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Re: FFTW3.0.1 wisdom

From: Dmitri A. Sergatskov
Subject: Re: FFTW3.0.1 wisdom
Date: Thu, 19 Feb 2004 10:20:06 -0700
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.6) Gecko/20040115

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.



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