Global Reduction Operations

void Intracomm.Reduce(Object sendbuf, int sendoffset,
                      Object recvbuf, int recvoffset,
                      int count, Datatype datatype,
                      Op op, int root)

sendbuf	send buffer array
sendoffset	initial offset in send buffer
recvbuf	receive buffer array
recvoffset	initial offset in receive buffer
count	number of items in send buffer
datatype	data type of each item in send buffer
op	reduce operation
dest	rank of root process

Combine elements in input buffer of each process using the reduce operation, and return the combined value in the output buffer of the root process. Java binding of the MPI operation MPI_REDUCE.

The predefined operations are available in Java as MPI.MAX, MPI.MIN, MPI.SUM, MPI.PROD, MPI.LAND, MPI.BAND, MPI.LOR, MPI.BOR, MPI.LXOR, MPI.BXOR, MPI.MINLOC and MPI.MAXLOC.

The handling of MINLOC and MAXLOC is modelled on the Fortran binding. The extra predefined types MPI.SHORT2, MPI.INT2, MPI.LONG2, MPI.FLOAT2, MPI.DOUBLE2 describe pairs of Java numeric primitive types.

Op.Op(User_function function, boolean commute)

function	user defined function
commute	true if commutative, false otherwise

Bind a user-defined global reduction operation to an Op object. Java binding of the MPI operation MPI_OP_CREATE. The abstract base class User_function is defined by

  class User_function {
    public abstract void Call(Object invec, int inoffset,
                              Object inoutvec, int inoutoffset,
                              int count, Datatype datatype) ;
  }

To define a new operation, the programmer should define a concrete subclass of User_function, implementing the Call method, then pass an object from this class to the Op constructor. The User_function.Call method plays exactly the same role as the function argument in the standard bindings of MPI. The actual arguments invec and inoutvec passed to call will be arrays containing count elements of the type specified in the datatype argument. Offsets in the arrays can be specified as for message buffers. The user-defined Call method should combine the arrays element by element, with results appearing in inoutvec.

void Op.finalize()

Destructor. Java binding of the MPI operation MPI_OP_FREE.

void Intracomm.Allreduce(Object sendbuf, int sendoffset,
                         Object recvbuf, int recvoffset,
                         int count, Datatype datatype,
                         Op op)

sendbuf	send buffer array
sendoffset	initial offset in send buffer
recvbuf	receive buffer array
recvoffset	initial offset in receive buffer
count	number of items in send buffer
datatype	data type of each item in send buffer
op	reduce operation

Same as Reduce except that the result appears in receive buffer of all processes in the group. Java binding of the MPI operation MPI_ALLREDUCE.