pthread_example.c

#include <pthread.h>
#include <stdlib.h>
#define MAX_THREADS 512
void *compute_pi (void *);
//....
int main() {
//...
pthread_t p_threads[MAX_THREADS];
pthread_attr_t attr;
pthread_attr_init (&attr);
for (i=0; i< num_threads; i++) {
hits[i] = i;
pthread_create(&p_threads[i], &attr, compute_pi,
(void *) &hits[i]);
}
for (i=0; i< num_threads; i++) {
pthread_join(p_threads[i], NULL);
total_hits += hits[i];
}
//...
}

void *compute_pi (void *s) {
int seed, i, *hit_pointer;
double rand_no_x, rand_no_y;
int local_hits;
hit_pointer = (int *) s;
seed = *hit_pointer;
local_hits = 0;
for (i = 0; i < sample_points_per_thread; i++) {
rand_no_x =(double)(rand_r(&seed))/(double)((2<<14)-1);
rand_no_y =(double)(rand_r(&seed))/(double)((2<<14)-1);
if (((rand_no_x - 0.5) * (rand_no_x - 0.5) +
(rand_no_y - 0.5) * (rand_no_y - 0.5)) < 0.25)
local_hits ++;
seed *= i;
}
*hit_pointer = local_hits;
pthread_exit(0);
}

Note the use of the function rand_r (instead of superior random
number generators such as drand48).
• Executing this on a 4-processor SGI Origin, we observe a 3.91 fold
speedup at 32 threads. This corresponds to a parallel efficiency of
0.98!
• We can also modify the program slightly to observe the effect of
false-sharing.
• The program can also be used to assess the secondary cache line
size.