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Re: Working patch for FFTW 3.0.x and Nd FFT's

From: Daniel J Sebald
Subject: Re: Working patch for FFTW 3.0.x and Nd FFT's
Date: Wed, 18 Feb 2004 12:52:56 -0600
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David Bateman wrote:

According to Daniel J Sebald <address@hidden> (on 02/18/04):
That was sort of my original concern, about the user getting stuck in some situation he or she doesn't know what is going on. (I'm wondering what could possibly be taking 12 hours, but that's a different matter.)

FFTW tests all possible ways it knows of to do a particular FFT and this
stores the information on the best one for the current architecture.
With the command "fftw-wisdom -c -v -o /etc/fftw/wisdom" it tests about
8 different ways FFTW might be called to do a particular FFT for the
following FFT sizes

    "1", "2", "4", "8", "16", "32", "64", "128", "256", "512", "1024",
    "2048", "4096", "8192", "16384", "32768", "65536", "131072",
    "262144", "524288", "1048576",

    "10", "100", "1000", "10000", "100000", "1000000",

    "2x2", "4x4", "8x8", "10x10", "16x16", "32x32", "64x64", "100x100",
    "128x128", "256x256", "512x512", "1000x1000", "1024x1024",

    "2x2x2", "4x4x4", "8x8x8", "10x10x10", "16x16x16", "32x32x32",
    "64x64x64", "100x100x100"

So its a lot of work.

That is what I suspected. The following points aren't critically important: I'm guessing that with the typical nonlinear dependence of computations based on FFT length, it is those very long FFT tests that are taking the longest time. (I also question the utility of a million point FFT, but let's say someone has a valid scenario for requiring such resolution.) One would think that some logic could be applied to the trends of algorithm performance for the shorter FFTs to deduce what would be the most efficient algorithm for the very long FFTs. Anyway, one often gets a feeling of the kind of information that is trying to be accumulated versus the amount of computer power that should be required, and somehow 12 hours seems like a long time to build a "wisdom table". This isn't Octave's concern though, I guess.

Why not the name fft_wisdom as opposed to fftw_wisdom? I realize that FFTW is being used, but if later some other FFT package is used then fft_wisdom could still apply. (I may have come in late on this discussion.)

My original patch had fft_wisdom, John preferred fftw_wisdom. He has CVS
write access, so he has the last say :-)




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