Re: [Discuss-gnuradio] Pathological FFT sizes

[Eric Blossom]

On Mon, Mar 09, 2009 at 08:53:26PM -0400, Marcus D. Leech wrote:
> 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. 

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