Select timing, please test this program (was: Signals in Gtk+)

[Sascha Ziemann <szi aibon ping de>]

Tim Janik <timj@gtk.org> writes:

| since you use system calls like select() or alarm() for your timers
| anyways, the highest accuracy to gain is 20ms or 50Hz on intel and
| 10ms or 100Hz on alpha, iirc. 

The Intel value is correct, but I have no idea why. Does anybody know?

I got the following results:

Linux Pentium II 333:
---------------------
starting test 1 with 5000 iterations ... done
starting test 2 with 5000 iterations ... done
results for: Linux olivia 2.0.35 #13 Tue Aug 18 09:25:29 MEST 1998 i686 unknown
select without timeout:    f1 =  154985.75 Hz     d1 =   0.006452 ms
select without reading:    f2 =      49.66 Hz     d2 =  20.135937 ms

Can please some people test the program on other systems? I would like
to know how this problem is handled there.

This program calculates the minimum delay for select:

/* Time-stamp: <1998-09-15 10:16:47 szi>
**
** select speed test
**
** please try this program on your system, and send me your results
** with a short note about the CPU.
**
** Sascha Ziemann <szi@aibon.ping.de> */

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/types.h>
#include <unistd.h>

double
get_time (void)
{
  struct timeval tv;
  struct timezone tz;

  gettimeofday (&tv, &tz);

  return tv.tv_sec + 1e-6 * tv.tv_usec;
}

void
fatal_error (char *msg)
{
  fprintf (stderr, "%s\n", msg);
  exit (1);
}
  
#define ITERATIONS 5000

void main (void)
{
  double         t0;
  double         t1;
  double         f1;
  double         f2;
  unsigned int   i;
  int            fd;
  fd_set         fds;
  struct timeval tv;
  unsigned char  data;

  setvbuf (stdout, NULL, _IONBF, 0);
  
  /* select without timeout */
  printf ("starting test 1 with %d iterations ... ", ITERATIONS);
  fd = open ("/dev/zero", O_RDONLY);
  t0 = get_time ();
  for (i=0; i<ITERATIONS; i++) {
    FD_ZERO(&fds);
    FD_SET(fd, &fds);
    if (select (fd+1, &fds, NULL, NULL, NULL) != 1)
      fatal_error ("select 1 failed");
    read (fd, &data, 1);
  }
  t1 = get_time ();
  f1 = ITERATIONS / (t1-t0);
  close (fd);
  printf ("done\n");
  
  /* select without reading */
  printf ("starting test 2 with %d iterations ... ", ITERATIONS);
  t0 = get_time ();
  for (i=0; i<ITERATIONS; i++) {
    tv.tv_sec  = 0;
    tv.tv_usec = 1;
    if (select (0, NULL, NULL, NULL, &tv) != 0)
      fatal_error ("select 2 failed");
  }
  t1 = get_time ();
  f2 = ITERATIONS / (t1-t0);
  printf ("done\n");

  /* report */
  printf ("results for: ");
  system ("uname -a");
  printf ("select without timeout:    f1 = %10.2f Hz     d1 = %10.6f ms\n",
	  f1, 1000/f1);
  printf ("select without reading:    f2 = %10.2f Hz     d2 = %10.6f ms\n",
	  f2, 1000/f2); 
}

-- 
/* In the beginning was the Word: */
typedef long SCM;