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cannam@167: <title>FFTW 3.3.8: Caveats in Using Wisdom</title>
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cannam@167: <a name="Caveats-in-Using-Wisdom"></a>
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cannam@167: <p>
cannam@167: 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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cannam@167: <hr>
cannam@167: <a name="Caveats-in-Using-Wisdom-1"></a>
cannam@167: <h3 class="section">3.4 Caveats in Using Wisdom</h3>
cannam@167: <a name="index-wisdom_002c-problems-with"></a>
cannam@167:
cannam@167: <blockquote>
cannam@167: <i>
cannam@167: <p>For in much wisdom is much grief, and he that increaseth knowledge
cannam@167: increaseth sorrow.
cannam@167: </i>
cannam@167: [Ecclesiastes 1:18]
cannam@167: <a name="index-Ecclesiastes"></a>
cannam@167: </p></blockquote>
cannam@167:
cannam@167: <a name="index-portability-1"></a>
cannam@167: <p>There are pitfalls to using wisdom, in that it can negate FFTW’s
cannam@167: ability to adapt to changing hardware and other conditions. For
cannam@167: example, it would be perfectly possible to export wisdom from a
cannam@167: program running on one processor and import it into a program running
cannam@167: on another processor. Doing so, however, would mean that the second
cannam@167: program would use plans optimized for the first processor, instead of
cannam@167: the one it is running on.
cannam@167: </p>
cannam@167: <p>It should be safe to reuse wisdom as long as the hardware and program
cannam@167: binaries remain unchanged. (Actually, the optimal plan may change even
cannam@167: between runs of the same binary on identical hardware, due to
cannam@167: differences in the virtual memory environment, etcetera. Users
cannam@167: seriously interested in performance should worry about this problem,
cannam@167: too.) It is likely that, if the same wisdom is used for two
cannam@167: different program binaries, even running on the same machine, the
cannam@167: plans may be sub-optimal because of differing code alignments. It is
cannam@167: therefore wise to recreate wisdom every time an application is
cannam@167: recompiled. The more the underlying hardware and software changes
cannam@167: between the creation of wisdom and its use, the greater grows
cannam@167: the risk of sub-optimal plans.
cannam@167: </p>
cannam@167: <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
cannam@167: using possibly-suboptimal wisdom (created on the same machine, but for a
cannam@167: different binary), the wisdom is likely to be better. For this reason,
cannam@167: we provide a function to import wisdom from a standard system-wide
cannam@167: location (<code>/etc/fftw/wisdom</code> on Unix):
cannam@167: <a name="index-wisdom_002c-system_002dwide"></a>
cannam@167: </p>
cannam@167: <div class="example">
cannam@167: <pre class="example">int fftw_import_system_wisdom(void);
cannam@167: </pre></div>
cannam@167: <a name="index-fftw_005fimport_005fsystem_005fwisdom"></a>
cannam@167:
cannam@167: <p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
cannam@167: by its own <code>man</code> page on Unix) with which users can create wisdom,
cannam@167: e.g. for a canonical set of sizes to store in the system wisdom file.
cannam@167: See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
cannam@167: <a name="index-fftw_002dwisdom-utility"></a>
cannam@167: </p>
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cannam@167: 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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