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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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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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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: How Many Threads to Use?</title>
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26
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27 <meta name="description" content="FFTW 3.3.8: How Many Threads to Use?">
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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="Thread-safety.html#Thread-safety" rel="next" title="Thread safety">
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38 <link href="Usage-of-Multi_002dthreaded-FFTW.html#Usage-of-Multi_002dthreaded-FFTW" rel="prev" title="Usage of Multi-threaded FFTW">
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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="How-Many-Threads-to-Use_003f"></a>
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72 <div class="header">
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73 <p>
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74 Next: <a href="Thread-safety.html#Thread-safety" accesskey="n" rel="next">Thread safety</a>, Previous: <a href="Usage-of-Multi_002dthreaded-FFTW.html#Usage-of-Multi_002dthreaded-FFTW" accesskey="p" rel="prev">Usage of Multi-threaded FFTW</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="How-Many-Threads-to-Use_003f-1"></a>
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78 <h3 class="section">5.3 How Many Threads to Use?</h3>
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79
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80 <a name="index-number-of-threads"></a>
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81 <p>There is a fair amount of overhead involved in synchronizing threads,
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82 so the optimal number of threads to use depends upon the size of the
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83 transform as well as on the number of processors you have.
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84 </p>
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85 <p>As a general rule, you don’t want to use more threads than you have
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86 processors. (Using more threads will work, but there will be extra
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87 overhead with no benefit.) In fact, if the problem size is too small,
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88 you may want to use fewer threads than you have processors.
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89 </p>
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90 <p>You will have to experiment with your system to see what level of
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91 parallelization is best for your problem size. Typically, the problem
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92 will have to involve at least a few thousand data points before threads
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93 become beneficial. If you plan with <code>FFTW_PATIENT</code>, it will
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94 automatically disable threads for sizes that don’t benefit from
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95 parallelization.
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96 <a name="index-FFTW_005fPATIENT-3"></a>
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97 </p>
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98
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99
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100
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101 </body>
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102 </html>
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