pi_reduce.f90
327098198a93complete

pi_reduce.f90
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			! ==========================================================================
			! This exercise is taken from the class Parallel Programming Workshop (MPI,
			! OpenMP and Advanced Topics) at HLRS given by Rolf Rabenseifner
			! ==========================================================================

			program pi_reduction

			use mpi

			implicit none

			integer, parameter :: n = 10000000
			double precision :: omp_t1, omp_t2
			double precision :: dx, x, sm, pif
			double precision, external :: f
			integer :: i
			integer :: ierror, prank, psize
			integer :: nlocal, istart, iend

			call MPI_init(ierror)
			call MPI_Comm_size(MPI_COMM_WORLD, psize, ierror)
			call MPI_Comm_rank(MPI_COMM_WORLD, prank, ierror)

			! calculate pi = integral [0..1] 4 / (1 + x*2) dx /
			omp_t1 = MPI_Wtime()

			nlocal = n / psize
			istart = 1 + nlocal * prank
			iend = nlocal * (prank + 1);

			dx = 1. / n
			sm = 0.0;

			do i = istart, iend
			x = (1. * i - 0.5) * dx
			sm = sm + f(x)
			enddo

			call MPI_Allreduce(MPI_IN_PLACE, sm, 1, MPI_DOUBLE, MPI_SUM, MPI_COMM_WORLD, ierror);

			pif = dx * sm

			omp_t2 = MPI_Wtime()

			if (prank.eq.0) then
			print *, 'computed pi =',pif
			print *, 'Running time = ', (omp_t2-omp_t1)
			endif

			call MPI_Finalize(ierror)

			end program pi_reduction


			double precision function f(a)
			implicit none
			double precision, intent(in) :: a
			f = 4. / (1. + (a**2))
			end function f