#include <mpi.h> int MPI_Allreduce(void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
INCLUDE 'mpif.h' MPI_ALLREDUCE(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR) <type> SENDBUF(*), RECVBUF(*) INTEGER COUNT, DATATYPE, OP, COMM, IERROR
#include <mpi.h> void MPI::Comm::Allreduce(const void* sendbuf, void* recvbuf, int count, const MPI::Datatype& datatype, const MPI::Op& op) const=0
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
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.
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.
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.
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.