Forum: GNU Radio Pathological FFT sizes

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Marcus D. Leech (Guest)
on 2009-03-10 02:54
(Received via mailing list)
I discovered, rather by accident, that the FFTW planner seems to get its
knickers royally in a twist for certain FFT sizes.  The code
    d_plan = fftwf_plan_dft_1d (fft_size,
                              reinterpret_cast<fftwf_complex
*>(d_inbuf),
                              reinterpret_cast<fftwf_complex
*>(d_outbuf),
                              forward ? FFTW_FORWARD : FFTW_BACKWARD,
                              FFTW_MEASURE);

Goes away and computes forever for FFT sizes of 3200000 and 6400000.  I
know that FFTW tries to compute
  and "optimized" algorithm based on size, etc.  I guess that there are
singularities in the optimizer??

I was trying to add various sizes other than {1,2,4,8}M bins, with 3.2M
and 6.4M being obvious intermediate
  sizes, which seems to have provoked this problem.   I'm running with
FFTW-3.2.

--
Marcus L.
Principal Investigator, Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
Eric B. (Guest)
on 2009-03-10 03:40
(Received via mailing list)
On Mon, Mar 09, 2009 at 08:53:26PM -0400, Marcus D. Leech wrote:
>   and "optimized" algorithm based on size, etc.  I guess that there are
> singularities in the optimizer??
>
> I was trying to add various sizes other than {1,2,4,8}M bins, with 3.2M
> and 6.4M being obvious intermediate
>   sizes, which seems to have provoked this problem.   I'm running with
> FFTW-3.2.

FFTW (when used with our wrapper) stores the best plans it finds in
the ~/.gr_fftw_wisdom file, and from then on just looks them up.
Since you're trying new sizes it runs its dynamic-programming based
planner.  The search space for giant FFTs is pretty big.  It should
only do the big search once for each size.  Be patient :-)

Eric
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