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

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Wed, Aug 30, 2017 at 08:07:41PM +0200, Thibaut BEYLER wrote:
> The monitoring software run on another server and monitor the chrony
> clients (which are operating as servers in that case) over NTP

So the same timestamping combination that the client is using to
synchronize its clock is used in the monitoring? I'm not sure if that
is a valid test. If there is a large asymmetry and the clock has a
large error, I don't think the monitoring client would see it, because
the asymmetry would cancel the error out in the opposite direction, in
which the monitoring client is making measurements.

In order to measure the error with SW timestamping it's necessary to
use something better, e.g. HW timestamping or a reference clock.

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.

> I can notice however that when using DK, KK or HK the reported jitter
> asymmetry is almost always all the time to +0.50 , with HH it's always 0
> 
> Could the asymmetric jitter correction added in chrony 3 produce wrong time
> in my setup ? Is it possible to disable this in order to test ?

With 3.2-pre2 you can disable it with "asymmetry 0.0" in the server
directive.

-- 
Miroslav Lichvar

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