I've been using MPI_Scatter / MPI_Gather for a variety of parallel computations. One thing I've noticed is that MPI_Barrier() is often called to synchronize processors, akin to the OpenMP barrier directive. I was tweaking my code for a project and commented out my MPI_Barrier() lines below, and found that the computations were still correct. Why is this the case? I can understand why the first MPI_Barrier() is needed- the other processors don't need to wait; as soon as they get the data from processor MASTER they can begin computations. But is MPI_Barrier ever needed AFTER an MPI_Gather, or does MPI_Gather already have an implicit barrier within?
Edit: does the size of the data being processed matter in this case?
MPI_Scatter(&sendingbuffer,sendingcount,MPI_FLOAT,receivingbuffer,sendcount,
MPI_INT,MASTER_ID,MPI_COMM_WORLD);
// PERFORM SOME COMPUTATIONS
MPI_Barrier(); //<--- I understand why this is needed
MPI_Gather(localdata,sendcount, MPI_INT, global,sendcount, MPI_INT, MASTER_ID, MPI_COMM_WORLD);
//MPI_Barrier(); <------ is this ever needed?