Question

I’m trying to get an intution about how MPI works. Therefore I started with a little example:

...
MPI_Comm_rank(MPI_COMM_WORLD, &tid);
MPI_Comm_size(MPI_COMM_WORLD, &nthreads);

int message = 2;

if(tid != 0)
{
    MPI_Recv(&rec, 1, MPI_INT, tid-1, 0, MPI_COMM_WORLD, &status);
    printf("Process %i receive %i\n", tid, rec);
}

if(tid != nthreads-1)
{
    message++;
    printf("Process %i sends %i\n", tid, message);
    MPI_Send(&message, 1, MPI_INT, tid+1, 0, MPI_COMM_WORLD);
}
...

That works fine despite the fact that message does seem to increase above 3. Why is that?

Answer 1

mpirun -np N command starts N MPI processes, not one process with N threads. In each process you have variable message and each process has its own variable set to 2. Next 0th process sends its message == 3 to 1st process. After it, 1st process sends his own message == 3 and 2nd catches it…