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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 19 Nov 2019 14:52:55 +0000
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3 <!-- This manual is for FFTW
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6 Copyright (C) 2003 Matteo Frigo.
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25 <title>FFTW 3.3.8: Caveats in Using Wisdom</title>
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71 <a name="Caveats-in-Using-Wisdom"></a>
72 <div class="header">
73 <p>
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>
75 </div>
76 <hr>
77 <a name="Caveats-in-Using-Wisdom-1"></a>
78 <h3 class="section">3.4 Caveats in Using Wisdom</h3>
79 <a name="index-wisdom_002c-problems-with"></a>
80
81 <blockquote>
82 <i>
83 <p>For in much wisdom is much grief, and he that increaseth knowledge
84 increaseth sorrow.
85 </i>
86 [Ecclesiastes 1:18]
87 <a name="index-Ecclesiastes"></a>
88 </p></blockquote>
89
90 <a name="index-portability-1"></a>
91 <p>There are pitfalls to using wisdom, in that it can negate FFTW&rsquo;s
92 ability to adapt to changing hardware and other conditions. For
93 example, it would be perfectly possible to export wisdom from a
94 program running on one processor and import it into a program running
95 on another processor. Doing so, however, would mean that the second
96 program would use plans optimized for the first processor, instead of
97 the one it is running on.
98 </p>
99 <p>It should be safe to reuse wisdom as long as the hardware and program
100 binaries remain unchanged. (Actually, the optimal plan may change even
101 between runs of the same binary on identical hardware, due to
102 differences in the virtual memory environment, etcetera. Users
103 seriously interested in performance should worry about this problem,
104 too.) It is likely that, if the same wisdom is used for two
105 different program binaries, even running on the same machine, the
106 plans may be sub-optimal because of differing code alignments. It is
107 therefore wise to recreate wisdom every time an application is
108 recompiled. The more the underlying hardware and software changes
109 between the creation of wisdom and its use, the greater grows
110 the risk of sub-optimal plans.
111 </p>
112 <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
113 using possibly-suboptimal wisdom (created on the same machine, but for a
114 different binary), the wisdom is likely to be better. For this reason,
115 we provide a function to import wisdom from a standard system-wide
116 location (<code>/etc/fftw/wisdom</code> on Unix):
117 <a name="index-wisdom_002c-system_002dwide"></a>
118 </p>
119 <div class="example">
120 <pre class="example">int fftw_import_system_wisdom(void);
121 </pre></div>
122 <a name="index-fftw_005fimport_005fsystem_005fwisdom"></a>
123
124 <p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
125 by its own <code>man</code> page on Unix) with which users can create wisdom,
126 e.g. for a canonical set of sizes to store in the system wisdom file.
127 See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
128 <a name="index-fftw_002dwisdom-utility"></a>
129 </p>
130 <hr>
131 <div class="header">
132 <p>
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>
134 </div>
135
136
137
138 </body>
139 </html>