MPI_Comm_split_type(3)

NAME
MPI_Comm_split_type - Creates new communicators based on colors and keys.

SYNTAX

C Syntax
#include <mpi.h>
int MPI_Comm_split_type(MPI_Comm comm, int split_type, int key,
	MPI_Info info, MPI_Comm *newcomm)

Fortran Syntax
INCLUDE 'mpif.h'
MPI_COMM_SPLIT_TYPE(COMM, SPLIT_TYPE, KEY, INFO, NEWCOMM, IERROR)
	INTEGER	COMM, SPLIT_TYPE, KEY, INFO, NEWCOMM, IERROR

INPUT PARAMETERS

comm
    Communicator (handle).

split_type
    Type of processes to be grouped together (integer).

key Control of rank assignment (integer).

info
    Info argument (handle).

OUTPUT PARAMETERS

newcomm
    New communicator (handle).

IERROR
    Fortran only: Error status (integer).

DESCRIPTION
This function partitions the group associated with comm into disjoint subgroups, based on the type specied by split_type. Each subgroup contains all processes of the same type. Within each subgroup, the processes are ranked in the order defined by the value of the argument key, with ties broken according to their rank in the old group. A new communicator is created for each subgroup and returned in newcomm. This is a collective call; all processes must provide the same split_type, but each process is permitted to provide different values for key. An exception to this rule is that a process may supply the type value MPI_UNDEFINED, in which case newcomm returns MPI_COMM_NULL.

SPLIT TYPES

MPI_COMM_TYPE_SHARED
    This type splits the communicator into subcommunicators, each of which can create a shared memory region.

NOTES
This is an extremely powerful mechanism for dividing a single communicating group of processes into k subgroups, with k chosen implicitly by the user (by the number of colors asserted over all the processes). Each resulting communicator will be nonoverlapping. Such a division could be useful for defining a hierarchy of computations, such as for multigrid or linear algebra.

Multiple calls to MPI_Comm_split_type can be used to overcome the requirement that any call have no overlap of the resulting communicators (each process is of only one color per call). In this way, multiple overlapping communication structures can be created. Creative use of the color and key in such splitting operations is encouraged.

Note that keys need not be unique. It is MPI_Comm_split_type's responsibility to sort processes in ascending order according to this key, and to break ties in a consistent way. If all the keys are specified in the same way, then all the processes in a given color will have the relative rank order as they did in their parent group. (In general, they will have different ranks.)

Essentially, making the key value zero for all processes of a given split_type means that one needn't really pay attention to the rank-order of the processes in the new communicator.

ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

SEE ALSO

MPI_Comm_create
MPI_Intercomm_create
MPI_Comm_dup
MPI_Comm_free
MPI_Comm_split