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author Chris Cannam <cannam@all-day-breakfast.com>
date Wed, 20 Mar 2013 15:35:50 +0000
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+<head>
+<title>Caveats in Using Wisdom - FFTW 3.3.3</title>
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+<a name="Caveats-in-Using-Wisdom"></a>
+<p>
+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>,
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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>
+
+   </body></html>
+