RE: [chrony-dev] makestep command sometimes makes chrony stop reading its sources

["Hattink, Tjalling (FINT)" <T.Hattink@xxxxxxxx>]

>On Wed, Jan 20, 2010 at 10:50:39AM +0100, Hattink, Tjalling (FINT)
wrote:
>> So far the results are very good under normal conditions. When a 
>> properly synchronised SHM and PPS clock is provided and the rtc clock

>> was already somewhat in sync the system converges in minutes after a 
>> cold boot! Also temperature changes are corrected a lot faster by 
>> Chrony than normally done by ntpd. Sometimes ntpd was off for 
>> milliseconds for hours when the temperature increased with 5 degrees,

>> but Chrony was only drifting 300 nSec for half an hour.
>
>Just curious, what vendor/model is the GPS card and what std dev is
>reported in sourcestats in normal conditions?

The GPS card is a commercial grade board from NovAtel, type OEMV3. It is
used for high-precision positioning on survey boats. According to specs
its time accuracy is 20 ns RMS. What I see in chronyc for the PPS signal
is a std dev of around 140 ns.

>
>Please note that currently chrony can't slew below microseconds, so
even
>when the reported std dev is very low, the real offset can't be better
>than 0.25 us on average. (the remaining offset is reported in the
tracking
>output).

I have to correct myself. Chrony was drifting 300 uSec for half an hour,
not 300 nSec ! I have to say that the system clock on this system is
really
bad. On other systems I've seen it drifting for only 100 uSec for 10
minutes.

I use a separate pc that reads out the ntp time from chrony every second
and compares it with its own PPS corrected time using a in-house
developed
application used for timekeeping our systems. It is quite precise and it
shows a std dev of 10 uSec for the ntp time for chrony.

>> I think there are some absolute timestamps used in chrony that are
not 
>> updated when makestep is issued.
>
>Correct, the timestamps used in scheduler aren't updated when makestep
is
>performed. The problem is that scheduler needs to use raw time (unlike
>pretty much everything else in chronyd) and adjusting only raw time
isn't
>reported by the ParameterChange system.
>
>Probably the easiest fix is to move the makestep code from system
drivers
>where it goes unnoticed to local module and do two adjustments, one to
>cancel the remaining slew and second to do the actual step. This also
makes
>the makestep command available on all systems.
>
>Please try the attached patch.
>

I'll try the patch and let you know of the results today.

--
Tjalling Hattink

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