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3.3 Words of Wisdom—Saving Plans

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cannam@95: FFTW implements a method for saving plans to disk and restoring them. cannam@95: In fact, what FFTW does is more general than just saving and loading cannam@95: plans. The mechanism is called wisdom. Here, we describe cannam@95: this feature at a high level. See FFTW Reference, for a less casual cannam@95: but more complete discussion of how to use wisdom in FFTW. cannam@95: cannam@95:

Plans created with the FFTW_MEASURE, FFTW_PATIENT, or cannam@95: FFTW_EXHAUSTIVE options produce near-optimal FFT performance, cannam@95: but may require a long time to compute because FFTW must measure the cannam@95: runtime of many possible plans and select the best one. This setup is cannam@95: designed for the situations where so many transforms of the same size cannam@95: must be computed that the start-up time is irrelevant. For short cannam@95: initialization times, but slower transforms, we have provided cannam@95: FFTW_ESTIMATE. The wisdom mechanism is a way to get the cannam@95: best of both worlds: you compute a good plan once, save it to cannam@95: disk, and later reload it as many times as necessary. The wisdom cannam@95: mechanism can actually save and reload many plans at once, not just cannam@95: one. cannam@95: cannam@95: cannam@95:

Whenever you create a plan, the FFTW planner accumulates wisdom, which cannam@95: is information sufficient to reconstruct the plan. After planning, cannam@95: you can save this information to disk by means of the function: cannam@95: 

     int fftw_export_wisdom_to_filename(const char *filename);
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(This function returns non-zero on success.) cannam@95: cannam@95:

The next time you run the program, you can restore the wisdom with cannam@95: fftw_import_wisdom_from_filename (which also returns non-zero on success), cannam@95: and then recreate the plan using the same flags as before. cannam@95: 

     int fftw_import_wisdom_from_filename(const char *filename);
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cannam@95: Wisdom is automatically used for any size to which it is applicable, as cannam@95: long as the planner flags are not more “patient” than those with which cannam@95: the wisdom was created. For example, wisdom created with cannam@95: FFTW_MEASURE can be used if you later plan with cannam@95: FFTW_ESTIMATE or FFTW_MEASURE, but not with cannam@95: FFTW_PATIENT. cannam@95: cannam@95:

The wisdom is cumulative, and is stored in a global, private cannam@95: data structure managed internally by FFTW. The storage space required cannam@95: is minimal, proportional to the logarithm of the sizes the wisdom was cannam@95: generated from. If memory usage is a concern, however, the wisdom can cannam@95: be forgotten and its associated memory freed by calling: cannam@95: 

     void fftw_forget_wisdom(void);
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cannam@95: Wisdom can be exported to a file, a string, or any other medium. cannam@95: For details, see Wisdom. cannam@95: cannam@95: cannam@95: