Fortran

Copy this code into reduced_loops.f.

      program main
      implicit none
      include 'mpif.h'

      integer i, rank, nprocs, count, start, stop, nloops
      integer total_nloops,err

      call MPI_Init(err)

c     get the number of processes, and the id of this process
      call MPI_Comm_rank(MPI_COMM_WORLD, rank, err)
      call MPI_Comm_size(MPI_COMM_WORLD, nprocs, err)

c     we want to perform 1000 iterations in total. Work out the 
c     number of iterations to perform per process...
      count = 1000 / nprocs

c     we use the rank of this process to work out which
c     iterations to perform.
      start = rank * count
      stop = start + count - 1

c     now perform the loop
      nloops = 0

      do i=start,stop
        nloops = nloops + 1
      enddo

      print *,"Process ",rank," performed ",nloops,
     .        " iterations of the loop."

      total_nloops = 0
      call MPI_Reduce( nloops, total_nloops, 1, MPI_INTEGER,
     .                 MPI_SUM, 0, MPI_COMM_WORLD, err )

      if (rank .eq. 0) then
c        ok - are there any more loops to run?
         nloops = 0

         if (total_nloops .lt. 1000) then
           do i=total_nloops,1000-1
              nloops = nloops + 1
           enddo
         endif

         print *,"Process 0 performed the remaining ",nloops,
     .           " iterations of the loop"
      endif

      call MPI_Finalize()

      end

In this code, MPI_Send and MPI_Recv have been replaced by a call to MPI_Reduce(void send, void receive, integer size, MPI_INTEGER, MPI_SUM, integer process, MPI_COMM_WORLD, integer err). This function reduces the data in the message in send using the MPI operation MPI_SUM and places the resulting message into receive. The messages are of size size, and you specify the data type as you do for MPI_Recv and MPI_Send, e.g. in this case we use MPI_INTEGER. The resulting message is placed only into receive on the process whose rank is given in process. Note that all processes in the MPI process team must call this function with the same arguments, or else the result is undefined. Note also that other reduction operations are possible, e.g. by replacing MPI_SUM with MPI_PROD.

Compile the code using;

mpif77 reduced_loops.f -o reduced_loops

This will give you an executable called reduced_loops.

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