Re: [chrony-users] Time offset on versions 3+ without hw timestamping

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On Thu, Aug 31, 2017 at 06:48:14PM +0200, Thibaut BEYLER wrote:
> > You could run a separate server instance of chronyd on a different
> > port with HW timestamping for the monitoring client. It needs to
> > support the interleaved mode to be able to get the server's HW
> > transmit timestamps.
> Ok i think i understand what you mean, i will try to configure a server
> instance to see if there is differencies. What source should i configure on
> that instance ?

Just the local clock, e.g. 'local stratum 10' in chrony.conf.

> I tried to force the jitter asymmetry to 0 and got good results again. Also
> with this settings test C are not failing anymore.

The question is if "good" really means more accurate here. I'm still
not sure what is your reference and how is measured the offset of the
clients, what timestamping is involved, etc. If the monitoring client
used DK timestamping, it wouldn't be surprising if DK on the machine
which is tested gave the smallest offset.

I think a good way to measure the accuracy of the clients would be to
get an independent stratum-1 NTP server which is known to be accurate
(e.g. the LeoNTP unit), add a network card which has HW timestamping
and fast reading of the clock (e.g. i210 or i350) to the client that
should be tested, connect it directly to the reference server and
measure the offset on the client with a separate chronyd instance
configured to not adjust the clock.

Something like this:

Production NTP server ---> Network ---> Client <---- Reference NTP server

> The jitter asymmetry goes especially very fast from 0 to +0.50 with the
> sources that have hw timestamping, here are some logs just after a chrony
> restarts for instance :

That suggests some of the timestamps (server's or client's) are not HW
timestamps, or that the client is not using the interleaved mode when
it needs to. An HH client with HH server connected to same switch with
low network traffic in my experience doesn't show any asymmetry. With
kernel and daemon timestamps that's normal.

A good way to confirm that all timestamps used for synchronization are
HW timestamps is to check the delay as reported in measurements.log or
chronyc ntpdata. If you know the the switch adds 20 microseconds, but
ntpdata shows delay larger than say 30 microseconds (assuming 1Gb
ethernet), you know something is wrong.

Miroslav Lichvar

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