[ntp:questions] slow SHM time source

unruh unruh at invalid.ca
Mon Aug 12 11:45:45 UTC 2013

On 2013-08-12, Nikolai Orekhov <nowhere at mail.ru> wrote:
> Hello, All!
> ntpd 4.2.6
> I've got a SHM driver source but time samples can come very rarely. May be

What is your shm driver source? What program are you using to feed shm?
What feeds data to that program?

> once per 30min or once per 1min.

You know this how?

> Also I've got an NTP server.

What does that mean? Perhaps if you put here the contents of your
ntp.conf file.

> 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.

Sure it is fine to hav as many servers as you want. ntpd is designed to
use them all. 

> 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
          ^^^^  its
What do you have in ntp.conf?

> 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
> ntpd.

???????????? That makes no sense at all. 

> Thanks, Nickolay

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