MPI_Allreduce(3)                   Open MPI                   MPI_Allreduce(3)



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

       ! 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.

       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.

       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 value
       of the function and Fortran routines in the last argument. C++
       functions 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.





1.4.1                            Jan 14, 2010                 MPI_Allreduce(3)
