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

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_COMM_SPLIT(COMM, COLOR, KEY, NEWCOMM, IERROR)
            INTEGER   COMM, COLOR, KEY, NEWCOMM, IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Comm_split(comm, color, key, newcomm, ierror)
            TYPE(MPI_Comm), INTENT(IN) :: comm
            INTEGER, INTENT(IN) :: color, key
            TYPE(MPI_Comm), INTENT(OUT) :: newcomm
            INTEGER, OPTIONAL, INTENT(OUT) :: 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 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

4.1.6                                             Sep 30, 2023                                 MPI_Comm_split(3)