NAME

       MPI_Reduce_scatter - Combines values and scatters the results.

SYNTAX

C Syntax

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

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
                 COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   RECVCOUNTS(*), DATATYPE, OP, COMM, 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).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       MPI_Reduce_scatter  first  does   an   element-wise   reduction   on   vector   of   count
       = S(i)revcounts[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.