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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 2cd0e3b3e1fd
children
rev   line source
Chris@42 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Caveats in Using Wisdom</title>
Chris@42 26
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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>