[ntp:questions] Seeing large jitter and delay in recent ntpd and recent Solaris

Chuck Swiger cswiger at mac.com
Wed Nov 9 23:44:20 UTC 2011


On Nov 9, 2011, at 2:47 PM, Chris Albertson wrote:
>> We have other real time processing occuring on the system and are sensitive to offset errors, so we want ntpd to run at the highest real time priority.
> 
> That will do little good and maybe some harm.  ntpd reads time stamped
> input.  Even if this sits in a buffer unprocessed it's OK because the
> critical work, the stamping is done inside an interrupt handler.

Agreed, Solaris ought to support and be using SO_TIMESTAMP, so it really doesn't matter whether ntpd itself is running at elevated priority.  Under normal circumstances, ntpd slews the clock and not steps it, so that just requires that it figure out the right slew rate for good timekeeping.

> Running a process at RT priority when it does not need to can take
> away CPU time from other processes and harm overall system
> performance.

Yep-- it might not be obvious, but RT can be very bad for overall system throughput compared with timesharing priority.

> If you are concerned about offset the solution is to use a good local
> reference clock

For best timekeeping, also setup PPS discipline.

Regards,
-- 
-Chuck



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