[ntp:questions] PSYCHO PC clock is advancing at 2 HR per second

Miroslav Lichvar mlichvar at redhat.com
Mon Mar 26 11:13:33 UTC 2012

On Fri, Mar 23, 2012 at 06:12:11PM +0100, Terje Mathisen wrote:
> Miroslav Lichvar wrote:
> >On Fri, Mar 23, 2012 at 11:49:19AM +0100, Terje Mathisen wrote:
> >But I think a much bigger problem with the clock filter and PLL
> >combination is that it can't drop more than 7 samples. When the
> >network is saturated, it's usually better to drop much more than. If
> >the increase in delay is 1 second and the clock is good to 10 ppm, it
> >could wait for days before accepting another sample.
> Oh but it can!
> Check out "huff-puff"!
> You can easily tell ntpd to coast past multi-hour periods of
> excessive delays/traffic.

With huff-puff it doesn't really coast, it just shifts the offset in
one direction by increase in the delay. This works well when the link
is saturated in one direction, but under normal conditions it makes the
timekeeping worse, so you need to consider if it's worth enabling.

If you want to see why ntpd can't drop more samples you can block the
NTP packets in firewall, e.g. in a cycle which allows 4 packets and
drops 60. The PLL will be unstable, frequency will be jumping up
and down, offset orders of magnitude higher. This is the reason why
some other NTP implementations were created.

Miroslav Lichvar

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