I have an "extreme" case below, for the sake of example.
Suppose one is running a MPI job with N >= 2 ranks, and at a certain moment the
code does the following:
.
.
.
If (rank == 0) {
MPI_Bcast(...);
}
.
.
.
std::cout << "Here A, rank = " << rank << std::endl;
MPI_Barrier(...);
std::cout << "Here B, rank = " << rank << std::endl;
.
.
.
I thought rank 0 would never print the message "Here A", because he MPI lib at
rank 0 would be stuck on the MPI_Bcast waiting for all other ranks to notify
(internally, in the MPI lib logic) that they have received the contents.
But this seems not to be the case. Instead, the code behaves as follows:
1. MPI_Bcast() returns the processing to rank 0, so it (rank 0) prints the
"Here A" message (and all the other ranks print "Here A" as well).
2. All ranks get to the barrier, and then all of them print the "Here B"
message afterwards.
Am I correct on the statements (1) and (2) above?
Thanks,
Ernesto.
Schlumberger-Private
