Intercommunicators

Communication Between Groups 1

• So far we have discussed communication between members of the same group.
  • this is called intragroup communication
• MPI also supports communication between members of different nonoverlapping groups.
  • this is called intergroup communication
• Intergroup communication may be used for
  • communication between different parallel modules in an application,
  • parallel client-server of computation,
  • supporting extension of MPI to dynamic process model.

Communication Between Groups 2

• The sender specifies the destination process by giving the rank relative to the remote group
• The receiver specifies the source process by giving the rank relative to the remote group
• Intercommunicators may be used for point-to-point communication only
  • no collective communication
  • no application topologies

Accessing Intercommunicators

• Can get size of remote group:

  mpi_comm_remote_size (comm, size, ierr)

• Can get handle to remote group:

  mpi_comm_remote_group (comm, group, ierr)

• The routines mpi_comm_size, mpi_comm_rank, and mpi_comm_group return information about the local group.

Intercommunicator Operations

• To create an intercommunicator we require:
  • a ``group leader'' for each group, known to members of its group,
  • an intracommunicator for communication between the 2 group leaders,
  • a tag for communicating safely between the 2 group leaders.
• Then an intercommunicator can be created using:

  mpi_intercomm_create (localcomm,
                        localleader, peercomm,
                        remoteleader, tag,
                        newintercomm, ierr)

• Can form an intracommunicator out of an intercommunicator:

  mpi_intercomm_merge (intercomm, high, newintracomm)

• high determines ranks in merged group.

Intercommunicator Operations