[OMPI users] Question on MPI_Comm_spawn timing

[Emani, Murali]

Hi all,

I am trying to evaluate the time taken for MPI_Comm_spawn operation in the
latest version of OpenMPI. Here a parent communicator (all processes, not
just the root) spawns one new child process (separate executable). The
code I¹m executing is

main(){
{
Š..
// MPI initialization
Š..
start1 = MPI_Wtime();
MPI_Comm_spawn(³./child", MPI_ARGV_NULL,1, MPI_INFO_NULL, 0,
MPI_COMM_WORLD, &inter_communicator, MPI_ERRCODES_IGNORE );
End = MPI_Wtime();

printf(³ spawn time: %f², (end-start));
MPI_Barrier(inter_communicator); // spawn is collective, but still want to
ensure it using a barrier
..
..
// MPI finalize
}


In child.c
main(){
{
Š..
// MPI initialization
Š..

MPI_Comm_get_parent(&parentcomm); // gets the inter-communicator
MPI_Barrier(parentcomm);
..
..
// MPI finalize
}

My observation is that the spawn time is very high (almost 80% of the
total program execution time). It increases exponentially with the number
of processes in the parent communicator. Is this method correct, and is
the MPI_Comm_spawn operation expensive.
I have also tried MPI_Comm_spawn_multiple but it still measures the same
time.

Could kindly someone guide me in this issue.

Thanks,
Murali