MPI_COMM_WORLD (see Section 3.9). This is a communicator defined by
MPI to permit all processes of your program at run time to communicate with each
other, be it point-to-point or collective.
In this section, you will learn to create new communicators. Two types of
communicators exist within MPI: intra-communicators and inter-communicators.
We will focus on intra-communicators which deal with communications
among processes within
individual communicators. Inter-communicators on the other hand deal with
communications between intra-communicators.
Intra-communicators are essentially subsets of processes of
MPI_COMM_WORLD.
The need for these new communicators is often driven
by the need to deal with, for instance, rows, columns or subblocks
of a matrix. These communicators are often used in conjunction with a
virtual topology -- more often than not a cartesian topology -- to
facilitate ease in implementation of parallel operations.
Furthermore, the use of communicators, and quite frequently together with
virtual topology, generally enhances the readability and maintainability of a program.
To this end, many routines are available in the MPI library to perform various communication-related tasks