3.12. An alternative to a butterfly-structured allreduce is a ring-pass structure. In a ring-pass, if there are p processes, each process q sends data to process q+1, except that process p-1 sends data to process 0. This is repeated until each process has the desired result. Thus, we can implement allreduce with the following code: sum = temp val = my val; for (i = 1; i < p; i++) { MPI Sendrecv replace(&temp val, 1, MPI INT, dest, sendtag, source, recvtag, comm, &status); sum += temp val; } a. Write an MPI program that implements this algorithm for allreduce. How does its performance compare to the butterfly-structured allreduce? b. Modify the MPI program you wrote in the first part so that it implements prefix sums. | |
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