101. 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.

Name Meaning

MPI_MAX maximum
MPI_MIN minimum
MPI_SUM sum
MPI_PROD product
MPI_LAND logical and
MPI_BAND bit-wise and
MPI_LOR logical or
MPI_BOR bit-wise or
MPI_LXOR logical exclusive or (xor)
MPI_BXOR bit-wise exclusive or (xor)
MPI_MAXLOC max value and location
MPI_MINLOC min value and location

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.

C integer: MPI_INT, MPI_LONG, MPI_SHORT,
MPI_UNSIGNED_SHORT, MPI_UNSIGNED,
MPI_UNSIGNED_LONG,
MPI_LONG_LONG_INT,
MPI_LONG_LONG (as synonym),
MPI_UNSIGNED_LONG_LONG,
MPI_SIGNED_CHAR, MPI_UNSIGNED_CHAR
Fortran integer: MPI_INTEGER
Floating point: MPI_FLOAT, MPI_DOUBLE, MPI_REAL,
MPI_DOUBLE_PRECISION
MPI_LONG_DOUBLE
Logical: MPI_LOGICAL
Complex: MPI_COMPLEX
Byte: MPI_BYTE

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

Op Allowed Types

MPI_MAX, MPI_MIN C integer, Fortran integer, Floating point
MPI_SUM, MPI_PROD C integer, Fortran integer, Floating point, Complex
MPI_LAND, MPI_LOR, MPI_LXOR C integer, Logical
MPI_BAND, MPI_BOR, MPI_BXOR C integer, Fortran integer, Byte

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 

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