[ntp:hackers] Does ntpd need to whine more ?

Mark Martinec Mark.Martinec at ijs.si
Mon Oct 3 22:57:19 UTC 2005


From Dave:
> >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 
cabinets. 
[...]
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. 


  Mark


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