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program coll_exer
use mpi
implicit none
integer, parameter :: n_mpi_tasks = 4
integer :: ntasks, rank, ierr, i, color, sub_comm
integer, dimension(2*n_mpi_tasks) :: sendbuf, recvbuf
integer, dimension(2*n_mpi_tasks**2) :: printbuf
call mpi_init(ierr)
call mpi_comm_size(MPI_COMM_WORLD, ntasks, ierr)
call mpi_comm_rank(MPI_COMM_WORLD, rank, ierr)
if (ntasks /= n_mpi_tasks) then
if (rank == 0) then
print *, "Run this program with ", n_mpi_tasks, " tasks."
end if
call mpi_abort(MPI_COMM_WORLD, -1, ierr)
end if
! Initialize message buffers
call init_buffers
! Print data that will be sent
call print_buffers(sendbuf)
! TODO: use a single collective communication call (and maybe prepare
! some parameters for the call)
! Print data that was received
! TODO: add correct buffer
call print_buffers(...)
call mpi_finalize(ierr)
contains
subroutine init_buffers
implicit none
integer :: i
do i = 1, 2*n_mpi_tasks
recvbuf(i) = -1
sendbuf(i) = i + 2*n_mpi_tasks * rank - 1
end do
end subroutine init_buffers
subroutine print_buffers(buffer)
implicit none
integer, dimension(:), intent(in) :: buffer
integer, parameter :: bufsize = 2*n_mpi_tasks
integer :: i
character(len=40) :: pformat
write(pformat,'(A,I3,A)') '(A4,I2,":",', bufsize, 'I3)'
call mpi_gather(buffer, bufsize, MPI_INTEGER, &
& printbuf, bufsize, MPI_INTEGER, &
& 0, MPI_COMM_WORLD, ierr)
if (rank == 0) then
do i = 1, ntasks
write(*,pformat) 'Task', i - 1, printbuf((i-1)*bufsize+1:i*bufsize)
end do
print *
end if
end subroutine print_buffers
end program coll_exer