[ntp:hackers] Does ntpd need to whine more ?
Mark.Martinec at ijs.si
Mon Oct 3 22:57:19 UTC 2005
> >Your comment about rapid temperature excursions leading to steps is very
> >relavent. I don't see that here in room temperature controlled
> >environments, but laptops could be another story.
From Poul-Henning Kamp:
> It depends wildly on the kind of hardware, with some correlation to
> price, but also a lot on the A/C at the installation.
> As I see it, the NTP clock is tuned for the good boys and makes
> a really rough ride for the regular boys because of it. Most of
> the hosts that poll my NTP server with > 256second poll rate
> do not send me timestamps back that justify it.
The A/C in a computer room with its 2 degC temperature fluctuation
can wreak havoc in the ms range. As Dave is asking for specific
proof, I have just the right example for the case.
See Fig.5 in my document: http://www.ijs.si/time/
The accompanying text says:
Example: The following diagram shows the behaviour of two similar NTP V4
stratum-2 servers located close together in the same air-conditioned room.
Their reference stratum-1 servers are accessed over WAN (about 50 ms
round-trip delay), resulting in a typical polling interval of 1024 seconds
and consequently large PLL time constant. The quartz oscillator of host P has
a relatively large temperature dependency: judging from the diagram the
temperature coefficient is about +1.2 ppm / K. The other host K has almost 20
times lower temperature coefficient (perhaps only apparently due to a better
chassis design) and is shown as a reference.
Despite air-conditioning the temperature changes near the computer cabinets
show daily peak-to-peak range of almost three degrees centigrade, the
temperature excursion on day 3 at noon (time = 3.5) was due to other reasons.
Fortunately the temperature changes are gradual and quite smooth -- no direct
airflow from the air-conditioning equipment was hitting the computer
The example clearly demonstrates that from +/- 5 ms up to 30 ms of time offset
error in this example is a direct consequence of relatively large temperature
coefficient of a quartz crystal coupled with temperature fluctuations in the
computer room, augmented by a relatively large PLL time constant, typical for
a WAN-synchronized stratum-2 server.
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