plucky (3) MPI_Ireduce_scatter.openmpi.3.gz

Provided by: openmpi-doc_5.0.7-1_all bug

SYNTAX

   C Syntax
          #include <mpi.h>

          int MPI_Reduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
               MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

          int MPI_Ireduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
               MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request)

          int MPI_Reduce_scatter_init(const void *sendbuf, void *recvbuf,
               const int recvcounts[], MPI_Datatype datatype, MPI_Op op,
               MPI_Comm comm, MPI_Info info, MPI_Request *request)

   Fortran Syntax
          USE MPI
          ! or the older form: INCLUDE 'mpif.h'
          MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
                       COMM, IERROR)
               <type>  SENDBUF(*), RECVBUF(*)
               INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, IERROR

          MPI_IREDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
                       COMM, REQUEST, IERROR)
               <type>  SENDBUF(*), RECVBUF(*)
               INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, REQUEST, IERROR

          MPI_REDUCE_SCATTER_INIT(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
                       COMM, INFO, REQUEST, IERROR)
               <type>  SENDBUF(*), RECVBUF(*)
               INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax
          USE mpi_f08
          MPI_Reduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
                       ierror)
               TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
               TYPE(*), DIMENSION(..) :: recvbuf
               INTEGER, INTENT(IN) :: recvcounts(*)
               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(sendbuf, recvbuf, recvcounts, datatype, op, comm,
                       request, ierror)
               TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
               TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
               INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
               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

          MPI_Reduce_scatter_init(sendbuf, recvbuf, recvcounts, datatype, op, comm,
                       info, request, ierror)
               TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
               TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
               INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
               TYPE(MPI_Datatype), INTENT(IN) :: datatype
               TYPE(MPI_Op), INTENT(IN) :: op
               TYPE(MPI_Comm), INTENT(IN) :: comm
               TYPE(MPI_Info), INTENT(IN) :: info
               TYPE(MPI_Request), INTENT(OUT) :: request
               INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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).

       • info: Info (handle, persistent).

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  first  does an element-wise reduction on vector of count = S(i)recvcounts[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 return result of the function and
       Fortran routines in the last argument.

       Before the error value is returned, the current MPI  error  handler  associated  with  the  communication
       object  (e.g.,  communicator, window, file) is called.  If no communication object is associated with the
       MPI call, then the call is considered attached to MPI_COMM_SELF and will call the  associated  MPI  error
       handler.   When   MPI_COMM_SELF   is   not  initialized  (i.e.,  before  MPI_Init/MPI_Init_thread,  after
       MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial  error  handler.
       The  initial  error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using
       the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info  key  to  MPI_Comm_spawn/‐
       MPI_Comm_spawn_multiple.   If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When
         called on a communicator, it acts as if MPI_Abort was called on  that  communicator.  If  called  on  a
         window  or file, acts as if MPI_Abort was called on a communicator containing the group of processes in
         the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler then MPI_Comm_set_errhandlerMPI_File_create_errhandler then MPI_File_set_errhandlerMPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_initMPI_Win_create_errhandler then MPI_Win_set_errhandler

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page for a full list of MPI error codes.

       See the Error Handling section of the MPI-3.1 standard for more information.

       2003-2025, The Open MPI Community

                                                  Feb 17, 2025                            MPI_IREDUCE_SCATTER(3)