Subj : NPTL, linuxthreads, and posix periodic timers (time shift)
To   : comp.programming.threads
From : =?ISO-8859-15?Q?st=E9phane_SARAGAGLIA?=
Date : Tue Jan 04 2005 03:03 pm

Hi all,

I am encountering troubles using posix timers with NPTL (debian sarge, 
2.6.8-1-686 kernel, libc-2.3.2.so, gcc 3.3.4).

I have already posted this message on comp.os.linux.development.system 
and comp.os.linux.development.applications without success. I think this 
list is more suitable, sorry for those who read all these lists...

By testing accuracy of timers, I am using a piece of code wich prints
the time when a timer signal is received :

-------------------------- SOURCE BEGIN -----------------------
#define POSIX_SOURCE 1
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <time.h>
#include <sys/time.h>

static volatile sig_atomic_t timer_tick = 0;

void sigalarm(int signo)
{
      struct timeval time0;
  struct tm      *ma_date;

  gettimeofday(&time0, NULL);
  ma_date = localtime( &time0.tv_sec );
  printf("%02d:%02d:%02d.%03d ",
              ma_date->tm_hour, ma_date->tm_min, ma_date->tm_sec,
              (unsigned)(time0.tv_usec/1000));
}

int main(int argc, char *argv[])
{
      clock_t clock = CLOCK_REALTIME;
      int signum = SIGRTMAX;
      int microsec = 100000;usec
      int status;
      struct timeval time0, time1;
      double dt;
      timer_t timer_id;
      struct itimerspec ts;
      struct sigevent se;
      struct sigaction act;
      sigset_t sigmask;

      /* Command-line arguments: */
      if (argc >= 2) microsec = atol(argv[1]);

      /* Set up signal handler: */
      sigfillset(&act.sa_mask);
      act.sa_flags = 0;
      act.sa_handler = sigalarm;
      sigaction(signum, &act, NULL);

      /* Set up timer: */
      memset(&se, 0, sizeof(se));
      se.sigev_notify = SIGEV_SIGNAL;
      se.sigev_signo = signum;
      se.sigev_value.sival_int = 0;

      if (timer_create(clock, &se, &timer_id) < 0) {
          perror("timer_create");
          return EXIT_FAILURE;
      }

      /* Start timer: */
      ts.it_interval.tv_sec = microsec / 1000000;
      ts.it_interval.tv_nsec = (microsec % 1000000) * 1000;
      ts.it_value = ts.it_interval;

  if (timer_settime(timer_id, 0, &ts, NULL) < 0) {
          perror("timer_settime");
          return EXIT_FAILURE;
      }

while (1){
sleep(10);
}
      timer_delete(timer_id);

      return EXIT_SUCCESS;
}
-------------------------- SOURCE END -----------------------

When linuxthreads is selected with (setenv LD_ASSUME_KERNEL 2.4.27), the
display I obtain is (DISPLAY1) :
-------------------------- DISPLAY1 BEGIN -----------------------
1) date :17:25:10.662
2) date :17:25:10.762
3) date :17:25:10.863  <-- .001s slow from prev tick : error accepted
4) date :17:25:10.963
5) date :17:25:11.063
6) date :17:25:11.163
7) date :17:25:11.263
8) date :17:25:11.363
9) date :17:25:11.462  <-- .001s fast from prev tick : error accepted
10) date :17:25:11.562
-------------------------- DISPLAY1 END -----------------------
Error from 1) to 10) is something like .001 seconds aswell : error accepted.
I have performed several tests with 2.4 kernel, and error from start 
tick seems to always be consistent whatever the timer duration : 
something equivalent to .001s.

When NPTL is selected with (setenv LD_ASSUME_KERNEL "" (default)), the
display I obtain is (DISPLAY2) :
-------------------------- DISPLAY2 BEGIN -----------------------
1) date :17:28:14.203
2) date :17:28:14.304  <-- .001s slow from prev tick : error accepted
3) date :17:28:14.405  <-- .001s slow from prev tick : error accepted
4) date :17:28:14.506  ...
5) date :17:28:14.607
6) date :17:28:14.708
7) date :17:28:14.809
8) date :17:28:14.910
9) date :17:28:15.011
10) date :17:28:15.112
-------------------------- DISPLAY2 END -----------------------
Error from 1) to 10) is something like .009, and will be more and more 
consequential with time : error not accepted.

I would like to use NPTL, but the results displayed by DISPLAY2 are
alarming me...
I have tried using 2.6.7 and 2.6.9 kernels, and I obtain the same results...

Why such a difference between 2.6 and 2.4 kernel ?
Is there something wrong with my version of NPTL ?
Are posix timers not suitable to my needs ?
Is there something wrong with my source ?
Is there a problem on librt.so library ?

Regards

.