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(\fIcomm\fP, \fIgroup\fP, \fInewcomm\fP) is equivalent to a call to MPI_Comm_split(\fIcomm\fP, \fIcolor\fP,\fI key\fP, \fInewcomm\fP), where all members of \fIgroup\fP provide \fIcolor\fP = 0 and \fIkey\fP = rank in group, and all processes that are not members of \fIgroup\fP provide \fIcolor\fP = MPI_UNDEFINED. The function MPI_Comm_split allows more general partitioning of a group into one or more subgroups with optional reordering.
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The value of \fIcolor\fP must be nonnegative or MPI_UNDEFINED.
.SH NOTES
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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.
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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.
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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.)
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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.
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.
