Computing p with Message Passing

• Notes:

  • thread identification first
  • explicit work sharing
  • all variables are private
  • explicit data exchange (reduce)
  • all code is parallel
  • number of processors is specified outside of code

#include <mpi.h>

#define N 1000000

main()

{

double pi, l, ls = 0.0, w = 1.0/N; int i, mid, nth;

MPI_init(&argc, &argv);

MPI_comm_rank(MPI_COMM_WORLD,&mid);

MPI_comm_size(MPI_COMM_WORLD,&nth);

for(i=mid; i<N; i += nth) {

l = (i+0.5)*w;

ls += 4.0/(1.0+l*l);

}

MPI_reduce(&ls,&pi,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);

if(mid == 0) printf(“pi is %f\n”,pi*w);

MPI_finalize();

}

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