Implied synchronization with MPI_BCAST for both sender and receivers?

Question

When calling MPI_BCAST, is there any implied synchronization? For example, if the sender process were to get to the MPI_BCAST before others could it do the BCAST and then continue without any acknowledgements? Some recent tests with code like:

program test
include 'mpif.h'

integer ierr, tid, tmp

call MPI_INIT(ierr)
call MPI_COMM_RANK(MPI_COMM_WORLD, tid, ierr)

tmp = tid

if(tid.eq.0) then
  call MPI_BCAST(tmp,1,MPI_INTEGER,MPI_ROOT,MPI_COMM_WORLD, ierr)
else

endif

write(*,*) tid,'done'
call MPI_FINALIZE(ierr)

end

shows that with two threads they both reach completion, despite only the sender doing a call to MPI_BCAST.

Output:

1 done           0
0 done           0

Could this be a problem with the MPI installation I'm working with (MPICH), or is this standard behavior for MPI?

Answer 1

Bcast is a collective communication call, and as such blocks. More precisely, it blocks until all processes in the specified communicator have made a matching call to Bcast, at which point communication occurs and execution continues.

Your code is too simplified for debugging purposes. Can you post a working minimal example that demonstrates the problem?