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MPI_Reduce_scatter(3OpenMPI)                                                    MPI_Reduce_scatter(3OpenMPI)



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]  ele-ments elements
       ments  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  occu-pied occupied
       pied 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 per-formed performed
       formed 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.




Open MPI 1.2                                   September 2006                   MPI_Reduce_scatter(3OpenMPI)