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MPI_Allreduce(3) man page (version 1.3.4)

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NAME

       MPI_Allreduce  - Combines values from all processes and distributes the
       result back to all processes.

SYNTAX


C Syntax

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

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_ALLREDUCE(SENDBUF, RECVBUF, COUNT, DATATYPE, OP,
                 COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   COUNT, DATATYPE, OP, COMM, IERROR

C++ Syntax

       #include <mpi.h>
       void MPI::Comm::Allreduce(const void* sendbuf, void* recvbuf,
            int count, const MPI::Datatype& datatype, const
            MPI::Op& op) const=0

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).

OUTPUT PARAMETERS

       recvbuf   Starting address of receive buffer (choice).

       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

       ! 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.

       The first group defines  the  root  process.   The  root  process  uses
       MPI_ROOT  as the value of root.  All other processes in the first group
       use MPI_PROC_NULL as the value of root.  All processes  in  the  second
       group  use the rank of the root process in the first group as the value
       of root.
       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 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::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.

1.3.4                            Nov 11, 2009                 MPI_Allreduce(3)

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