NAME

       MPI_Reduce_scatter_block,  MPI_Ireduce_scatter_block  -  Combines  values and scatters the
       results in blocks.

SYNTAX

C Syntax

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

       int MPI_Ireduce_scatter_block(const void *sendbuf, void *recvbuf, int recvcount,
            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_BLOCK(SENDBUF, RECVBUF, RECVCOUNT, DATATYPE, OP,
                 COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   RECVCOUNT, DATATYPE, OP, COMM, IERROR

       MPI_IREDUCE_SCATTER_BLOCK(SENDBUF, RECVBUF, RECVCOUNT, DATATYPE, OP,
                 COMM, REQUEST, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   RECVCOUNT, DATATYPE, OP, COMM, REQUEST, IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Ireduce_scatter_block(sendbuf, recvbuf, recvcount, datatype, op, comm,
                 ierror)
            TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
            TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
            INTEGER, INTENT(IN) :: recvcount
            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_block(sendbuf, recvbuf, recvcount, datatype, op, comm,
                 request, ierror)
            TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
            TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
            INTEGER, INTENT(IN) :: recvcount
            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

INPUT PARAMETERS

       sendbuf   Starting address of send buffer (choice).

       recvcount lement count per block (non-negative integer).

       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_block first does an element-wise reduction on vector  of  count  = n  *
       recvcount  elements  in the send buffer defined by sendbuf, count, and datatype, using the
       operation op, where n is the number of processes in the group of comm. Next, the resulting
       vector of results is split into n disjoint segments, where n is the number of processes in
       the group. Each segments contains recvcount elements. The ith segment is sent to process i
       and stored in the receive buffer defined by recvbuf, recvcount, 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.

       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_Reduce_scatter