| Re: [chrony-users] Time offset on versions 3+ without hw timestamping |
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William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
UBC, Vancouver,BC _|_ Program in Cosmology |____ unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 ____|____ and Gravity ______|_ www.theory.physics.ubc.ca/
On Wed, 30 Aug 2017, Miroslav Lichvar wrote:
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.
The problem is interrupt contention, and clock reading contention. I once
tried to have two different readers (chrony and ntpd, or two versions of
chrony) reading the interrupt at the same time and one of them was about 8us
out because it did not get the interrupt until the first one had finished.
Ie, in monitoring you do not want to do it "on the second" since it might
interfere with the other one.
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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