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### 3.4 Caveats in Using Wisdom

> 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. See Wisdom Utilities.

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