4.10. Reduce-Scatter

MPI includes variants of each of the reduce operations where the result is scattered to all processes in the group on return.

MPI_REDUCE_SCATTER( sendbuf, recvbuf, recvcounts, datatype, op, comm)
IN sendbuf	starting address of send buffer (choice)
OUT recvbuf	starting address of receive buffer (choice)
IN recvcounts	integer array specifying the number of elements in result distributed to each process. Array must be identical on all calling processes.
IN datatype	data type of elements of input buffer (handle)
IN op	operation (handle)
IN comm	communicator (handle)

int MPI_Reduce_scatter(void* sendbuf, void* recvbuf, int *recvcounts, MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, IERROR

MPI_REDUCE_SCATTER first does an element-wise reduction on vector of elements in the send buffer defined by sendbuf, count and datatype. Next, the resulting vector of results is split into n disjoint segments, where n is the number of members in the group. Segment i contains recvcounts[i] elements. The ith segment is sent to process i and stored in the receive buffer defined by recvbuf, recvcounts[i] and datatype.

Advice to implementors.

The MPI_REDUCE_SCATTER routine is functionally equivalent to: A MPI_REDUCE operation function with count equal to the sum of recvcounts[i] followed by MPI_SCATTERV with sendcounts equal to recvcounts. However, a direct implementation may run faster. ( End of advice to implementors.)