Re: [Xenomai-help] Huge clock drift

[Jan Kiszka <jan.kiszka@domain.hid>]

[-- Attachment #1: Type: text/plain, Size: 1241 bytes --]

On 2011-05-27 21:11, Gilles Chanteperdrix wrote:
> On 05/27/2011 08:29 PM, Jonas Witt wrote:
>> Sorry, I missed the NTP-part. I am not using NTP. Just plain timer 
>> queries on a single system.
>>
>> My clock source is tsc which is the same for Xenomai I suppose.
>>
>> I wonder how a Xenomai task, even if it occupies 50% or even 90% of a 4 
>> milliseconds time slice can interfere with the tsc. The tsc is not 
>> incremented via an interrupt, is it? But I do not know much about the 
>> inner workings of these functions.
> 
> The problem is not the clocksource, the problem is the timer interrupt.
> The kernel expects 1 timer tick every millisecond.

Not on archs that are CONFIG_NO_HZ capable.

> When you run a
> real-time task during 2 milliseconds and prevent the kernel from
> receiving the timer interrupts, you certainly disrupt its timekeeping.
> If you want to do this, switch the Linux kernel frequency (CONFIG_HZ) to
> 100.

Time keeping can perfectly bridge a lot of missing ticks as far as the
underlying clocksource allows. And that's quite a bit with the x86 TSC.

I wonder if the TSC is sufficiently stable here. Forgot to disable
CONFIG_CPU_FREQ? How does your .config look like?

Jan


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 259 bytes --]