Re: Posix periodic time first tick

[Per Oberg <pero@wolfram.com>]

----- Den 10 aug 2020, på kl 9:52, Petr Červenka grugh@centrum.cz skrev:

> Hello.

> The problem is only when I use Xenomai linking flags and only with
> thestart time. Period is OK.

> That aprox. 16s time delay can be "compensated" by substracting of any
> number of seconds from the start time.

You could try using CLOCK_REALTIME or CLOCK_HOST_REALTIME instead of CLOCK_MONOTONIC. I use timerfd and similar heuristics without issues afaik.


>From the top of my head I don't recall exactly how Xenomai timers correspond to Linux timers but you may have an issue if you check your starting time a non RT thread and use that value in a RT thread, depending on your choice of timer.  

Check out: https://gitlab.denx.de/Xenomai/xenomai/-/wikis/Porting_To_Xenomai_POSIX

I have had some issues with time-keeping of my machines and I had to do a bit of manual tuning for it to work properly (talking 0.x pct drifts but also a bit of a offset). For debugging I used the  following:

* Hard coding freq using boot time kernel argument: "clocksource=tsc tsc=reliable xenomai.clockfreq=2807909955"
* Using (in my case ) "adjtimex -f 2080674" to adjust the time-keeping scaling
* "clocktest" from Xenomai test-suite. 
 Right now I have (for a setting tuned to some machine and then run on another)
CPU      ToD offset [us] ToD drift [us/s]      warps max delta [us]
--- -------------------- ---------------- ---------- --------------
  0          110881217.1       316239.964          0            0.0
  1          110881224.7       316239.943          0            0.0
  2          110881216.8       316239.928          0            0.0
  3          110881217.4       316239.921          0            0.0


Which is about 110s and drifting 0.3s per second. 

Regards
Per Öberg

> // Create timer
> timer_fd = timerfd_create(CLOCK_MONOTONIC, 0);

> // Get current time
> struct timespec start;
> clock_gettime(CLOCK_MONOTONIC, &start);

> // Set periodic
> struct itimerspec timer_conf;
> memset(&timer_conf, 0, sizeof (timer_conf));
> timer_conf.it_value = timespec_add(start, -15 * NANOSEC_PER_SEC);
> // compensation of the delay
> ...
> timerfd_settime(timer_fd, TFD_TIMER_ABSTIME, &timer_conf, NULL);

> There is strange unexpected difference/time shift between product of
> clock_gettime and timerfd_settime. Only with Xenomai posix wrappers.

> Best regards

> Petr Cervenka

> Dne 10.08.2020 v 8:45 Per Oberg napsal(a):
> > ----- Den 7 aug 2020, på kl 17:12, xenomai xenomai@xenomai.org skrev:

> >> Hello.
> >> I have created small example of periodic task in posix.
> >> If I compile it without xenomai it works fine.
> >> When I add Xenomai linking flags, the first tick of periodic task take around
> >> 16s. After that it continues as it should.
> >> Used clock type for timer_fd is CLOCK_MONOTONIC. The example should in
> >> attachment.
> >> Do you have any suggestions?
>> Just had a extremely quick look at it. Are you sure you don't get any rounding
> > errors when doing your period arithmetics ?

> >> Best regards
> >> -------------- next part --------------
> >> A non-text attachment was scrubbed...
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> > Per Öberg