program example8
      include 'mpif.h'
      integer ierr, rank, size, i, n, lmax, NMAX, NTIMES
      parameter (NMAX = 1 000 000, NTIMES = 10)
      double precision time_start, time, bandwidth, max
      real*8 a(NMAX)
      integer status(MPI_STATUS_SIZE)

      call MPI_INIT(ierr)
      call MPI_COMM_SIZE(MPI_COMM_WORLD, size, ierr)
      call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierr)

      time_start = MPI_WTIME(ierr)
      n = 0
      max = 0.0
      lmax = 0
      do while(n .le. NMAX)
         time_start = MPI_WTIME(ierr)
         do i = 1, NTIMES
            if(rank .eq. 0) then
               call MPI_SEND(a, n, MPI_DOUBLE_PRECISION, 1, 1, 
     &                       MPI_COMM_WORLD, ierr)
               call MPI_RECV(a, n, MPI_DOUBLE_PRECISION, 1, 1,
     &                       MPI_COMM_WORLD, status, ierr)
            end if
            if(rank .eq. 1) then
               call MPI_RECV(a, n, MPI_DOUBLE_PRECISION, 0, 1,
     &                       MPI_COMM_WORLD, status, ierr)
               call MPI_SEND(a, n, MPI_DOUBLE_PRECISION, 0, 1,
     &                       MPI_COMM_WORLD, ierr)
            end if
         enddo
         time = (MPI_WTIME(ierr)-time_start)/2/NTIMES
         bandwidth = (8*n*1.d0/(2**20))/time
         if(max .lt. bandwidth) then
            max = bandwidth
            lmax = 8*n
         end if
         if(rank .eq. 0) then
            if(n .eq. 0) then 
               print *, 'latency = ', time, ' seconds'
            else 
               print *, 8*n, ' bytes, bandwidth =', bandwidth,
     &                  ' Mb/s'
            end if
         end if
         if(n .eq. 0) then
            n = 1
         else 
            n = 2*n
         end if
      end do
      if(rank .eq. 0) then
         print *, 'max bandwidth =', max, ' Mb/s , length =', 
     &            lmax, ' bytes'
      end if
      call MPI_FINALIZE(ierr)
      end