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<h3 class="section">3.4 Caveats in Using Wisdom</h3>

<p><a name="index-wisdom_002c-problems-with-133"></a>
<blockquote>
<i>For in much wisdom is much grief, and he that increaseth knowledge
increaseth sorrow.
</i>[Ecclesiastes 1:18]
<a name="index-Ecclesiastes-134"></a></blockquote>

   <p><a name="index-portability-135"></a>There are pitfalls to using wisdom, in that it can negate FFTW's
ability to adapt to changing hardware and other conditions. For
example, it would be perfectly possible to export wisdom from a
program running on one processor and import it into a program running
on another processor.  Doing so, however, would mean that the second
program would use plans optimized for the first processor, instead of
the one it is running on.

   <p>It should be safe to reuse wisdom as long as the hardware and program
binaries remain unchanged. (Actually, the optimal plan may change even
between runs of the same binary on identical hardware, due to
differences in the virtual memory environment, etcetera.  Users
seriously interested in performance should worry about this problem,
too.)  It is likely that, if the same wisdom is used for two
different program binaries, even running on the same machine, the
plans may be sub-optimal because of differing code alignments.  It is
therefore wise to recreate wisdom every time an application is
recompiled.  The more the underlying hardware and software changes
between the creation of wisdom and its use, the greater grows
the risk of sub-optimal plans.

   <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
using possibly-suboptimal wisdom (created on the same machine, but for a
different binary), the wisdom is likely to be better.  For this reason,
we provide a function to import wisdom from a standard system-wide
location (<code>/etc/fftw/wisdom</code> on Unix):
<a name="index-wisdom_002c-system_002dwide-136"></a>
<pre class="example">     int fftw_import_system_wisdom(void);
</pre>
   <p><a name="index-fftw_005fimport_005fsystem_005fwisdom-137"></a>
FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
by its own <code>man</code> page on Unix) with which users can create wisdom,
e.g. for a canonical set of sizes to store in the system wisdom file.
See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
<a name="index-fftw_002dwisdom-utility-138"></a>

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author	Chris Cannam
date	Wed, 20 Mar 2013 15:35:50 +0000
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