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
- comm
- Communicator (handle).
- color
- Control of subset assignment (nonnegative integer).
- key
- Control of rank assignment (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_UNDEFINED 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