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The current practice in many codes 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. As such, most current message passing libraries have no equivalent argument to the communicator. It is implicitly all the processes as ranked by the system.
This practice can be followed in MPI by using the predefined communicator MPI_COMM_WORLD wherever a communicator argument is required. Thus, using current practice in MPI is very easy. Users that are content with it can ignore most of the information in this chapter. However, everyone should seriously consider understanding the potential risks in using MPI_COMM_WORLD to avoid unexpected behavior of their programs.