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128 lines
5.7 KiB
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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<title>Caveats in Using Wisdom (FFTW 3.3.10)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<span id="Caveats-in-Using-Wisdom"></span><div class="header">
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<p>
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Previous: <a href="Words-of-Wisdom_002dSaving-Plans.html" accesskey="p" rel="prev">Words of Wisdom-Saving Plans</a>, Up: <a href="Other-Important-Topics.html" accesskey="u" rel="up">Other Important Topics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Caveats-in-Using-Wisdom-1"></span><h3 class="section">3.4 Caveats in Using Wisdom</h3>
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<span id="index-wisdom_002c-problems-with"></span>
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<blockquote>
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<i>
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<p>For in much wisdom is much grief, and he that increaseth knowledge
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increaseth sorrow.
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</i>
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[Ecclesiastes 1:18]
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<span id="index-Ecclesiastes"></span>
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</p></blockquote>
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<span id="index-portability-1"></span>
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<p>There are pitfalls to using wisdom, in that it can negate FFTW’s
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ability to adapt to changing hardware and other conditions. For
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example, it would be perfectly possible to export wisdom from a
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program running on one processor and import it into a program running
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on another processor. Doing so, however, would mean that the second
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program would use plans optimized for the first processor, instead of
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the one it is running on.
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</p>
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<p>It should be safe to reuse wisdom as long as the hardware and program
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binaries remain unchanged. (Actually, the optimal plan may change even
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between runs of the same binary on identical hardware, due to
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differences in the virtual memory environment, etcetera. Users
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seriously interested in performance should worry about this problem,
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too.) It is likely that, if the same wisdom is used for two
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different program binaries, even running on the same machine, the
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plans may be sub-optimal because of differing code alignments. It is
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therefore wise to recreate wisdom every time an application is
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recompiled. The more the underlying hardware and software changes
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between the creation of wisdom and its use, the greater grows
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the risk of sub-optimal plans.
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</p>
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<p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
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using possibly-suboptimal wisdom (created on the same machine, but for a
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different binary), the wisdom is likely to be better. For this reason,
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we provide a function to import wisdom from a standard system-wide
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location (<code>/etc/fftw/wisdom</code> on Unix):
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<span id="index-wisdom_002c-system_002dwide"></span>
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</p>
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<div class="example">
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<pre class="example">int fftw_import_system_wisdom(void);
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</pre></div>
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<span id="index-fftw_005fimport_005fsystem_005fwisdom"></span>
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<p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
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by its own <code>man</code> page on Unix) with which users can create wisdom,
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e.g. for a canonical set of sizes to store in the system wisdom file.
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See <a href="Wisdom-Utilities.html">Wisdom Utilities</a>.
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<span id="index-fftw_002dwisdom-utility"></span>
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</p>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Words-of-Wisdom_002dSaving-Plans.html" accesskey="p" rel="prev">Words of Wisdom-Saving Plans</a>, Up: <a href="Other-Important-Topics.html" accesskey="u" rel="up">Other Important Topics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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