MPIReducescatter(3OpenMPI) MPIReducescatter(3OpenMPI)

NAME
       MMPPIIRReedduucceessccaatttteerr - Combines values and scatters the results.


SSYYNNTTAAXX
CC SSyynnttaaxx
       #include 
       int MPIReducescatter(void *sendbuf, void *recvbuf, int *recvcounts,
            MPIDatatype datatype, MPIOp op, MPIComm comm)


FFoorrttrraann SSyynnttaaxx
       INCLUDE 'mpif.h'
       MPIREDUCESCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
                 COMM, IERROR)
                SENDBUF(*), RECVBUF(*)
            INTEGER   RECVCOUNTS(*), DATATYPE, OP, COMM, IERROR


CC++++ SSyynnttaaxx
       #include 
       void MPI::Comm::Reducescatter(const void* sendbuf, void* recvbuf,
            int recvcounts[], const MPI::Datatype& datatype,
            const MPI::Op& op) const


IINNPPUUTT PPAARRAAMMEETTEERRSS
       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).


OOUUTTPPUUTT PPAARRAAMMEETTEERRSS
       recvbuf   Starting address of receive buffer (choice).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       MPIReducescatter  first  does  an element-wise reduction on vector of
       count = S(i)revcounts[i] elements in the send buffer defined  by  send-
       buf,  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.



UUSSEE OOFF IINN-PPLLAACCEE OOPPTTIIOONN
       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 MPIINPLACE as the value of the send-
       buf.  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.


WWHHEENN CCOOMMMMUUNNIICCAATTOORR IISS AANN IINNTTEERR-CCOOMMMMUUNNIICCAATTOORR
       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 argu-
       ment,  and the sum of the recvcounts values should be the same for both
       groups.


NNOOTTEESS OONN CCOOLLLLEECCTTIIVVEE OOPPEERRAATTIIOONNSS
       The reduction functions ( MPIOp ) do not return an error value.  As  a
       result,  if  the  functions  detect an error, all they can do is either
       call MPIAbort or silently skip the problem.  Thus, if you  change  the
       error handler from MPIERRORSAREFATAL to something else, for example,
       MPIERRORSRETURN , 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.


EERRRROORRSS
       Almost  all MPI routines return an error value; C routines as the value
       of the function and Fortran routines in the last  argument.  C++  func-
       tions  do  not  return  errors.  If the default error handler is set to
       MPI::ERRORSTHROWEXCEPTIONS, 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
       MPICommseterrhandler; the predefined error handler MPIERRORSRETURN
       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    MPIReducescatter(3OpenMPI)