plucky (3) MPI_Allreduce_init.openmpi.3.gz

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

SYNTAX

   C Syntax
          #include <mpi.h>

          int MPI_Allreduce(const void *sendbuf, void *recvbuf, int count,
                            MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

          int MPI_Iallreduce(const void *sendbuf, void *recvbuf, int count,
                             MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
                             MPI_Request *request)

          int MPI_Allreduce_init(const void *sendbuf, void *recvbuf, int count,
                                 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_ALLREDUCE(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
               <type>  SENDBUF(*), RECVBUF(*)
               INTEGER COUNT, DATATYPE, OP, COMM, IERROR

          MPI_IALLREDUCE(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)
               <type>  SENDBUF(*), RECVBUF(*)
               INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, IERROR

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

   Fortran 2008 Syntax
          USE mpi_f08
          MPI_Allreduce(sendbuf, recvbuf, count, datatype, op, comm, ierror)
               TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
               TYPE(*), DIMENSION(..) :: recvbuf
               INTEGER, INTENT(IN) :: count
               TYPE(MPI_Datatype), INTENT(IN) :: datatype
               TYPE(MPI_Op), INTENT(IN) :: op
               TYPE(MPI_Comm), INTENT(IN) :: comm
               INTEGER, OPTIONAL, INTENT(OUT) :: ierror

          MPI_Iallreduce(sendbuf, recvbuf, count, datatype, op, comm, request,
                       ierror)
               TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
               TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
               INTEGER, INTENT(IN) :: count
               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_Allreduce_init(sendbuf, recvbuf, count, datatype, op, comm, info, request,
                       ierror)
               TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
               TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
               INTEGER, INTENT(IN) :: count
               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).

       • count: Number of elements in send buffer (integer).

       • datatype: Datatype of elements of send buffer (handle).

       • op: Operation (handle).

       • comm: Communicator (handle).

       • info: Info (handle, persistent only).

OUTPUT PARAMETERS

recvbuf: Starting address of receive buffer (choice).

       • request: Request (handle, non-blocking only).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION

       Same as MPI_Reduce except that the result appears in the receive buffer of all the group members.

       Example 1: A routine that computes the product of a vector and an array that  are  distributed  across  a
       group of processes and returns the answer at all nodes (compare with Example 2, with MPI_Reduce, below).

          SUBROUTINE PAR_BLAS2(m, n, a, b, c, comm)
          REAL a(m), b(m,n)    ! local slice of array
          REAL c(n)            ! result
          REAL sum(n)
          INTEGER n, comm, i, j, ierr

          ! local sum
          DO j= 1, n
            sum(j) = 0.0
            DO i = 1, m
              sum(j) = sum(j) + a(i)*b(i,j)
            END DO
          END DO

          ! global sum
          CALL MPI_ALLREDUCE(sum, c, n, MPI_REAL, MPI_SUM, comm, ierr)

          ! return result at all nodes
          RETURN

       Example  2:  A  routine  that computes the product of a vector and an array that are distributed across a
       group of processes and returns the answer at node zero.

          SUBROUTINE PAR_BLAS2(m, n, a, b, c, comm)
          REAL a(m), b(m,n)    ! local slice of array
          REAL c(n)            ! result
          REAL sum(n)
          INTEGER n, comm, i, j, ierr

          ! local sum
          DO j= 1, n
            sum(j) = 0.0
            DO i = 1, m
              sum(j) = sum(j) + a(i)*b(i,j)
            END DO
          END DO

          ! global sum
          CALL MPI_REDUCE(sum, c, n, MPI_REAL, MPI_SUM, 0, comm, ierr)

          ! return result at node zero (and garbage at the other nodes)
          RETURN

USE OF IN-PLACE OPTION

       When the communicator is an intracommunicator, you can perform  an  all-reduce  operation  in-place  (the
       output  buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of sendbuf at all
       processes.

       Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.

       Because the in-place option converts the receive buffer into a send-and-receive buffer, a Fortran binding
       that includes INTENT must mark these as INOUT, not OUT.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

       When  the  communicator  is an inter-communicator, the reduce operation occurs in two phases. The data is
       reduced from all the members of the first group and received by all the members of the second group. Then
       the  data  is  reduced  from  all  the members of the second group and received by all the members of the
       first. The operation exhibits a symmetric, full-duplex behavior.

       When the communicator is an intra-communicator, these groups are the same, and the operation occurs in  a
       single phase.

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.

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_ALLREDUCE_INIT(3)