61 lines
2.1 KiB
C
61 lines
2.1 KiB
C
|
// Working version of threaded picalc. Uses a mutex to lock the global
|
||
|
|
// variable total_hits so that one thread accesses it at a time. This
|
||
|
|
// version is inefficient due to the busy waiting and contention
|
||
|
|
// associated with mutex locking.
|
||
|
|
|
||
|
|
#include <stdio.h>
|
||
|
|
#include <stdlib.h>
|
||
|
|
#include <pthread.h>
|
||
|
|
|
||
|
|
// Global variables shared by all threads
|
||
|
|
int total_hits=0;
|
||
|
|
int points_per_thread = -1;
|
||
|
|
pthread_mutex_t lock;
|
||
|
|
|
||
|
|
void *compute_pi(void *arg){
|
||
|
|
long thread_id = (long) arg;
|
||
|
|
unsigned int rstate = 123456789 * thread_id; // give each thread its own starting point
|
||
|
|
for (int i = 0; i < points_per_thread; i++) {
|
||
|
|
double x = ((double) rand_r(&rstate)) / ((double) RAND_MAX);
|
||
|
|
double y = ((double) rand_r(&rstate)) / ((double) RAND_MAX);
|
||
|
|
if (x*x + y*y <= 1.0){
|
||
|
|
pthread_mutex_lock(&lock); // lock global variable
|
||
|
|
total_hits++; // update
|
||
|
|
pthread_mutex_unlock(&lock); // unlock global variable
|
||
|
|
}
|
||
|
|
}
|
||
|
|
return NULL;
|
||
|
|
}
|
||
|
|
|
||
|
|
int main(int argc, char **argv) {
|
||
|
|
if(argc < 2){
|
||
|
|
printf("usage: %s <num_samples> [num_threads]\n",argv[0]);
|
||
|
|
printf(" num_samples: int, how many sample points to try, higher gets closer to pi\n");
|
||
|
|
printf(" num_threads: int, number of threads to use for the computation, default 4\n");
|
||
|
|
return -1;
|
||
|
|
}
|
||
|
|
|
||
|
|
int npoints = atoi(argv[1]); // number of samples
|
||
|
|
int num_threads = argc>2 ? atoi(argv[2]) : 4; // number of threads
|
||
|
|
|
||
|
|
points_per_thread = npoints / num_threads; // init global variables
|
||
|
|
pthread_mutex_init(&lock, NULL); // create a lock
|
||
|
|
|
||
|
|
pthread_t threads[num_threads]; // track each thread
|
||
|
|
|
||
|
|
for(long p=0; p<num_threads; p++){ // launch each thread
|
||
|
|
pthread_create(&threads[p],NULL,compute_pi, (void *) (p+1));
|
||
|
|
}
|
||
|
|
|
||
|
|
for(int p=0; p<num_threads; p++){ // wait for each thread to finish
|
||
|
|
pthread_join(threads[p], (void **) NULL);
|
||
|
|
}
|
||
|
|
|
||
|
|
pthread_mutex_destroy(&lock); // get rid of the lock
|
||
|
|
|
||
|
|
double pi_est = ((double)total_hits) / npoints * 4.0;
|
||
|
|
printf("npoints: %8d\n",npoints);
|
||
|
|
printf("hits: %8d\n",total_hits);
|
||
|
|
printf("pi_est: %f\n",pi_est);
|
||
|
|
}
|