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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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2 <html>
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3 <!-- This manual is for FFTW
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4 (version 3.3.8, 24 May 2018).
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5
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6 Copyright (C) 2003 Matteo Frigo.
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7
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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9
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10 Permission is granted to make and distribute verbatim copies of this
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11 manual provided the copyright notice and this permission notice are
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12 preserved on all copies.
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13
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14 Permission is granted to copy and distribute modified versions of this
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15 manual under the conditions for verbatim copying, provided that the
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16 entire resulting derived work is distributed under the terms of a
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17 permission notice identical to this one.
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18
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19 Permission is granted to copy and distribute translations of this manual
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20 into another language, under the above conditions for modified versions,
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22 approved by the Free Software Foundation. -->
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23 <!-- Created by GNU Texinfo 6.3, http://www.gnu.org/software/texinfo/ -->
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24 <head>
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25 <title>FFTW 3.3.8: Usage of Multi-threaded FFTW</title>
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26
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27 <meta name="description" content="FFTW 3.3.8: Usage of Multi-threaded FFTW">
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33 <link href="index.html#Top" rel="start" title="Top">
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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36 <link href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" rel="up" title="Multi-threaded FFTW">
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37 <link href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" rel="next" title="How Many Threads to Use?">
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38 <link href="Installation-and-Supported-Hardware_002fSoftware.html#Installation-and-Supported-Hardware_002fSoftware" rel="prev" title="Installation and Supported Hardware/Software">
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64 -->
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65 </style>
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66
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67
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68 </head>
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69
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70 <body lang="en">
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71 <a name="Usage-of-Multi_002dthreaded-FFTW"></a>
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72 <div class="header">
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73 <p>
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74 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> [<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>
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75 </div>
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76 <hr>
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77 <a name="Usage-of-Multi_002dthreaded-FFTW-1"></a>
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78 <h3 class="section">5.2 Usage of Multi-threaded FFTW</h3>
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79
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80 <p>Here, it is assumed that the reader is already familiar with the usage
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81 of the uniprocessor FFTW routines, described elsewhere in this manual.
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82 We only describe what one has to change in order to use the
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83 multi-threaded routines.
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84 </p>
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85 <a name="index-OpenMP-1"></a>
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86 <p>First, programs using the parallel complex transforms should be linked
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87 with <code>-lfftw3_threads -lfftw3 -lm</code> on Unix, or <code>-lfftw3_omp
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88 -lfftw3 -lm</code> if you compiled with OpenMP. You will also need to link
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89 with whatever library is responsible for threads on your system
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90 (e.g. <code>-lpthread</code> on GNU/Linux) or include whatever compiler flag
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91 enables OpenMP (e.g. <code>-fopenmp</code> with gcc).
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92 <a name="index-linking-on-Unix"></a>
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93 </p>
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94
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95 <p>Second, before calling <em>any</em> FFTW routines, you should call the
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96 function:
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97 </p>
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98 <div class="example">
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99 <pre class="example">int fftw_init_threads(void);
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100 </pre></div>
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101 <a name="index-fftw_005finit_005fthreads"></a>
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102
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103 <p>This function, which need only be called once, performs any one-time
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104 initialization required to use threads on your system. It returns zero
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105 if there was some error (which should not happen under normal
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106 circumstances) and a non-zero value otherwise.
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107 </p>
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108 <p>Third, before creating a plan that you want to parallelize, you should
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109 call:
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110 </p>
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111 <div class="example">
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112 <pre class="example">void fftw_plan_with_nthreads(int nthreads);
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113 </pre></div>
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114 <a name="index-fftw_005fplan_005fwith_005fnthreads"></a>
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115
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116 <p>The <code>nthreads</code> argument indicates the number of threads you want
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117 FFTW to use (or actually, the maximum number). All plans subsequently
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118 created with any planner routine will use that many threads. You can
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119 call <code>fftw_plan_with_nthreads</code>, create some plans, call
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120 <code>fftw_plan_with_nthreads</code> again with a different argument, and
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121 create some more plans for a new number of threads. Plans already created
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122 before a call to <code>fftw_plan_with_nthreads</code> are unaffected. If you
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123 pass an <code>nthreads</code> argument of <code>1</code> (the default), threads are
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124 disabled for subsequent plans.
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125 </p>
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126 <a name="index-OpenMP-2"></a>
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127 <p>With OpenMP, to configure FFTW to use all of the currently running
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128 OpenMP threads (set by <code>omp_set_num_threads(nthreads)</code> or by the
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129 <code>OMP_NUM_THREADS</code> environment variable), you can do:
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130 <code>fftw_plan_with_nthreads(omp_get_max_threads())</code>. (The ‘<samp>omp_</samp>’
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131 OpenMP functions are declared via <code>#include <omp.h></code>.)
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132 </p>
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133 <a name="index-thread-safety"></a>
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134 <p>Given a plan, you then execute it as usual with
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135 <code>fftw_execute(plan)</code>, and the execution will use the number of
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136 threads specified when the plan was created. When done, you destroy
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137 it as usual with <code>fftw_destroy_plan</code>. As described in
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138 <a href="Thread-safety.html#Thread-safety">Thread safety</a>, plan <em>execution</em> is thread-safe, but plan
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139 creation and destruction are <em>not</em>: you should create/destroy
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140 plans only from a single thread, but can safely execute multiple plans
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141 in parallel.
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142 </p>
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143 <p>There is one additional routine: if you want to get rid of all memory
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144 and other resources allocated internally by FFTW, you can call:
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145 </p>
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146 <div class="example">
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147 <pre class="example">void fftw_cleanup_threads(void);
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148 </pre></div>
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149 <a name="index-fftw_005fcleanup_005fthreads"></a>
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150
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151 <p>which is much like the <code>fftw_cleanup()</code> function except that it
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152 also gets rid of threads-related data. You must <em>not</em> execute any
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153 previously created plans after calling this function.
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154 </p>
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155 <p>We should also mention one other restriction: if you save wisdom from a
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156 program using the multi-threaded FFTW, that wisdom <em>cannot be used</em>
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157 by a program using only the single-threaded FFTW (i.e. not calling
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158 <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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159 </p>
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160 <hr>
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161 <div class="header">
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162 <p>
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163 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> [<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>
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164 </div>
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165
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166
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167
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168 </body>
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169 </html>
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