Chris@10: Previous: FFTW MPI Reference, Chris@10: Up: Distributed-memory FFTW with MPI Chris@10:
Chris@10: The FFTW MPI interface is callable from modern Fortran compilers
Chris@10: supporting the Fortran 2003 iso_c_binding standard for calling
Chris@10: C functions. As described in Calling FFTW from Modern Fortran,
Chris@10: this means that you can directly call FFTW's C interface from Fortran
Chris@10: with only minor changes in syntax. There are, however, a few things
Chris@10: specific to the MPI interface to keep in mind:
Chris@10:
Chris@10:
fftw3.f03 as in Overview of Fortran interface, you should include 'fftw3-mpi.f03' (after
Chris@10: use, intrinsic :: iso_c_binding as before). The
Chris@10: fftw3-mpi.f03 file includes fftw3.f03, so you should
Chris@10: not include them both yourself. (You will also want to
Chris@10: include the MPI header file, usually via include 'mpif.h' or
Chris@10: similar, although though this is not needed by fftw3-mpi.f03
Chris@10: per se.) (To use the ‘fftwl_’ long double extended-precision routines in supporting compilers, you should include fftw3f-mpi.f03 in addition to fftw3-mpi.f03. See Extended and quadruple precision in Fortran.)
Chris@10:
Chris@10: integer types; there is
Chris@10: no MPI_Comm type, nor is there any way to access a C
Chris@10: MPI_Comm. Fortunately, this is taken care of for you by the
Chris@10: FFTW Fortran interface: whenever the C interface expects an
Chris@10: MPI_Comm type, you should pass the Fortran communicator as an
Chris@10: integer.1
Chris@10:
Chris@10: ptrdiff_t in C, you should use integer(C_INTPTR_T) in
Chris@10: Fortran (see FFTW Fortran type reference).
Chris@10:
Chris@10: fftw_execute_dft becomes fftw_mpi_execute_dft,
Chris@10: etcetera. See Using MPI Plans.
Chris@10:
Chris@10: For example, here is a Fortran code snippet to perform a distributed
Chris@10: L × M complex DFT in-place. (This assumes you have already
Chris@10: initialized MPI with MPI_init and have also performed
Chris@10: call fftw_mpi_init.)
Chris@10:
Chris@10:
use, intrinsic :: iso_c_binding Chris@10: include 'fftw3-mpi.f03' Chris@10: integer(C_INTPTR_T), parameter :: L = ... Chris@10: integer(C_INTPTR_T), parameter :: M = ... Chris@10: type(C_PTR) :: plan, cdata Chris@10: complex(C_DOUBLE_COMPLEX), pointer :: data(:,:) Chris@10: integer(C_INTPTR_T) :: i, j, alloc_local, local_M, local_j_offset Chris@10: Chris@10: ! get local data size and allocate (note dimension reversal) Chris@10: alloc_local = fftw_mpi_local_size_2d(M, L, MPI_COMM_WORLD, & Chris@10: local_M, local_j_offset) Chris@10: cdata = fftw_alloc_complex(alloc_local) Chris@10: call c_f_pointer(cdata, data, [L,local_M]) Chris@10: Chris@10: ! create MPI plan for in-place forward DFT (note dimension reversal) Chris@10: plan = fftw_mpi_plan_dft_2d(M, L, data, data, MPI_COMM_WORLD, & Chris@10: FFTW_FORWARD, FFTW_MEASURE) Chris@10: Chris@10: ! initialize data to some function my_function(i,j) Chris@10: do j = 1, local_M Chris@10: do i = 1, L Chris@10: data(i, j) = my_function(i, j + local_j_offset) Chris@10: end do Chris@10: end do Chris@10: Chris@10: ! compute transform (as many times as desired) Chris@10: call fftw_mpi_execute_dft(plan, data, data) Chris@10: Chris@10: call fftw_destroy_plan(plan) Chris@10: call fftw_free(cdata) Chris@10:Chris@10:
Note that when we called fftw_mpi_local_size_2d and
Chris@10: fftw_mpi_plan_dft_2d with the dimensions in reversed order,
Chris@10: since a L × M Fortran array is viewed by FFTW in C as a
Chris@10: M × L array. This means that the array was distributed over
Chris@10: the M dimension, the local portion of which is a
Chris@10: L × local_M array in Fortran. (You must not use an
Chris@10: allocate statement to allocate an L × local_M array,
Chris@10: however; you must allocate alloc_local complex numbers, which
Chris@10: may be greater than L * local_M, in order to reserve space for
Chris@10: intermediate steps of the transform.) Finally, we mention that
Chris@10: because C's array indices are zero-based, the local_j_offset
Chris@10: argument can conveniently be interpreted as an offset in the 1-based
Chris@10: j index (rather than as a starting index as in C).
Chris@10:
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If instead you had used the ior(FFTW_MEASURE,
Chris@10: FFTW_MPI_TRANSPOSED_OUT) flag, the output of the transform would be a
Chris@10: transposed M × local_L array, associated with the same
Chris@10: cdata allocation (since the transform is in-place), and which
Chris@10: you could declare with:
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complex(C_DOUBLE_COMPLEX), pointer :: tdata(:,:) Chris@10: ... Chris@10: call c_f_pointer(cdata, tdata, [M,local_L]) Chris@10:Chris@10:
where local_L would have been obtained by changing the
Chris@10: fftw_mpi_local_size_2d call to:
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Chris@10:
alloc_local = fftw_mpi_local_size_2d_transposed(M, L, MPI_COMM_WORLD, & Chris@10: local_M, local_j_offset, local_L, local_i_offset) Chris@10:Chris@10:
[1] Technically, this is because you aren't
Chris@10: actually calling the C functions directly. You are calling wrapper
Chris@10: functions that translate the communicator with MPI_Comm_f2c
Chris@10: before calling the ordinary C interface. This is all done
Chris@10: transparently, however, since the fftw3-mpi.f03 interface file
Chris@10: renames the wrappers so that they are called in Fortran with the same
Chris@10: names as the C interface functions.