Re: [Xenomai-help] Huge clock drift

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

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On 2011-05-30 09:43, Jonas Witt wrote:
> Am 30.05.2011 09:07, schrieb Jan Kiszka:
>> On 2011-05-30 09:03, Pavel Machek wrote:
>>> On Sat 2011-05-28 16:32:45, Jan Kiszka wrote:
>>>> 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.
>>> Umm. NO_HZ is only active while system is idle. Kernel will still
>>> expect the periodic ticks when CPU is busy....
>>>
>>> (I'm not sure how the compensation works; perhaps it can compensate
>>> even while busy..)
>> See update_wall_time, the !CONFIG_ARCH_USES_GETTIMEOFFSET includes no
>> fixed tick length.
>>
>> Again, this is also important for Linux when running over hypervisors
>> which tend to miss ticks on overcommitment as well.
>>
>> Jan
> 
> Thanks for the active discussion of the issue. I attached my config.
> CONFIG_NO_HZ is activated and I think I disabled all power management
> and frequency scaling correctly. Do you think it is worth trying a
> kernel with fixed Hz as Gilles suggested? Actually the 1ms Xenomai load
> seems to play at least some role in the issue.

For sure, I may also be proven wrong by plain reality.

In addition, enable CONFIG_PM and ACPI with the exception of
ACPI_PROCESSOR. Who knows what your BIOS is doing in the absence of OS
support for this.

Jan


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