Reduction

• Reduction; the all-to-one reduction operation is performed in MPI using the MPI_Reduce function.

  int MPI_Reduce(void *sendbuf, void *recvbuf, int count, 
          MPI_Datatype datatype, MPI_Op op, int target, 
          MPI_Comm comm)
  

  • combines the elements stored in the buffer sendbuf of each process in the group,
  • using the operation specified in op,
  • returns the combined values in the buffer recvbuf of the process with rank target.
• Both the sendbuf and recvbuf must have the same number of count items of type datatype.
• When count is more than one, then the combine operation is applied element-wise on each entry of the sequence.
• Note that all processes must provide a recvbuf array, even if they are not the target of the reduction operation.

Figure 2: Diagram for Reduce.

• MPI provides a list of predefined operations that can be used to combine the elements stored in sendbuf (See Table 1).
  

  Table 1: Predefined reduction operations.

  Operation	Meaning	Datatypes
  MPI_MAX	Maximum	C integers and floating point
  MPI_MIN	Minimum	C integers and floating point
  MPI_SUM	Sum	C integers and floating point
  MPI_PROD	Product	C integers and floating point
  MPI_LAND	Logical AND	C integers
  MPI_BAND	Bit-wise AND	C integers and byte
  MPI_LOR	Logical OR	C integers
  MPI_BOR	Bit-wise OR	C integers and byte
  MPI_LXOR	Logical XOR	C integers
  MPI_BXOR	Bit-wise XOR	C integers and byte
  MPI_MAXLOC	max-min value-location	Data-pairs
  MPI_MINLOC	min-min value-location	Data-pairs

  
  
• MPI also allows programmers to define their own operations.