fftw3: Caveats in Using Wisdom
3.4 Caveats in Using Wisdom
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For in much wisdom is much grief, and he that increaseth knowledge
increaseth sorrow. [Ecclesiastes 1:18]
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.
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.
Nevertheless, if the choice is between using 'FFTW_ESTIMATE' 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 ('/etc/fftw/wisdom' on Unix):
int fftw_import_system_wisdom(void);
FFTW also provides a standalone program, 'fftw-wisdom' (described by
its own 'man' page on Unix) with which users can create wisdom, e.g.
for a canonical set of sizes to store in the system wisdom file. 
Wisdom Utilities.
Info Catalog
fftw3: Words of Wisdom-Saving Plans
fftw3: Other Important Topics