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Chris@82: <title>FFTW 3.3.8: Caveats in Using Wisdom</title>
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Chris@82: <a name="Caveats-in-Using-Wisdom"></a>
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Chris@82: 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> [<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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Chris@82: <hr>
Chris@82: <a name="Caveats-in-Using-Wisdom-1"></a>
Chris@82: <h3 class="section">3.4 Caveats in Using Wisdom</h3>
Chris@82: <a name="index-wisdom_002c-problems-with"></a>
Chris@82:
Chris@82: <blockquote>
Chris@82: <i>
Chris@82: <p>For in much wisdom is much grief, and he that increaseth knowledge
Chris@82: increaseth sorrow.
Chris@82: </i>
Chris@82: [Ecclesiastes 1:18]
Chris@82: <a name="index-Ecclesiastes"></a>
Chris@82: </p></blockquote>
Chris@82:
Chris@82: <a name="index-portability-1"></a>
Chris@82: <p>There are pitfalls to using wisdom, in that it can negate FFTW’s
Chris@82: ability to adapt to changing hardware and other conditions. For
Chris@82: example, it would be perfectly possible to export wisdom from a
Chris@82: program running on one processor and import it into a program running
Chris@82: on another processor. Doing so, however, would mean that the second
Chris@82: program would use plans optimized for the first processor, instead of
Chris@82: the one it is running on.
Chris@82: </p>
Chris@82: <p>It should be safe to reuse wisdom as long as the hardware and program
Chris@82: binaries remain unchanged. (Actually, the optimal plan may change even
Chris@82: between runs of the same binary on identical hardware, due to
Chris@82: differences in the virtual memory environment, etcetera. Users
Chris@82: seriously interested in performance should worry about this problem,
Chris@82: too.) It is likely that, if the same wisdom is used for two
Chris@82: different program binaries, even running on the same machine, the
Chris@82: plans may be sub-optimal because of differing code alignments. It is
Chris@82: therefore wise to recreate wisdom every time an application is
Chris@82: recompiled. The more the underlying hardware and software changes
Chris@82: between the creation of wisdom and its use, the greater grows
Chris@82: the risk of sub-optimal plans.
Chris@82: </p>
Chris@82: <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
Chris@82: using possibly-suboptimal wisdom (created on the same machine, but for a
Chris@82: different binary), the wisdom is likely to be better. For this reason,
Chris@82: we provide a function to import wisdom from a standard system-wide
Chris@82: location (<code>/etc/fftw/wisdom</code> on Unix):
Chris@82: <a name="index-wisdom_002c-system_002dwide"></a>
Chris@82: </p>
Chris@82: <div class="example">
Chris@82: <pre class="example">int fftw_import_system_wisdom(void);
Chris@82: </pre></div>
Chris@82: <a name="index-fftw_005fimport_005fsystem_005fwisdom"></a>
Chris@82:
Chris@82: <p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
Chris@82: by its own <code>man</code> page on Unix) with which users can create wisdom,
Chris@82: e.g. for a canonical set of sizes to store in the system wisdom file.
Chris@82: See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
Chris@82: <a name="index-fftw_002dwisdom-utility"></a>
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