[ntp:questions] Re: Strange scattergram

Karel Sandler sandler at ujf.cas.cz
Wed May 17 21:02:48 UTC 2006


"Brian Utterback" wrote:
> Karel Sandler wrote:
>> Hello,
>> to better know the network quality between my S2 and its peers, the wedge 
>> scattergrams were used. That strange one has the upper limb prolonged 
>> down to the left side of the head. Only few samples (status 9314) are 
>> located there but their existence is beyond all my understanding.
>> Hope my S2 works well, only one plot (one peer from seven) shows up that 
>> feature - lx.ujf.cas.cz/ntp-lx/antitail.png .
>> --
>> K. Sandler
>
> I wouldn't worry too much about the continuation of the line down and
> towards the left. That is probably due to the fact that the systems
> clock is in the process of slewing and is in the "overshoot" part of
> the phase locked loop. Of much greater concern is the complete asymmetry
> of the wedge plot. The typical cause for having the wedge plot bunched
> up towards one limb or the other is an asymmetric network path between
> the client and the server. However, looking at the offset plots
> at http://lx.ujf.cas.cz/ntp-lx it looks to me like your ntpd is not
> disciplining the frequency at all. Your system clock runs fast and
> ntpd adjusts it back in what look to me to be about a two hour cycle.
> I think that is why all of the offsets are positive in the wedge plot,
> because your clock runs fast.

Thanks, Brien, for your response. As I understand, you mean that due to a 
lack of discipline a datagrams can get false timestamps sometimes. Well, but 
the offset of my server rarely exceeds 2ms and the majority of the upper 
limb population lies far higher. As I understand, only the asymmetry can 
explain that - the upward times are longer then the downward times here. 
Moreover, other wedge plots (for other peers) don't have that huge 
asymmetry. Two similar asymetric wedge plots I get also for another my 
server . One of its peers, I surely know, was overloaded, the second peer is 
from the same location as that one from my original post. And, again, it has 
the antitail, although the offset behaviour of my second server is quite 
smooth.
Yes, I am not satisfied with the shape of the lx offset curve though the 
values itself are good for S2. Here you can see the last loopstats picture: 
lx.ujf.cas.cz/ntp-lx/loop.png. Any comment would be appreciated.
--
K. Sandler 





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