Re: [chrony-dev] chronyd not recovering after time stepped.

[Bryan Christianson <bryan@xxxxxxxxxxxxx>]

> On 28/08/2015, at 9:13 am, Bryan Christianson <bryan@xxxxxxxxxxxxx> wrote:
> 
> 
>> On 27/08/2015, at 11:43 pm, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
>> 
>> On Thu, Aug 27, 2015 at 11:35:02PM +1200, Bryan Christianson wrote:
>>>> On 27/08/2015, at 11:24 pm, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
>>>> That is odd. How long is the polling interval? It shouldn't take more
>>>> than 6-10 new measurements after the step for the frequency to get close
>>>> again to the right value and correct any remaining offset.
>>> 
>>> The polling interval is 8 seconds. And indeed, when I restart chronyd it only takes a few cycles to correct the offset.
>>> I'm not seeing any significant change in frequency, maybe 1ppm (from 17.5 ppm) for just a few samples after the step and then back to 17.5.
>> 
>> Ok, that probably means it's something in the driver or the kernel.
>> It might help us to see what values at what time is adjtime() getting
>> and what it returns.
> 
> I think I figured it out. When the time is stepped there is a big spike in offset_sd. This causes the drift removal interval to be increased to a very large value (aprox 2 hours in the trace I just looked at) and then applied when the current drift cycle completes.
> 
> So for the next 2 hours, clock adjustments in start_adjust() are having a predicted offset of 30 ms or so. When the very long drift removal interval expires, then things go back to normal - offset_sd is now small and the system rapidly recovers.
> 
> I think the drift removal cycle should be restarted if the newly calculated interval is significantly different from the value of current_drift_removal_interval
> 

Alternatively, and much simpler, we set an upper bound on drift_removal_interval of say 200sec


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