Strange behaviour when doing a fftw3 MPI Fourier transform from complex to real

Consider a problem where you want to transform a spectral representation of f(x,y) ~ cos(x) to coordinate space. So exp(i*x) + exp(-i*x) ----> f(x,y), where f(x,y) = some_factor * cos(x) .

In column-major layout (my example is written below in Fortran 2003/8) the spectral array would be initialised as,

src(2,1)=(1.,0.)  .

After the transform one should get a tgt array of reals (doubles), where all columns are identical and each column behaves as a cos(x) function.

However , using the code given below, I get a tgt array where only the odd columns behave correctly, while even are all zero.

What, if anything am I doing wrong?

My fftw3 version is 3.4.4 .

    program c2r

  ! compiled using: mpif90 -O0 -g -I/usr/include c2rnot.F90 -o c2rnot -L /usr/lib/x86_64-linux-gnu -lfftw3_mpi -lfftw3
  use, intrinsic :: iso_c_binding
  use MPI

  implicit none
  include 'fftw3-mpi.f03'
  integer :: ierr,id

  ! logical dimensions of the transform
  integer(C_INTPTR_T), parameter :: N1 = 32
  integer(C_INTPTR_T), parameter :: N2 = 32

  ! helper variables: storage size, extents along the slow dim, offsets
  integer(C_INTPTR_T) :: alloc_local, slicec,slicer,offc,offr

  ! pointers to planner, real (image) and complex (original) arrays
  type(C_PTR) :: plan, ctgt, csrc

  ! fortran array representation of csrc ...
  complex(8), pointer :: src(:,:)
  ! ..., ctgt and a total array to hold the mpi-gathered result
  real(8), pointer :: tgt(:,:),total(:,:)

  integer(C_INTPTR_T) :: i,y0

  ! init mpi and fftw ...
  call mpi_init(ierr)

  call mpi_comm_rank(MPI_COMM_WORLD,id,ierr)

  call fftw_mpi_init()
  ! done with init

  ! calculate local complex array storage requirements and layout
  alloc_local = fftw_mpi_local_size_2d(N2,N1/2+1, &
        & MPI_COMM_WORLD,slicec,offc)
  ! allocate local complex array
  csrc = fftw_alloc_complex(alloc_local)
  ! fortran representation
  call c_f_pointer(csrc, src, [N1/2+1,slicec])

  ! allocate local real array
  ctgt = fftw_alloc_real(2*alloc_local)
  ! for non-transposed arrays slices are the same
  ! fortran representation
  call c_f_pointer(ctgt, tgt, [2*(N1/2+1),slicer])

  ! allocate global container (holds gathered tgt arrays)
  if (id==0) allocate(total(N1,N2))

  ! N1 - fast, N2 - slow dimension, in: src, out: tgt
  plan =  fftw_mpi_plan_dft_c2r_2d(N2,N1,src,tgt, MPI_COMM_WORLD, FFTW_MEASURE)


  ! ~ exp(i*x) + exp(-i*x) which should turn into ~ cos(x)

  ! do transform
  call fftw_mpi_execute_dft_c2r(plan,src,tgt)

  ! collect tgt-s into total in process 0
  call mpi_gather(tgt(1:n1,1:slicer),slicer*n1,mpi_double,total,slicer*n1,mpi_double,0,mpi_comm_world,ierr)

  ! The scalar field total has x: 1..N1, y:1..N2 layout. Print
  ! behaviour along the x-axis for a given, constant y. The result
  ! should be ~ cos(x) function for any y0 in [1..N2]. However, I get
  ! such result only for odd y0. For even, total(i,y0) is 0.
  if (id == 0) then
     do i=1,n1
        print *, i, total(i,y0)

  call mpi_finalize(ierr)
end program c2r


Remember that all processus execute the code in mpi.

So as you write src(2,1)=(1.D0,0.D0) you are setting size frequencies, one for each process.

Therefore, your code seems to work if a single process is used mpiexec -np 1 c2rnot. But if you run many processus mpiexec -np 2 c2rnot, you get something else...

Could you try if(id==0) src(2,1)=(1.D0,0.D0) ? id is the rank, as in your code.

You may also use MPI_REAL8 in mpi_gather() : my compiler complained about mpi_double having no IMPLICIT type.

Good luck ! Bye,

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