[chrony-users] Chrony's offset calculation mechanism

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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!


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