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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 pro-
cesses 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_UNEDE-
FINED as the color.
A call to MPI_Comm_create(comm, group, newcomm) is equivalent to a call
NOTES
This is an extremely powerful mechanism for dividing a single communi-
cating 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 require-
ment that any call have no overlap of the resulting communicators (each
process is of only one color per call). In this way, multiple overlap-
ping 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 par-
ent 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++ func-
tions 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
Open MPI 1.2 September 2006 MPI_Comm_split(3OpenMPI)
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