[ntp:questions] Polling interval in FreeBSD vs. Windows

Miroslav Lichvar mlichvar at redhat.com
Wed Jan 19 08:42:36 UTC 2011


On Tue, Jan 18, 2011 at 09:48:08PM +0000, David Woolley wrote:
> That was my point.  Unruh's main issue is that, on modern LANs, the
> dominant low frequency error is in the local clock, rather than the
> measurements.

That's the theory behind NTP.

> It's more complicated.  I don't think the current version ages the
> samples,

It still does, see clock_filter() in ntp_proto.c:

	dtemp = clock_phi * (current_time - peer->update);

> I
> think the filter will take out more like 7 in 8 for gaussian input,
> but the expected input pattern isn't actually gaussian, either.

I think the delay is assumed to be exponentially distributed (and
that's what I use in my simulations). It would be interesting to
analyse real data from rawstats.

Also, to answer the question whether PLL has a good lock, you can use
the runs tests. Run this command on your loopstats and we'll see if
the offset is random or not.

awk '{ n++; r += $3 * l < 0; l = $3 } END { print r " / " n }'

-- 
Miroslav Lichvar



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