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)
  

• MPI_Reduce combines the elements stored in the buffer sendbuf of each process in the group, using the operation specified in op, and 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. Note that all processes must provide a recvbuf array, even if they are not the target of the reduction operation.
• When count is more than one, then the combine operation is applied element-wise on each entry of the sequence.
• MPI provides a list of predefined operations that can be used to combine the elements stored in sendbuf (See Table 1). MPI also allows programmers to define their own operations.
  

  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