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

Dave Hart hart at ntp.org
Wed Nov 9 02:32:27 UTC 2011

On Tue, Nov 8, 2011 at 23:26, Chuck Swiger <cswiger at mac.com> wrote:
> If you've set ntpd's priority above 100 via realtime scheduling class,
> then it has priority over the system kernel threads which service
> network interrupts.  select() might be legitimately returning zero
> because ntpd is running before the NTP packet gets processed
> by the network stack.
> If you continue to run ntpd bound to processor 0, but change priority
> to 59 fixed-priority, does it see the packets then?

Chuck advice to ensure ntpd's priority is lower than the
interrupt-processing thread(s) seems wise to me.  Moreover, there's
not a lot to be gained by running ntpd at elevated priority when your
synchronized to network peers on systems which support SO_TIMESTAMP,
as most Unix systems do and Solaris does.  With SO_TIMESTAMP, ntpd's
packet receive timestamp comes from a timeval (usec resolution)
captured by the network stack at interrupt time or shortly thereafter,
rather than by ntpd querying the clock after select() returns.

When using a reference clock, or on a system without SO_TIMESTAMP such
as Windows, elevated priority improves the latency of the receive

Dave Hart

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