5.1.2.0.3 Intra-communicators.

The most commonly used means for message passing in MPI is via intra-communicators. Intra-communicators contain an instance of a group, contexts of communication for both point-to-point and collective communication, and the ability to include virtual topology and other attributes. These features work as follows:

Advice to users. The current practice in many communication libraries is that there is a unique, predefined communication universe that includes all processes available when the parallel program is initiated; the processes are assigned consecutive ranks. Participants in a point-to-point communication are identified by their rank; a collective communication (such as broadcast) always involves all processes. This practice can be followed in MPI by using the predefined communicator MPI_COMM_WORLD. Users who are satisfied with this practice can plug in MPI_COMM_WORLD wherever a communicator argument is required, and can consequently disregard the rest of this chapter.(End of advice to users.)

MPI-Standard for MARMOT