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2 <head>
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3 <title>Usage of Multi-threaded FFTW - FFTW 3.3.3</title>
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5 <meta name="description" content="FFTW 3.3.3">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" title="Multi-threaded FFTW">
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9 <link rel="prev" href="Installation-and-Supported-Hardware_002fSoftware.html#Installation-and-Supported-Hardware_002fSoftware" title="Installation and Supported Hardware/Software">
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10 <link rel="next" href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" title="How Many Threads to Use?">
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11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.3, 25 November 2012).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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23
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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45 --></style>
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46 </head>
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47 <body>
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48 <div class="node">
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49 <a name="Usage-of-Multi-threaded-FFTW"></a>
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50 <a name="Usage-of-Multi_002dthreaded-FFTW"></a>
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51 <p>
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52 Next: <a rel="next" accesskey="n" href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f">How Many Threads to Use?</a>,
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53 Previous: <a rel="previous" accesskey="p" href="Installation-and-Supported-Hardware_002fSoftware.html#Installation-and-Supported-Hardware_002fSoftware">Installation and Supported Hardware/Software</a>,
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54 Up: <a rel="up" accesskey="u" href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a>
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55 <hr>
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56 </div>
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57
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58 <h3 class="section">5.2 Usage of Multi-threaded FFTW</h3>
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59
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60 <p>Here, it is assumed that the reader is already familiar with the usage
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61 of the uniprocessor FFTW routines, described elsewhere in this manual.
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62 We only describe what one has to change in order to use the
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63 multi-threaded routines.
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64
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65 <p><a name="index-OpenMP-332"></a>First, programs using the parallel complex transforms should be linked
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66 with <code>-lfftw3_threads -lfftw3 -lm</code> on Unix, or <code>-lfftw3_omp
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67 -lfftw3 -lm</code> if you compiled with OpenMP. You will also need to link
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68 with whatever library is responsible for threads on your system
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69 (e.g. <code>-lpthread</code> on GNU/Linux) or include whatever compiler flag
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70 enables OpenMP (e.g. <code>-fopenmp</code> with gcc).
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71 <a name="index-linking-on-Unix-333"></a>
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72
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73 <p>Second, before calling <em>any</em> FFTW routines, you should call the
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74 function:
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75
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76 <pre class="example"> int fftw_init_threads(void);
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77 </pre>
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78 <p><a name="index-fftw_005finit_005fthreads-334"></a>
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79 This function, which need only be called once, performs any one-time
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80 initialization required to use threads on your system. It returns zero
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81 if there was some error (which should not happen under normal
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82 circumstances) and a non-zero value otherwise.
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83
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84 <p>Third, before creating a plan that you want to parallelize, you should
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85 call:
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86
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87 <pre class="example"> void fftw_plan_with_nthreads(int nthreads);
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88 </pre>
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89 <p><a name="index-fftw_005fplan_005fwith_005fnthreads-335"></a>
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90 The <code>nthreads</code> argument indicates the number of threads you want
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91 FFTW to use (or actually, the maximum number). All plans subsequently
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92 created with any planner routine will use that many threads. You can
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93 call <code>fftw_plan_with_nthreads</code>, create some plans, call
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94 <code>fftw_plan_with_nthreads</code> again with a different argument, and
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95 create some more plans for a new number of threads. Plans already created
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96 before a call to <code>fftw_plan_with_nthreads</code> are unaffected. If you
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97 pass an <code>nthreads</code> argument of <code>1</code> (the default), threads are
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98 disabled for subsequent plans.
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99
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100 <p><a name="index-OpenMP-336"></a>With OpenMP, to configure FFTW to use all of the currently running
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101 OpenMP threads (set by <code>omp_set_num_threads(nthreads)</code> or by the
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102 <code>OMP_NUM_THREADS</code> environment variable), you can do:
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103 <code>fftw_plan_with_nthreads(omp_get_max_threads())</code>. (The ‘<samp><span class="samp">omp_</span></samp>’
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104 OpenMP functions are declared via <code>#include <omp.h></code>.)
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105
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106 <p><a name="index-thread-safety-337"></a>Given a plan, you then execute it as usual with
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107 <code>fftw_execute(plan)</code>, and the execution will use the number of
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108 threads specified when the plan was created. When done, you destroy
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109 it as usual with <code>fftw_destroy_plan</code>. As described in
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110 <a href="Thread-safety.html#Thread-safety">Thread safety</a>, plan <em>execution</em> is thread-safe, but plan
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111 creation and destruction are <em>not</em>: you should create/destroy
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112 plans only from a single thread, but can safely execute multiple plans
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113 in parallel.
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114
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115 <p>There is one additional routine: if you want to get rid of all memory
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116 and other resources allocated internally by FFTW, you can call:
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117
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118 <pre class="example"> void fftw_cleanup_threads(void);
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119 </pre>
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120 <p><a name="index-fftw_005fcleanup_005fthreads-338"></a>
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121 which is much like the <code>fftw_cleanup()</code> function except that it
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122 also gets rid of threads-related data. You must <em>not</em> execute any
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123 previously created plans after calling this function.
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124
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125 <p>We should also mention one other restriction: if you save wisdom from a
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126 program using the multi-threaded FFTW, that wisdom <em>cannot be used</em>
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127 by a program using only the single-threaded FFTW (i.e. not calling
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128 <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>.
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129
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130 <!-- -->
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131 </body></html>
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132
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