Communicators

In MPI context, a communicator is a special object representing a group of processes that participate in communication. When a MPI routine is called, the communication will involve some or all of the processes in a communicator.

In C and Fortran, all MPI routines expect a communicator as one of the arguments. In Python, most MPI routines are implemented as methods of a communicator object.

A single process can belong to multiple communicators and will have an unique ID (rank) in each of the communicators.

User-defined communicators

All processes start in a global communicator called MPI_COMM_WORLD (or
MPI.COMM_WORLD in mpi4py), but the user can also define their own custom
communicators as needed.

• By default a single, universal communicator exists to which all
  processes belong (MPI.COMM_WORLD)

A new communicator is created as a collective operation of an existing
communicator. For example, to split the processes in a communicator into
smaller sub-groups, one could do the following:

comm = MPI.COMM_WORLD
rank = comm.Get_rank()

color = rank % 4

local_comm = comm.Split(color)
local_rank = local_comm.Get_rank()

print("Global rank: %d Local rank: %d" % (rank, local_rank))

A distinct label (called color, which is actually just an integer number between 0-3) is assigned to each process based on its rank in the original communicator. New communicators are then created based on this value, so that all processes with the same “color” end up in the same communicator.

If effect, this splits the processes in the original communicator into 4 sub-groups that each share a new communicator within the sub-group.

Can you think of cases where multiple communicators can be useful?