MPI Programming and Subroutine Reference

Appendix D. Reduction Operations

Predefined Reduction Operations

The following is a list of the predefined operations for use with MPI_REDUCE, MPI_ALLREDUCE, MPI_REDUCE_SCATTER and MPI_SCAN. To invoke a predefined operation, place any of the following in op.

Reduction Operation	Description
MPI_MAX	maximum
MPI_MIN	minimum
MPI_SUM	sum
MPI_PROD	product
MPI_LAND	logical AND
MPI_BAND	bitwise AND
MPI_LOR	logical OR
MPI_BOR	bitwise OR
MPI_LXOR	logical XOR
MPI_BXOR	bitwise XOR
MPI_MAXLOC	max value and location
MPI_MINLOC	min value and location

Reduction Operations - Valid Datatype Arguments Operations

The reduction operations have the following basic datatype arguments.

Type	Valid Datatype Arguments
C integer	MPI_INT MPI_LONG MPI_LONG_LONG_INT MPI_SHORT MPI_UNSIGNED MPI_UNSIGNED_LONG MPI_UNSIGNED_LONG_LONG MPI_UNSIGNED_SHORT
FORTRAN integer	MPI_INTEGER MPI_INTEGER8
Floating point	MPI_DOUBLE MPI_DOUBLE_PRECISION MPI_FLOAT MPI_LONG_DOUBLE MPI_REAL
Logical	MPI_LOGICAL
Complex	MPI_COMPLEX
Byte	MPI_BYTE
C Pair	MPI_DOUBLE_INT MPI_FLOAT_INT MPI_LONG_INT MPI_LONG_DOUBLE_INT MPI_SHORT_INT MPI_2INT
FORTRAN Pair	MPI_2DOUBLE_PRECISION MPI_2INTEGER MPI_2REAL

op Option - Valid Datatypes

The following are the valid datatypes for each op option.

Type	Valid Datatypes For op Option
C integer	MPI_BAND MPI_BOR MPI_BXOR MPI_LAND MPI_LOR MPI_LXOR MPI_MAX MPI_MIN MPI_SUM MPI_PROD
FORTRAN integer	MPI_BAND MPI_BOR MPI_BXOR MPI_MAX MPI_MIN MPI_PROD MPI_SUM
Floating point	MPI_MAX MPI_MIN MPI_PROD MPI_SUM
Logical	MPI_LAND MPI_LOR MPI_LXOR
Complex	MPI_PROD MPI_SUM
Byte	MPI_BAND MPI_BOR MPI_BXOR
C Pair	MPI_MAXLOC MPI_MINLOC
FORTRAN Pair	MPI_MAXLOC MPI_MINLOC

Examples

Examples of user-defined reduction functions for integer vector addition.

C Example

void int_sum (int *in, int *inout,
     int *len, MPI_Datatype *type);
 
{
   int i
   for (i=0; i<*len; i++)  {
      inout[i] + = in[i];
   }
}

FORTRAN Example

      SUBROUTINE INT_SUM(IN,INOUT,LEN,TYPE)
 
      INTEGER IN(*),INOUT(*),LEN,TYPE,I
 
      DO I = 1,LEN
         INOUT(I) = IN(I) + INOUT(I)
      ENDDO
      END

User-supplied reduction operations have four arguments:

• The first argument, in, is an array or scalar variable. The length, in elements, is specified by the third argument, len.

  This argument is an input array to be reduced.

• The second argument, inout, is an array or scalar variable. The length, in elements, is specified by the third argument, len.

  This argument is an input array to be reduced and the result of the reduction will be placed here.

• The third argument, len is the number of elements in in and inout to be reduced.

• The fourth argument type is the datatype of the elements to be reduced.

Users may code their own reduction operations, with the restriction that the operations must be associative. Also, C programmers should note that the values of len and type will be passed as pointers. No communication calls are allowed in user-defined reduction operations. See "Limitations In Setting The Thread Stacksize" in Appendix G. "Programming Considerations for User Applications in POE" for thread stacksize considerations when using the threaded MPI library.