[ntp:questions] slow SHM time source

Nikolai Orekhov nowhere at mail.ru
Mon Aug 12 07:51:19 UTC 2013

Hello, All!

ntpd 4.2.6

I've got a SHM driver source but time samples can come very rarely. May be
once per 30min or once per 1min.
Also I've got an NTP server.
I know that it's not good to have two servers so I've marked them both
"true". Also "maxpoll 3" and "iburst"  is set on both of them.

The problem is when SHM driver receives a sample ( only one sample ) than
somehow it's jitter becomes = 0.01 and it becomes sys.peer instead of ntp
server which becomes candidate. Still it's dispersion is about 3900.

How ntpd can calculate jitter on only one sample? Why it selects SHM with
dispersion 3900 over ntp with significantly lower dispersion?
Can I somehow set priority on ntp server and prohibit this driver from
selection when ntp is reachable?

I know that if ntpd will query shmem slower than samples will be received
everything will be fine. The problem is I don't know exactly how often the
samples will come and also a user can change it's period without notifying

Thanks, Nickolay

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