Re: [chrony-users] Accurately measuring clock drift

[Rob Janssen <chrony-users@xxxxxxxxx>]

Miroslav,

That serial port example uses gpsd.  I wonder how that compares to using
"ldattach 18 /dev/ttyS0" and using /dev/pps0 in chrony.
That avoids the path through a user process and via SHM (which has only
microsecond resolution).

I use chrony this way on some servers (also for simulcast) and I do poll and
graph the offset of System Time displayed by chronyc tracking.  These graphs
show that the offset peaks differ quite a lot by type of machine, with e.g.
some HP ProLiant machines peaking up to 5us offset while (older and slower)
Dell servers peak up to 200ns.  No idea why.

I don't know if these numbers can be compared to what is shown by Thangalin
and in the docs you mention, as these are all different measurement methods.
Also of course the fact that the system time remains stable relative to the
observed PPS input does not mean at all that it is accurate: there could be a
systemic offset (due to hardware and software delays) that is impossible to
measure this way.  But when one would have "the same" machines all over the
network, that would not matter in the use case of simulcast.

Rob

On 6/20/23 09:48, Miroslav Lichvar wrote:
> - What else would we need to do to achieve sub-30 μs clock drift (or
>>    sub-10 μs)?
> That depends on the hardware. There might be a better way to timestamp
> the PPS that doesn't involve interrupts. Is it an x86_64 machine?
> See these two examples:
>
> https://chrony.tuxfamily.org/examples.html#_server_using_reference_clock_on_serial_port
> https://chrony.tuxfamily.org/examples.html#_server_using_reference_clock_on_nic
>


-- 
To unsubscribe email chrony-users-request@xxxxxxxxxxxxxxxxxxxx 
with "unsubscribe" in the subject.
For help email chrony-users-request@xxxxxxxxxxxxxxxxxxxx 
with "help" in the subject.
Trouble?  Email listmaster@xxxxxxxxxxxxxxxxxxxx.