103. Predefined Reduction Operations

The following predefined operations are supplied for MPI_REDUCE and related functions MPI_ALLREDUCE, MPI_REDUCE_SCATTER, MPI_SCAN, and MPI_EXSCAN. These operations are invoked by placing the following in op.

The two operations MPI_MINLOC and MPI_MAXLOC are discussed separately in Section MINLOC and MAXLOC . For the other predefined operations, we enumerate below the allowed combinations of op and datatype arguments. First, define groups of MPI basic datatypes in the following way.

Now, the valid datatypes for each option is specified below.

The following examples use intracommunicators.
Example A routine that computes the dot product of two vectors that are distributed across a group of processes and returns the answer at node zero.

SUBROUTINE PAR_BLAS1(m, a, b, c, comm) 
REAL a(m), b(m)       ! local slice of array 
REAL c                ! result (at node zero) 
REAL sum 
INTEGER m, comm, i, ierr 
 
! local sum 
sum = 0.0 
DO i = 1, m 
   sum = sum + a(i)*b(i) 
END DO 
 
! global sum 
CALL MPI_REDUCE(sum, c, 1, MPI_REAL, MPI_SUM, 0, comm, ierr) 
RETURN 

Example 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