I am creating an MPI program, where I am trying to send the same data to all processes as soon as they finish their calculation. The processes can have large differences in their computation time, so I don't want that one processor waits for another.
The root process is guaranteed to always send first.
I know that MPI_Bcast acts as a Barries, so I experimented with MPI_IBcast:
program main
use mpi
implicit none
integer rank, nprcos, ierror, a(10), req
call MPI_INIT(ierror)
call MPI_COMM_SIZE(MPI_COMM_WORLD, nprcos, ierror)
call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierror)
a = rank
if(rank /= 2) then
call MPI_IBCAST(a, size(a), MPI_INTEGER, 0, MPI_COMM_WORLD, req, ierror)
call MPI_WAIT(req, MPI_STATUS_IGNORE, IERROR)
endif
write (*,*) 'Hello World from process: ', rank, 'of ', nprcos, "a = ", a(1)
call MPI_FINALIZE(ierror)
end program main
From my experiments it seems, that irregardless of which rank is "boycotting" the MPI_IBcast it always works on all the others:
> $ mpifort test.f90 && mpiexec --mca btl tcp,self -np 4 ./a.out
Hello World from process: 2 of 4 a = 2
Hello World from process: 1 of 4 a = 0
Hello World from process: 0 of 4 a = 0
Hello World from process: 3 of 4 a = 0
Is this a guaranteed behavior or is this just specific to my OpenMPI implementation? How else could I implement this? I can only think of loop over MPI_Isends.
No, this is not guaranteed, all ranks in the communicator should participate. Within MPI this is the definition of a collective communication.