequently
Chris@42 119 created with any planner routine will use that many threads. You can
Chris@42 120 call <code>fftw_plan_with_nthreads</code>, create some plans, call
Chris@42 121 <code>fftw_plan_with_nthreads</code> again with a different argument, and
Chris@42 122 create some more plans for a new number of threads. Plans already created
Chris@42 123 before a call to <code>fftw_plan_with_nthreads</code> are unaffected. If you
Chris@42 124 pass an <code>nthreads</code> argument of <code>1</code> (the default), threads are
Chris@42 125 disabled for subsequent plans.
Chris@42 126 </p>
Chris@42 127 <a name="index-OpenMP-2"></a>
Chris@42 128 <p>With OpenMP, to configure FFTW to use all of the currently running
Chris@42 129 OpenMP threads (set by <code>omp_set_num_threads(nthreads)</code> or by the
Chris@42 130 <code>OMP_NUM_THREADS</code> environment variable), you can do:
Chris@42 131 <code>fftw_plan_with_nthreads(omp_get_max_threads())</code>. (The &lsquo;<samp>omp_</samp>&rsquo;
Chris@42 132 OpenMP functions are declared via <code>#include &lt;omp.h&gt;</code>.)
Chris@42 133 </p>
Chris@42 134 <a name="index-thread-safety"></a>
Chris@42 135 <p>Given a plan, you then execute it as usual with
Chris@42 136 <code>fftw_execute(plan)</code>, and the execution will use the number of
Chris@42 137 threads specified when the plan was created. When done, you destroy
Chris@42 138 it as usual with <code>fftw_destroy_plan</code>. As described in
Chris@42 139 <a href="Thread-safety.html#Thread-safety">Thread safety</a>, plan <em>execution</em> is thread-safe, but plan
Chris@42 140 creation and destruction are <em>not</em>: you should create/destroy
Chris@42 141 plans only from a single thread, but can safely execute multiple plans
Chris@42 142 in parallel.
Chris@42 143 </p>
Chris@42 144 <p>There is one additional routine: if you want to get rid of all memory
Chris@42 145 and other resources allocated internally by FFTW, you can call:
Chris@42 146 </p>
Chris@42 147 <div class="example">
Chris@42 148 <pre class="example">void fftw_cleanup_threads(void);
Chris@42 149 </pre></div>
Chris@42 150 <a name="index-fftw_005fcleanup_005fthreads"></a>
Chris@42 151
Chris@42 152 <p>which is much like the <code>fftw_cleanup()</code> function except that it
Chris@42 153 also gets rid of threads-related data. You must <em>not</em> execute any
Chris@42 154 previously created plans after calling this function.
Chris@42 155 </p>
Chris@42 156 <p>We should also mention one other restriction: if you save wisdom from a
Chris@42 157 program using the multi-threaded FFTW, that wisdom <em>cannot be used</em>
Chris@42 158 by a program using only the single-threaded FFTW (i.e. not calling
Chris@42 159 <code>fftw_init_threads</code>). See <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>.
Chris@42 160 </p>
Chris@42 161 <hr>
Chris@42 162 <div class="header">
Chris@42 163 <p>
Chris@42 164 Next: <a href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" accesskey="n" rel="next">How Many Threads to Use?</a>, Previous: <a href="Installation-and-Supported-Hardware_002fSoftware.html#Installation-and-Supported-Hardware_002fSoftware" accesskey="p" rel="prev">Installation and Supported Hardware/Software</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@42 165 </div>
Chris@42 166
Chris@42 167
Chris@42 168
Chris@42 169 </body>
Chris@42 170 </html>