[ntp:questions] Sine wave offset patterns

unruh unruh at invalid.ca
Tue Sep 10 18:17:58 UTC 2013


On 2013-09-10, E-Mail Sent to this address will be added to the BlackLists <Null at BlackList.Anitech-Systems.invalid> wrote:
> Jared Watkins wrote:
>> I've had a chance to try a few things since I posted this.
>>   I don't think it's temperature.
>>  I have plots of that and it's very stable.
>> I think it's a combination of network latency to the
>>  upstream peer combined with varying system jitter.
>> I plotted the jitter and saw that it's a sawtooth pattern
>>   that varies over the same interval with peaks and valleys
>>   lining up with the offset swings.
>>  On 'good' hosts it fluctuates between about .5 and a max of 1ms
>>   and on bad hosts it's more like 1-4ms of jitter.
>>  A really good host had jitter swings of between about .2 to .5 ms
>>   and a corresponding offset swing of about +- 8ms.
>>
>> I don't know for certain this is cause and effect
>>  but I've not been able to find anything else.
>
> Unless you decreased the maxpoll,
>  I would have thought the eventual long poll times would
>   have smoothed that out.

"long polls(10)" are 20 min. Even with the 1 in 8 of the clock filter,
that is still just over 2 hours.
 
>
> The only time I've seen anything like that (twice, in 2 different states),
>  was on a overloaded (grossly over sold bandwidth) cable TV ISP network,
>  you could watch the latencies (and packet loss) climb,
>  as kids got out of school, then later as others got home from work;
>   I would have thought ISPs would have improved their
>    infrastructure beyond that by now.

Why would they. The fact that he is seeing worse behaviour on distant
sources makes one suspect that it is a "one way" latency issue. There is
no reason why the machine's own behaviour should depend on how far away
the source is. 

>



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