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(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 acts as if it 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 processes in the group of
comm.
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.
