I had the same problem when i tried to convert the latency
test example to C++. I thought to be smart and use nano seconds
everywhere this resulted in increasing latency. As far as
i can tell is it due to rounding errors when converting from
ticks to nanoseconds and back. I advice you to try the original
latency_calibration example and see if it happens there too.
- Erwin
Sebastien GAUME wrote:
>
> Hello,
>
> I'm doing basic tests with a periodic task : I'm measuring when the task
> really wakes up which enables me to know much more about the jitter.
>
> I sometimes obtain excellent results (the jitter value is about 1 us)
> but I sometimes come across quite surprising results. By surprising
> results, I mean that the jitter is increasing more and more ; for
> instance, after 30 s, the jitter is about 100 us !
>
> These problems occure on the two PCs I am using : one is a P166 MMX with
> 96M Ram, the other is a PII 350 with 64M Ram. On both PC, I am using
> RTAI 1.6 with linux kernel 2.2.17.
>
> I first thought the trouble was coming from the CPU frequency. That's
> why I used your CPU frequency calibration. Unfortunately, the results
> became as bad as the earlier were, and even worse sometimes.
>
> The latency calibration was realized on both PC, but it did not change
> anything. I even tried to realize the latency calibration offered by
> RTAI 24.1.4. but the results go on being either excellent either bad.
>
> I provide you the test code. So, if you have any idea of where the
> problem comes from, do not hesitate e-mailing me. Thank you.
>
> Sebastien
>
> #include <linux/module.h>
> #include <asm/io.h>
>
> #include <rtai.h>
> #include <rtai_fifos.h>
> #include <rtai_sched.h>
>
> #define TICK_PERIOD 1000000
>
> #define STACK_SIZE 4000
>
> #define LOAD 16550
>
> RT_TASK tache_periodique[1];
>
> RTIME now, tick_period;
>
> struct donnees_
> {
> int tache; // id tache
> int passage; // nombre de passages
> long long temps; // temps entre 2 passages
> long long duree; // duree de la tache
> };
>
> void periodic_task(int id_tache)
> {
> int compteur, cpt_load;
> long long time_cpu, intermediate_time_cpu, end_time, task_duration;
> struct donnees_ donnees;
>
> // recopie id de la tache
> donnees.tache = id_tache;
>
> // initialisation compteur nb de reveils de la tache
> compteur = 0;
>
> while(1)
> {
> // recuperation date de reveil de la tache
> intermediate_time_cpu = rt_get_cpu_time_ns();
>
> // calcul de la date de reveil en partant de la date de lancement de
> l'application
> time_cpu = intermediate_time_cpu - count2nano(now);
>
> // recopie des donnes concernant la tache dans la structure donnees
> donnees.passage = compteur;
> donnees.temps = time_cpu;
>
> // chargement du cpu
> rt_busy_sleep(200000);
>
> // recuperation date de fin de la tache
> end_time = rt_get_cpu_time_ns();
>
> // calcul de la duree de la tache
> task_duration = end_time - intermediate_time_cpu;
>
> // recopie de la duree de la tache dans
> donnees.duree = task_duration;
>
> // envoi de la structure donnees (par fifo) // tous les 500 passages
> if (compteur % 500 == 0)
> {
> if (rtf_put(0, &donnees, sizeof(donnees)) < sizeof(donnees))
> {
> rt_printk("impossible d'ecrire dans la fifo \n");
> rt_task_suspend(&tache_periodique[id_tache]);
> }
> }
>
> // on incremente le compteur nb de reveils de la tache
> compteur++;
>
> //attente prochaine periode de reveil
> rt_task_wait_period();
> }
> }
>
> int init_module(void)
> {
> //creation de la fifo
> rtf_create(0, 100000);
>
> // initialisation des taches
> rt_task_init(&tache_periodique[0], periodic_task, 0, STACK_SIZE, 5, 0,
> 0);
>
> // definition du mode (oneshot)
> rt_set_oneshot_mode();
>
> // lancement du timer
> tick_period = start_rt_timer(nano2count(TICK_PERIOD));
>
> // recuperation de la date actuelle
> now = rt_get_time();
>
> rt_printk("now %d \n", count2nano(now));
>
> // lancement de la tache periodique
> rt_task_make_periodic(&tache_periodique[0], now+nano2count(500000),
> nano2count(700000));
>
> return 0;
> }
>
> void cleanup_module(void)
> {
> // arret du timer
> stop_rt_timer();
>
> // destruction de la fifo
> rtf_destroy(0);
>
> // destruction des taches
> rt_task_delete(&tache_periodique[0]);
>
> printk("Et c'est fini !!!!!!!!\n\n");
> }
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