Re: Strange computational time with fftn

[Sergei Steshenko]

On 17/04/2020 22:33, Marco Caliari wrote:

Hi all,

anybody can explain me the strange peak in elapsed time I see in
computing fftn for dimension 100^3 with the default planner "estimate"?
Moreover, ml is twice as fast.
I attach the script and the result of the plot.
I'm on Linux Mint 19.3, octave 5.1.1 or 7.0.0, libfftw3 3.3.7

Thank you very much,

Marco

The problem is "estimate". I.e. FFTW does a really good job choosing codelets performing FFT of actual size on actual machine when it uses "measure". Because measure is brute force choosing of proper codelets.

"estimate" just guesses.

I suggest to spend time once using "measure" for your expected sizes on the actual machine - FFTW wisdom is preserved, so next times you won't spend time to create FFTW plans using "measure".

--Sergei.