[Bug c++/53292] multi-threaded (OpenMP) is slower than single-threaded

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-05-09 
15:01:24 UTC ---
Just try equivalent pthread program and you'll note the same behavior.

#include <pthread.h>
#include <stdlib.h>

double *p;
int c;

void *tf (void *x)
{
  int i, s = ((long) x) * c, e = s + c;
  for (i = s; i < e; i++)
    p[i] = 1.0;
  return NULL;
}

int
main (int argc, char **argv)
{
  int n = atoi (argv[1]), i;
  int sz = atoi (argv[2]);
  if (n > 32 || n < 1 || sz < 128 || (sz % n) != 0)
    return 1;
  p = malloc (sz * sizeof (double));
  if (p == NULL)
    return 1;
  c = sz / n;
  pthread_t t[32];
  for (i = 1; i < n; i++)
    pthread_create (&t[i], NULL, tf, (void *)(long) i);
  tf ((void *) 0L);
  for (i = 1; i < n; i++)
    pthread_join (t[i], NULL);
  return 0;
}