[ntp:questions] Sine wave offset patterns

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Tue Sep 10 17:06:40 UTC 2013

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.

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.

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