NAME

       MPI_Reduce_scatter, MPI_Ireduce_scatter - Combines values and scatters the results.

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Reduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
            MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

       int MPI_Ireduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
            MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request)

Fortran Syntax

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
                 COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   RECVCOUNTS(*), DATATYPE, OP, COMM, IERROR

       MPI_IREDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
                 COMM, REQUEST, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   RECVCOUNTS(*), DATATYPE, OP, COMM, REQUEST, IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Reduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
                 ierror)
            TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
            TYPE(*), DIMENSION(..) :: recvbuf
            INTEGER, INTENT(IN) :: recvcounts(*)
            TYPE(MPI_Datatype), INTENT(IN) :: datatype
            TYPE(MPI_Op), INTENT(IN) :: op
            TYPE(MPI_Comm), INTENT(IN) :: comm
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

       MPI_Ireduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
                 request, ierror)
            TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
            TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
            INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
            TYPE(MPI_Datatype), INTENT(IN) :: datatype
            TYPE(MPI_Op), INTENT(IN) :: op
            TYPE(MPI_Comm), INTENT(IN) :: comm
            TYPE(MPI_Request), INTENT(OUT) :: request
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

C++ Syntax

       #include <mpi.h>
       void MPI::Comm::Reduce_scatter(const void* sendbuf, void* recvbuf,
            int recvcounts[], const MPI::Datatype& datatype,
            const MPI::Op& op) const

INPUT PARAMETERS

       sendbuf   Starting address of send buffer (choice).

       recvcounts
                 Integer  array  specifying  the number of elements in result distributed to each process. Array
                 must be identical on all calling processes.

       datatype  Datatype of elements of input buffer (handle).

       op        Operation (handle).

       comm      Communicator (handle).

OUTPUT PARAMETERS

       recvbuf   Starting address of receive buffer (choice).

       request   Request (handle, non-blocking only).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       MPI_Reduce_scatter first does an element-wise reduction on vector of count  = S(i)recvcounts[i]  elements
       in  the  send  buffer  defined  by sendbuf, count, and datatype. Next, the resulting vector of results is
       split into n disjoint segments, where n is the number of processes  in  the  group.  Segment  i  contains
       recvcounts[i]  elements. The ith segment is sent to process i and stored in the receive buffer defined by
       recvbuf, recvcounts[i], and datatype.

USE OF IN-PLACE OPTION

       When the communicator is an intracommunicator, you can perform a reduce-scatter operation  in-place  (the
       output  buffer  is used as the input buffer).  Use the variable MPI_IN_PLACE as the value of the sendbuf.
       In this case, the input data is taken from the top of the receive buffer.  The area occupied by the input
       data may be either longer or shorter than the data filled by the output data.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

       When  the  communicator  is  an  inter-communicator,  the  reduce-scatter operation occurs in two phases.
       First, the result of the reduction performed on the data provided by the processes in the first group  is
       scattered  among  the processes in the second group.  Then the reverse occurs: the reduction performed on
       the data provided by the processes in the second group is scattered among  the  processes  in  the  first
       group.  For each group, all processes provide the same recvcounts argument, and the sum of the recvcounts
       values should be the same for both groups.

NOTES ON COLLECTIVE OPERATIONS

       The reduction functions ( MPI_Op ) do not return an error value.  As a result, if the functions detect an
       error,  all  they  can do is either call MPI_Abort or silently skip the problem.  Thus, if you change the
       error handler from MPI_ERRORS_ARE_FATAL to something else, for example, MPI_ERRORS_RETURN , then no error
       may be indicated.

       The  reason for this is the performance problems in ensuring that all collective routines return the same
       error value.

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.