[ntp:questions] Re: ntpd PLL and clock overshoot

Richard B. Gilbert rgilbert88 at comcast.net
Tue Oct 17 19:00:28 UTC 2006


David L. Mills wrote:
> Richard,
> 
> You say the initial offset starts out at +90 ms and then "overshoots to 
> -9 ms". It can't do that; the PLL/FLL impulse response with default poll 
> interval crosses zero in about 3000 s, then <overshoots> about 6 
> percent. Even if you set the poll interval to the minimum 16 s, the zero 
> crossing would be about 750 s with identical overshoot.
> 
> What you describe looks like the clock is being set directly, not 
> disciplined by the PLL/FLL loop, or the adjtime() system call is broken, 
> as discussed previously. The discipline loop acts as a lowpass filter; 
> it cannot torque the offset "quickly" as you describe.
> 
> I do assume your ntpd code is relatively recent, like in the last year 
> or two. While some details of the ntpd initial training states have 
> changed in minor ways, the semi-linear PLL/FLL loop has been unchanged 
> for some time. The old xntpd code was seriously broken in this area and 
> displayed ntpq data that could be in serious error.
> 
> Dave
> 
> Richard B. Gilbert wrote:
> 
>> David Woolley wrote:
>>
>>> In article <egr25e$kd3$1 at scrotar.nss.udel.edu>,
>>> David L. Mills <mills at udel.edu> wrote:

Pretty recent:

sunblok_$ ntpq -crv
assID=0 status=04c4 leap_none, sync_uhf_clock, 12 events, 
event_peer/strat_chg,
version="ntpd 4.2.1p241-RC at 1.1498 Wed Apr 26 20:16:54 EDT 2006 (1)",
processor="sun4u", system="SunOS/5.8", leap=00, stratum=1,
precision=-21, rootdelay=0.000, rootdispersion=0.371, peer=42873,
refid=GPS, reftime=c8dfa4cd.4c7f32bc  Tue, Oct 17 2006 14:51:57.298,
poll=4, clock=c8dfa4d4.d4e8a36a  Tue, Oct 17 2006 14:52:04.831, state=4,
offset=0.000, frequency=12.473, jitter=0.001, noise=0.001,
stability=0.000

I'll save the log file and the stats files the next time I'm forced to 
restart it.  We used to have almost monthly power outrages which 
necessitated a a warm restart but a neighbor was persuaded to trim some 
trees that were actually touching the power lines and it's been months 
since the power went off!




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