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Add null config files
author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 02 Mar 2020 14:03:47 +0000
parents bd3cc4d1df30
children
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cannam@167 3 <!-- This manual is for FFTW
cannam@167 4 (version 3.3.8, 24 May 2018).
cannam@167 5
cannam@167 6 Copyright (C) 2003 Matteo Frigo.
cannam@167 7
cannam@167 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@167 24 <head>
cannam@167 25 <title>FFTW 3.3.8: Caveats in Using Wisdom</title>
cannam@167 26
cannam@167 27 <meta name="description" content="FFTW 3.3.8: Caveats in Using Wisdom">
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cannam@167 38 <link href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" rel="prev" title="Words of Wisdom-Saving Plans">
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cannam@167 68 </head>
cannam@167 69
cannam@167 70 <body lang="en">
cannam@167 71 <a name="Caveats-in-Using-Wisdom"></a>
cannam@167 72 <div class="header">
cannam@167 73 <p>
cannam@167 74 Previous: <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" accesskey="p" rel="prev">Words of Wisdom-Saving Plans</a>, Up: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="u" rel="up">Other Important Topics</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>
cannam@167 75 </div>
cannam@167 76 <hr>
cannam@167 77 <a name="Caveats-in-Using-Wisdom-1"></a>
cannam@167 78 <h3 class="section">3.4 Caveats in Using Wisdom</h3>
cannam@167 79 <a name="index-wisdom_002c-problems-with"></a>
cannam@167 80
cannam@167 81 <blockquote>
cannam@167 82 <i>
cannam@167 83 <p>For in much wisdom is much grief, and he that increaseth knowledge
cannam@167 84 increaseth sorrow.
cannam@167 85 </i>
cannam@167 86 [Ecclesiastes 1:18]
cannam@167 87 <a name="index-Ecclesiastes"></a>
cannam@167 88 </p></blockquote>
cannam@167 89
cannam@167 90 <a name="index-portability-1"></a>
cannam@167 91 <p>There are pitfalls to using wisdom, in that it can negate FFTW&rsquo;s
cannam@167 92 ability to adapt to changing hardware and other conditions. For
cannam@167 93 example, it would be perfectly possible to export wisdom from a
cannam@167 94 program running on one processor and import it into a program running
cannam@167 95 on another processor. Doing so, however, would mean that the second
cannam@167 96 program would use plans optimized for the first processor, instead of
cannam@167 97 the one it is running on.
cannam@167 98 </p>
cannam@167 99 <p>It should be safe to reuse wisdom as long as the hardware and program
cannam@167 100 binaries remain unchanged. (Actually, the optimal plan may change even
cannam@167 101 between runs of the same binary on identical hardware, due to
cannam@167 102 differences in the virtual memory environment, etcetera. Users
cannam@167 103 seriously interested in performance should worry about this problem,
cannam@167 104 too.) It is likely that, if the same wisdom is used for two
cannam@167 105 different program binaries, even running on the same machine, the
cannam@167 106 plans may be sub-optimal because of differing code alignments. It is
cannam@167 107 therefore wise to recreate wisdom every time an application is
cannam@167 108 recompiled. The more the underlying hardware and software changes
cannam@167 109 between the creation of wisdom and its use, the greater grows
cannam@167 110 the risk of sub-optimal plans.
cannam@167 111 </p>
cannam@167 112 <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
cannam@167 113 using possibly-suboptimal wisdom (created on the same machine, but for a
cannam@167 114 different binary), the wisdom is likely to be better. For this reason,
cannam@167 115 we provide a function to import wisdom from a standard system-wide
cannam@167 116 location (<code>/etc/fftw/wisdom</code> on Unix):
cannam@167 117 <a name="index-wisdom_002c-system_002dwide"></a>
cannam@167 118 </p>
cannam@167 119 <div class="example">
cannam@167 120 <pre class="example">int fftw_import_system_wisdom(void);
cannam@167 121 </pre></div>
cannam@167 122 <a name="index-fftw_005fimport_005fsystem_005fwisdom"></a>
cannam@167 123
cannam@167 124 <p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
cannam@167 125 by its own <code>man</code> page on Unix) with which users can create wisdom,
cannam@167 126 e.g. for a canonical set of sizes to store in the system wisdom file.
cannam@167 127 See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
cannam@167 128 <a name="index-fftw_002dwisdom-utility"></a>
cannam@167 129 </p>
cannam@167 130 <hr>
cannam@167 131 <div class="header">
cannam@167 132 <p>
cannam@167 133 Previous: <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" accesskey="p" rel="prev">Words of Wisdom-Saving Plans</a>, Up: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="u" rel="up">Other Important Topics</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>
cannam@167 134 </div>
cannam@167 135
cannam@167 136
cannam@167 137
cannam@167 138 </body>
cannam@167 139 </html>