MPI_Reduce_scatter(3)              Open MPI              MPI_Reduce_scatter(3)



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.




1.4.1                            Jan 14, 2010            MPI_Reduce_scatter(3)
