Question

I am looking at the code here which I am doing for practice.

http://www.mcs.anl.gov/research/projects/mpi/usingmpi/examples/simplempi/main.html

I am confused about the part shown here.

MPI::COMM_WORLD.Reduce(&mypi, &pi, 1, MPI::DOUBLE, MPI::SUM, 0); 
if (rank == 0) 
    cout << "pi is approximately " << pi 
         << ", Error is " << fabs(pi - PI25DT) 
         << endl; 

My question is does the mpi reduce function know when all the other processes (in this case the programs with rank 1-3) have finished and that its result is complete?

Answer 1

All collective communication calls (Reduce, Gather, Scatter, etc) are blocking.