MPI_Comm_split(3)                  Open MPI                  MPI_Comm_split(3)



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


SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Comm_split(MPI_Comm comm, int color, int key,
            MPI_Comm *newcomm)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_COMM_SPLIT(COMM, COLOR, KEY, NEWCOMM, IERROR)
            INTEGER   COMM, COLOR, KEY, NEWCOMM, IERROR


C++ Syntax
       #include <mpi.h>
       MPI::Intercomm MPI::Intercomm::Split(int color, int key) const

       MPI::Intracomm MPI::Intracomm::Split(int color, int key) const


INPUT PARAMETERS
       comm      Communicator (handle).

       color     Control of subset assignment (nonnegative integer).

       key       Control of rank assigment (integer).


OUTPUT PARAMETERS
       newcomm   New communicator (handle).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       This function partitions the group associated with comm into disjoint
       subgroups, one for each value of color. Each subgroup contains all
       processes of the same color. 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. A process may supply
       the color value MPI_UNDEFINED, in which case newcomm returns
       MPI_COMM_NULL. This is a collective call, but each process is permitted
       to provide different values for color and key.

       When you call MPI_Comm_split on an inter-communicator, the processes on
       the left with the same color as those on the right combine to create a
       new inter-communicator.  The key argument describes the relative rank
       of processes on each side of the inter-communicator.  The function
       returns MPI_COMM_NULL for  those colors that are specified on only one
       side of the inter-communicator, or for those that specify
       MPI_UNEDEFINED as the color.

       A call to MPI_Comm_create(comm, group, newcomm) is equivalent to a call
       to MPI_Comm_split(comm, color, key, newcomm), where all members of
       group provide color = 0 and key = rank in group, and all processes that
       are not members of group provide color = MPI_UNDEFINED. The function
       MPI_Comm_split allows more general partitioning of a group into one or
       more subgroups with optional reordering.

       The value of color must be nonnegative or MPI_UNDEFINED.


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 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, for a fixed color, the keys need not be unique. It is
       MPI_Comm_split'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
       color 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. C++
       functions do not return errors. If the default error handler is set to
       MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
       will be used to throw an MPI:Exception object.

       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





1.4.1                            Jan 14, 2010                MPI_Comm_split(3)
