Dear Matt,
I cannot rewrite this example using the MatCreateFFT calls because it is not clear to me which is the shape and the ordering of the vectors produced by MatScatterPetscToFFTW. Furthermore, if I understand correctly this function returns the c2r / r2c Fourier transforms, and not the c2c transforms (unless PETSc is configured for complex data).
However, I have rewritten the example using FFTW instead of PFFT (attached), and it runs fine both in serial and in parallel (with a  cpu count that should be a power of two). However, I prefer using PFFT over FFTW, so if somebody has comments on this library, it would be great to know.
Thank you,
Giuseppe




On 11/23/2015 03:09 PM, Matthew Knepley wrote:
On Mon, Nov 23, 2015 at 1:31 AM, Giuseppe Pitton <gpitton@sissa.it> wrote:
Dear users and developers,
I am trying to interface PETSc with the parallel fast Fourier transform library PFFT (https://www-user.tu-chemnitz.de/~potts/workgroup/pippig/software.php.en), based in turn on FFTW. My plan is to build a spectral differentiation code, and in the attached files you can see a simple example.

Is it possible to write this example using the existing calls?

  http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatCreateFFT.html

That way we would have a baseline we could both run that works, and then we could look
at something broken.

  Thanks,

     Matt
 
The code works correctly in serial, but in parallel there are some problems regarding the output of the results, I think due to some differences in the way PETSc and PFFT store data, but I'm not sure if this is really the issue.
In the attached code, the number of processors used should be specified at compile time in the variable "ncpus". As long as ncpus = 1, everything works fine, but if ncpus = 2 or an higher power of 2, the code terminates correctly but the results show some artifacts, as you can see from the generated hdf5 file, named "output-pfft.h5".
In the makefile the variables PFFTINC, PFFTLIB, FFTWINC and FFTWLIB should be set correctly.
Thank you,
Giuseppe




--
What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener