Communicators

• A key concept used throughout MPI is that of the communication domain.
• A communication domain is a set of processes that are allowed to communicate with each other.
• Information about communication domains is stored in variables of type MPI_Comm, that are called communicators.
• These communicators are used as arguments to all message transfer MPI routines.
• They uniquely identify the processes participating in the message transfer operation.

• In general, all the processes may need to communicate with each other.
• For this reason, MPI defines a default communicator called
  MPI_COMM_WORLD which includes all the processes involved.
• However, in many cases we want to perform communication only within (possibly overlapping) groups of processes.
• By using a different communicator for each such group, we can ensure that no messages will ever interfere with messages destined to any other group.