[chrony-users] Chrony's offset calculation mechanism

[Thomas Koehler <mailbox.koehler@xxxxxxxxx>]

Hello List,

while experimenting with capturing a timing gps receiver's PPS-Output
highly accurate (~ 30ns pps jitter + ~30ns capturing jitter), I
configured chrony and NTPD hooked up to my linux pps source.

Chrony gave me way better results with +- 100ns offset 50% of the time
and +- 300ns offset 99% of the time.
NTPD values are each about 5 times higher.

I tried to verify this values by putting out the PPS timestamps directly
in my PPS kernel module.

There I see values of up to +- 1500ns, no matter if I use chrony or NTPD.

Interestingly the offset values NTPD reports are exactly the numbers I
see in the Kernel module.


I tried to dive into the code and look up the offset calculation. It
seems a bit complicated to me.

Is there any source of documentation for chrony where an in depth look
is given on topics like tracking offset calculations?


Thank you and best Regards!

Thomas

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