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MPI Group and Communicator Management

A group is an ordered set of processes. Each process in a group is associated with a unique integer rank. Rank values start at zero and go to N-1, where N is the number of processes in the group. In MPI, a group is represented within system memory as an object. It is accessible to the programmer only by a "handle". A group is always associated with a communicator object.

A communicator encompasses a group of processes that may communicate with each other. All MPI messages must specify a communicator. In the simplest sense, the communicator is an extra "tag" that must be included with MPI calls. Like groups, communicators are represented within system memory as objects and are accessible to the programmer only by "handles". For example, the handle for the communicator that comprises all tasks is MPI_COMM_WORLD, and the MPI_COMM_NULL is the NULL communicator (some functions can return NULL).

Primary purposes of group and communicator objects:

  • Allow you to organize tasks, based upon function, into task groups.
  • Enable Collective Communications operations across a subset of related tasks.
  • Provide basis for implementing user defined virtual topologies.
  • Provide for safe communications.

