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