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Current fftw source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:40:26 +0100
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cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
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cannam@127 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@127 25 <title>FFTW 3.3.5: Caveats in Using Wisdom</title>
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cannam@127 69 </head>
cannam@127 70
cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="Caveats-in-Using-Wisdom"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="Caveats-in-Using-Wisdom-1"></a>
cannam@127 79 <h3 class="section">3.4 Caveats in Using Wisdom</h3>
cannam@127 80 <a name="index-wisdom_002c-problems-with"></a>
cannam@127 81
cannam@127 82 <blockquote>
cannam@127 83 <i><p>For in much wisdom is much grief, and he that increaseth knowledge
cannam@127 84 increaseth sorrow.
cannam@127 85 </i>[Ecclesiastes 1:18]
cannam@127 86 <a name="index-Ecclesiastes"></a>
cannam@127 87 </p></blockquote>
cannam@127 88
cannam@127 89 <a name="index-portability-1"></a>
cannam@127 90 <p>There are pitfalls to using wisdom, in that it can negate FFTW&rsquo;s
cannam@127 91 ability to adapt to changing hardware and other conditions. For
cannam@127 92 example, it would be perfectly possible to export wisdom from a
cannam@127 93 program running on one processor and import it into a program running
cannam@127 94 on another processor. Doing so, however, would mean that the second
cannam@127 95 program would use plans optimized for the first processor, instead of
cannam@127 96 the one it is running on.
cannam@127 97 </p>
cannam@127 98 <p>It should be safe to reuse wisdom as long as the hardware and program
cannam@127 99 binaries remain unchanged. (Actually, the optimal plan may change even
cannam@127 100 between runs of the same binary on identical hardware, due to
cannam@127 101 differences in the virtual memory environment, etcetera. Users
cannam@127 102 seriously interested in performance should worry about this problem,
cannam@127 103 too.) It is likely that, if the same wisdom is used for two
cannam@127 104 different program binaries, even running on the same machine, the
cannam@127 105 plans may be sub-optimal because of differing code alignments. It is
cannam@127 106 therefore wise to recreate wisdom every time an application is
cannam@127 107 recompiled. The more the underlying hardware and software changes
cannam@127 108 between the creation of wisdom and its use, the greater grows
cannam@127 109 the risk of sub-optimal plans.
cannam@127 110 </p>
cannam@127 111 <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
cannam@127 112 using possibly-suboptimal wisdom (created on the same machine, but for a
cannam@127 113 different binary), the wisdom is likely to be better. For this reason,
cannam@127 114 we provide a function to import wisdom from a standard system-wide
cannam@127 115 location (<code>/etc/fftw/wisdom</code> on Unix):
cannam@127 116 <a name="index-wisdom_002c-system_002dwide"></a>
cannam@127 117 </p>
cannam@127 118 <div class="example">
cannam@127 119 <pre class="example">int fftw_import_system_wisdom(void);
cannam@127 120 </pre></div>
cannam@127 121 <a name="index-fftw_005fimport_005fsystem_005fwisdom"></a>
cannam@127 122
cannam@127 123 <p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
cannam@127 124 by its own <code>man</code> page on Unix) with which users can create wisdom,
cannam@127 125 e.g. for a canonical set of sizes to store in the system wisdom file.
cannam@127 126 See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
cannam@127 127 <a name="index-fftw_002dwisdom-utility"></a>
cannam@127 128 </p>
cannam@127 129 <hr>
cannam@127 130 <div class="header">
cannam@127 131 <p>
cannam@127 132 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>
cannam@127 133 </div>
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