Managing Groups, Contexts, and Communicators

A distinguishing feature of the MPI standard is that it includes a mechanism for creating separate worlds of communication, accomplished through communicators, contexts, and groups.

A communicator specifies a group of processes that will conduct communication operations within a specified context without affecting or being affected by operations occurring in other groups or contexts elsewhere in the program. A communicator also guarantees that, within any group and context, point-to-point and collective communication are isolated from each other.

A group is an ordered collection of processes. Each process has a rank in the group; the rank runs from 0 to n-1. A process can belong to more than one group; its rank in one group has nothing to do with its rank in any other group.

A context is the internal mechanism by which a communicator guarantees safe communication space to the group.

At program startup, two default communicators are defined: MPI_COMM_WORLD, which has as a process group all the processes of the job; and MPI_COMM_SELF, which is equivalent to an identity communicator. The process group that corresponds to MPI_COMM_WORLD is not predefined, but can be accessed using MPI_COMM_GROUP. One MPI_COMM_SELF communicator is defined for each process, each of which has rank zero in its own communicator. For many programs, these are the only communicators needed.

Communicators are of two kinds: intracommunicators, which conduct operations within a given group of processes; and intercommunicators, which conduct operations between two groups of processes.

Communicators provide a caching mechanism, which allows an application to attach attributes to communicators. Attributes can be user data or any other kind of information.

New groups and new communicators are constructed from existing ones. Group constructor routines are local, and their execution does not require interprocessor communication. Communicator constructor routines are collective, and their execution may require interprocess communication.

Users who do not need any communicator other than the default MPI_COMM_WORLD communicator -- that is, who do not need any sub- or supersets of processes -- can simply plug in MPI_COMM_WORLD wherever a communicator argument is requested. In these circumstances, users can ignore this section and the associated routines. (These routines can be identified from the listing in Appendix A, Sun MPI and Sun MPI I/O Routines.)