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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 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Chris@42 2 <html>
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: Caveats in Using Wisdom</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: Caveats in Using Wisdom">
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Chris@42 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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Chris@42 69 </head>
Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Caveats-in-Using-Wisdom"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Caveats-in-Using-Wisdom-1"></a>
Chris@42 79 <h3 class="section">3.4 Caveats in Using Wisdom</h3>
Chris@42 80 <a name="index-wisdom_002c-problems-with"></a>
Chris@42 81
Chris@42 82 <blockquote>
Chris@42 83 <i><p>For in much wisdom is much grief, and he that increaseth knowledge
Chris@42 84 increaseth sorrow.
Chris@42 85 </i>[Ecclesiastes 1:18]
Chris@42 86 <a name="index-Ecclesiastes"></a>
Chris@42 87 </p></blockquote>
Chris@42 88
Chris@42 89 <a name="index-portability-1"></a>
Chris@42 90 <p>There are pitfalls to using wisdom, in that it can negate FFTW&rsquo;s
Chris@42 91 ability to adapt to changing hardware and other conditions. For
Chris@42 92 example, it would be perfectly possible to export wisdom from a
Chris@42 93 program running on one processor and import it into a program running
Chris@42 94 on another processor. Doing so, however, would mean that the second
Chris@42 95 program would use plans optimized for the first processor, instead of
Chris@42 96 the one it is running on.
Chris@42 97 </p>
Chris@42 98 <p>It should be safe to reuse wisdom as long as the hardware and program
Chris@42 99 binaries remain unchanged. (Actually, the optimal plan may change even
Chris@42 100 between runs of the same binary on identical hardware, due to
Chris@42 101 differences in the virtual memory environment, etcetera. Users
Chris@42 102 seriously interested in performance should worry about this problem,
Chris@42 103 too.) It is likely that, if the same wisdom is used for two
Chris@42 104 different program binaries, even running on the same machine, the
Chris@42 105 plans may be sub-optimal because of differing code alignments. It is
Chris@42 106 therefore wise to recreate wisdom every time an application is
Chris@42 107 recompiled. The more the underlying hardware and software changes
Chris@42 108 between the creation of wisdom and its use, the greater grows
Chris@42 109 the risk of sub-optimal plans.
Chris@42 110 </p>
Chris@42 111 <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
Chris@42 112 using possibly-suboptimal wisdom (created on the same machine, but for a
Chris@42 113 different binary), the wisdom is likely to be better. For this reason,
Chris@42 114 we provide a function to import wisdom from a standard system-wide
Chris@42 115 location (<code>/etc/fftw/wisdom</code> on Unix):
Chris@42 116 <a name="index-wisdom_002c-system_002dwide"></a>
Chris@42 117 </p>
Chris@42 118 <div class="example">
Chris@42 119 <pre class="example">int fftw_import_system_wisdom(void);
Chris@42 120 </pre></div>
Chris@42 121 <a name="index-fftw_005fimport_005fsystem_005fwisdom"></a>
Chris@42 122
Chris@42 123 <p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
Chris@42 124 by its own <code>man</code> page on Unix) with which users can create wisdom,
Chris@42 125 e.g. for a canonical set of sizes to store in the system wisdom file.
Chris@42 126 See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
Chris@42 127 <a name="index-fftw_002dwisdom-utility"></a>
Chris@42 128 </p>
Chris@42 129 <hr>
Chris@42 130 <div class="header">
Chris@42 131 <p>
Chris@42 132 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>
Chris@42 133 </div>
Chris@42 134
Chris@42 135
Chris@42 136
Chris@42 137 </body>
Chris@42 138 </html>