Re: FFTW3.0.1 wisdom

[Dmitri A. Sergatskov]

David Bateman wrote:
> Be careful here, because it might not be just processor dependent, but
> also dependent on the amount of memory available. FFTW might use some
> technique where it allocates large blocks of memeory to speed up FFT's
> but if you use say wisdom generated on a machine with 512MB on one with
> 128MB uou might, even with the same processor you might be surprised...
> These are just my speculations, I don't know what FFTW really does
> internally .....

In Steve Johnson paper you can see a plot where they tested
PIV with wisdom file generated on PPC970 and PPC970 with a wisdom
file generated on PIV -- the relative worst drop in performance
was ~30% and in few cases it did not change at all.
I did some simple tests of fftw with and without wisdom file
and for _large_ problem difference is quite small. And fftw3
without any wisdom outperforms fftw2.
For some small cases I get up to ~50% improvement in fftw3.
Without it fftw3 is ~30% slower than fftw2 and with it
fftw3 30% faster than fftw2. The tests were not exhaustive, but
it so far it looks that fftw3 is robust,  even "wrong" wisdom will
not degrade performance too much.

> D.

Dmitri.



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