Parallel sum 1 bn numbers pthreads

psum.c

/* 

Adapted from:
https://computing.llnl.gov/tutorials/pthreads/samples/arrayloops.c
http://stackoverflow.com/questions/2962785/c-using-clock-to-measure-time-in-multi-threaded-programs 

Run as:
gcc -Ofast -pthread psum.c -lm && ./a.out 

*/

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>

#define NTHREADS     16
#define ARRAYSIZE   1000000000
#define ITERATIONS   ARRAYSIZE / NTHREADS

double  sum=0, *a;
pthread_mutex_t sum_mutex;


void *do_work(void *tid)
{
  int i, start, *mytid, end;
  double mysum=0;

  mytid = (int *) tid;
  start = (*mytid * ITERATIONS);
  end = start + ITERATIONS;

  for (i=start; i < end ; i++) {
    mysum = mysum + a[i];
    /* mysum = mysum + sin(a[i]); */
    }

  pthread_mutex_lock (&sum_mutex);
  sum = sum + mysum;
  pthread_mutex_unlock (&sum_mutex);
  pthread_exit(NULL);
}


int main(int argc, char *argv[])
{
  int i, start, tids[NTHREADS];
  pthread_t threads[NTHREADS];
  pthread_attr_t attr;
  struct timespec start_tm, finish_tm;
  double elapsed;
  double mysum;

  a = malloc(ARRAYSIZE*sizeof(double));
  for (i=0;i<ARRAYSIZE;i++){
    a[i] = 1;
  }

  clock_gettime(CLOCK_MONOTONIC, &start_tm);

  pthread_mutex_init(&sum_mutex, NULL);
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
  for (i=0; i<NTHREADS; i++) {
    tids[i] = i;
    pthread_create(&threads[i], &attr, do_work, (void *) &tids[i]);
    }

  for (i=0; i<NTHREADS; i++) {
    pthread_join(threads[i], NULL);
  }

  pthread_attr_destroy(&attr);
  pthread_mutex_destroy(&sum_mutex);

  clock_gettime(CLOCK_MONOTONIC, &finish_tm);
  elapsed = (finish_tm.tv_sec - start_tm.tv_sec);
  elapsed += (finish_tm.tv_nsec - start_tm.tv_nsec) / 1000000000.0;

  printf ("p = %d \n", NTHREADS);
  printf ("n = %e \n", (double)ARRAYSIZE);
  printf ("s = %e \n", sum);
  printf("time: %f\n", elapsed);


  clock_gettime(CLOCK_MONOTONIC, &start_tm);

  mysum = 0;
  for (i=0; i < ARRAYSIZE ; i++) {
    mysum = mysum + a[i];
    /* mysum = mysum + sin(a[i]); */
  }

  clock_gettime(CLOCK_MONOTONIC, &finish_tm);
  elapsed = (finish_tm.tv_sec - start_tm.tv_sec);
  elapsed += (finish_tm.tv_nsec - start_tm.tv_nsec) / 1000000000.0;
  printf ("s0 = %e \n", mysum);
  printf("time0: %f\n", elapsed);

  pthread_exit (NULL);
}