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author Chris Cannam
date Wed, 07 Sep 2016 10:40:32 +0100
parents 26056e866c29
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Chris@19 1 <html lang="en">
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Chris@19 3 <title>Caveats in Using Wisdom - FFTW 3.3.4</title>
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Chris@19 48 <a name="Caveats-in-Using-Wisdom"></a>
Chris@19 49 <p>
Chris@19 50 Previous:&nbsp;<a rel="previous" accesskey="p" href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>,
Chris@19 51 Up:&nbsp;<a rel="up" accesskey="u" href="Other-Important-Topics.html#Other-Important-Topics">Other Important Topics</a>
Chris@19 52 <hr>
Chris@19 53 </div>
Chris@19 54
Chris@19 55 <h3 class="section">3.4 Caveats in Using Wisdom</h3>
Chris@19 56
Chris@19 57 <p><a name="index-wisdom_002c-problems-with-133"></a>
Chris@19 58 <blockquote>
Chris@19 59 <i>For in much wisdom is much grief, and he that increaseth knowledge
Chris@19 60 increaseth sorrow.
Chris@19 61 </i>[Ecclesiastes 1:18]
Chris@19 62 <a name="index-Ecclesiastes-134"></a></blockquote>
Chris@19 63
Chris@19 64 <p><a name="index-portability-135"></a>There are pitfalls to using wisdom, in that it can negate FFTW's
Chris@19 65 ability to adapt to changing hardware and other conditions. For
Chris@19 66 example, it would be perfectly possible to export wisdom from a
Chris@19 67 program running on one processor and import it into a program running
Chris@19 68 on another processor. Doing so, however, would mean that the second
Chris@19 69 program would use plans optimized for the first processor, instead of
Chris@19 70 the one it is running on.
Chris@19 71
Chris@19 72 <p>It should be safe to reuse wisdom as long as the hardware and program
Chris@19 73 binaries remain unchanged. (Actually, the optimal plan may change even
Chris@19 74 between runs of the same binary on identical hardware, due to
Chris@19 75 differences in the virtual memory environment, etcetera. Users
Chris@19 76 seriously interested in performance should worry about this problem,
Chris@19 77 too.) It is likely that, if the same wisdom is used for two
Chris@19 78 different program binaries, even running on the same machine, the
Chris@19 79 plans may be sub-optimal because of differing code alignments. It is
Chris@19 80 therefore wise to recreate wisdom every time an application is
Chris@19 81 recompiled. The more the underlying hardware and software changes
Chris@19 82 between the creation of wisdom and its use, the greater grows
Chris@19 83 the risk of sub-optimal plans.
Chris@19 84
Chris@19 85 <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
Chris@19 86 using possibly-suboptimal wisdom (created on the same machine, but for a
Chris@19 87 different binary), the wisdom is likely to be better. For this reason,
Chris@19 88 we provide a function to import wisdom from a standard system-wide
Chris@19 89 location (<code>/etc/fftw/wisdom</code> on Unix):
Chris@19 90 <a name="index-wisdom_002c-system_002dwide-136"></a>
Chris@19 91 <pre class="example"> int fftw_import_system_wisdom(void);
Chris@19 92 </pre>
Chris@19 93 <p><a name="index-fftw_005fimport_005fsystem_005fwisdom-137"></a>
Chris@19 94 FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
Chris@19 95 by its own <code>man</code> page on Unix) with which users can create wisdom,
Chris@19 96 e.g. for a canonical set of sizes to store in the system wisdom file.
Chris@19 97 See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
Chris@19 98 <a name="index-fftw_002dwisdom-utility-138"></a>
Chris@19 99
Chris@19 100 </body></html>
Chris@19 101