When do I need to use MPI_Barrier()?

Question

I wonder when do I need to use barrier? Do I need it before/after a scatter/gather for example? Or should OMPI ensure all processes have reached that point before scatter/ga

Answer 1

One use of MPI_Barrier is for example to control access to an external resource such as the filesystem, which is not accessed using MPI. For example, if you want each process to write stuff to a file in sequence, you could do it like this:

int rank, size;
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
for ( int ii = 0; ii < size; ++ii ) {
    if ( rank == ii ) {
        // my turn to write to the file
        writeStuffToTheFile();
    }
    MPI_Barrier(MPI_COMM_WORLD);
}

That way, you can be sure that no two processes are concurrently calling writeStuffToTheFile.