oracular (3) MPI_Comm_split.openmpi.3.gz

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