The following predefined operations are supplied for MPI_REDUCE and related functions MPI_ALLREDUCE, MPI_REDUCE_SCATTER, and MPI_SCAN. 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 xor

[ MPI_BXOR] bit-wise 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 Sec. 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

[ 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

** 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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MPI-1.1 of June 12, 1995

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