fftw_flags

set method for fft planner algorithm selection

Calling Sequence

[a,[S]]=fftw_flags(flag)

Arguments

:flag a string or an integer. Specifies the planner algorithm. See

below.

: :a an integer. The planner code. See below. : :S a character string. The planner name. :

Description

This function enables the selection of the algorithm used to determine the fftw planner algorithm. The planner is used to determine an efficient way to compute the fft.

Warning: the default value “FFTW_MEASURE” gives quite efficient plans. Try to change it only if the fft efficiency is really important and you have a lot of simmilar calls to do.

Accepted entries are :

• FFTW_ESTIMATE or 64. Specifies that, instead of actual measurements of different algorithms, a simple heuristic is used to pick a (probably sub-optimal) plan quickly. With this flag, the input/output arrays are not overwritten during planning. It is the defailt value
• “FFTW_MEASURE” or 0. tells FFTW to find an optimized plan by actually computing several FFTs and measuring their execution time. Depending on your machine, this can take some time (often a few seconds). planning option.
• FFTW_PATIENT or 32. It is like “FFTW_MEASURE”, but considers a wider range of algorithms and often produces a “more optimal” plan (especially for large transforms), but at the expense of several times longer planning time (especially for large transforms).
• FFTW_EXHAUSTIVE or 8. It is like “FFTW_PATIENT”, but considers an even wider range of algorithms, including many that we think are unlikely to be fast, to produce the most optimal plan but with a substantially increased planning time.

Remark : when using FFTW_MEASURE/FFTW_PATIENT/FFTW_EXHAUSTIVE you must call two times fftw. (first call for initialisation, second and others calls for computation)

Examples

A=`rand`_(1,2^9+2^15+2^17);
`fftw_forget_wisdom`_();
fftw_flags("FFTW_ESTIMATE");
`timer`_();y=`fft`_(A);`timer`_()//first call determines the plan
`timer`_();y=`fft`_(A);`timer`_() //subsequent similar calls
fftw_flags("FFTW_MEASURE");
`fftw_forget_wisdom`_();
`timer`_();y=`fft`_(A);`timer`_()//first call determines the plan
`timer`_();y=`fft`_(A);`timer`_() //subsequent similar calls

See Also

• fft fast Fourier transform.
• set_fftw_wisdom set fftw wisdom
• get_fftw_wisdom return fftw wisdom
• fftw_forget_wisdom Reset fftw wisdom