source: CIVL/mods/dev.civl.abc/examples/mpi/mpi_prime.c

/******************************************************************************
* FILE: mpi_prime.c
* DESCRIPTION:
*   Generates prime numbers.  All tasks distribute the work evenly, taking
*   every nth number, where n is the stride computed as:  (rank *2) + 1
*   so that even numbers are automatically skipped.  The method of using
*   stride is preferred over contiguous blocks of numbers, since numbers
*   in the higher range require more work to compute and may result in 
*   load imbalance.  This program demonstrates embarrassing parallelism.
*   Collective communications calls are used to reduce the only two data
*   elements requiring communications: the number of primes found and
*   the largest prime.
* AUTHOR: Blaise Barney 11/25/95 - adapted from version contributed by 
*   Richard Ng &  Wong Sze Cheong during MHPCC Singapore Workshop (8/22/95).
* LAST REVISED: 04/13/05
******************************************************************************/
#include <mpi.h>
#include <civlmpi.cvh>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define LIMIT     8    /* Increase this to find more primes */
#define FIRST     0           /* Rank of first task */

int isprime(int n) {
  int i,squareroot;
  if (n>10) {
   squareroot = (int) sqrt(n);
   for (i=3; i<=squareroot; i=i+2)
      if ((n%i)==0)
         return 0;
   return 1;
   }
  /* Assume first four primes are counted elsewhere. Forget everything else */
  else
    return 0;
}


int main (int argc, char *argv[])
{
  int   
    ntasks,               /* total number of tasks in partitiion */
    rank,                 /* task identifier */
    n,                    /* loop variable */
    pc,                   /* prime counter */
    pcsum,                /* number of primes found by all tasks */
    foundone,             /* most recent prime found */
    maxprime,             /* largest prime found */
    mystart,              /* where to start calculating */
    stride;               /* calculate every nth number */

  double start_time,end_time;
        
  MPI_Init(&argc,&argv);
  MPI_Comm_rank(MPI_COMM_WORLD,&rank);
  MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
  if (((ntasks%2) !=0) || ((LIMIT%ntasks) !=0)) {
    printf("Sorry - this exercise requires an even number of tasks.\n");
    printf("evenly divisible into %d.  Try 4 or 8.\n",LIMIT);
    MPI_Finalize();
    exit(0);
  }
  
  start_time = MPI_Wtime();   /* Initialize start time */
  mystart = (rank*2)+1;       /* Find my starting point - must be odd number */
  stride = ntasks*2;          /* Determine stride, skipping even numbers */
  pc=0;                       /* Initialize prime counter */
  foundone = 0;               /* Initialize */
  
  /******************** task with rank 0 does this part ********************/
  if (rank == FIRST) {
    printf("Using %d tasks to scan %d numbers\n",ntasks,LIMIT);
    pc = 4;                  /* Assume first four primes are counted here */
    for (n=mystart; n<=LIMIT; n=n+stride) {
      if (isprime(n)) {
         pc++;
         foundone = n;
         /***** Optional: print each prime as it is found
         printf("%d\n",foundone);
         *****/
         }
    }
    MPI_Reduce(&pc,&pcsum,1,MPI_INT,MPI_SUM,FIRST,MPI_COMM_WORLD);
    MPI_Reduce(&foundone,&maxprime,1,MPI_INT,MPI_MAX,FIRST,MPI_COMM_WORLD);
    end_time=MPI_Wtime();
    printf("Done. Largest prime is %d Total primes %d\n",maxprime,pcsum);
    printf("Wallclock time elapsed: %.2lf seconds\n",end_time-start_time);
  }


/******************** all other tasks do this part ***********************/
if (rank > FIRST) {
  for (n=mystart; n<=LIMIT; n=n+stride) {
    if (isprime(n)) {
      pc++;
      foundone = n;
         /***** Optional: print each prime as it is found
         printf("%d\n",foundone);
         *****/
         }
  }
  MPI_Reduce(&pc,&pcsum,1,MPI_INT,MPI_SUM,FIRST,MPI_COMM_WORLD);
  MPI_Reduce(&foundone,&maxprime,1,MPI_INT,MPI_MAX,FIRST,MPI_COMM_WORLD);
 }

 MPI_Finalize();
}