[ntp:questions] Sine wave offset patterns

Jared Watkins jared.watkins at gmail.com
Tue Sep 10 05:48:20 UTC 2013


On Sep 9, 2013, at 10:02 PM, David Taylor wrote:

> On 09/09/2013 23:10, E-Mail Sent to this address will be added to the BlackLists wrote:
>> Jared Watkins wrote:
>>> ...
>>> In pretty much all cases though when graphing the offsets
>>>   I'm seeing sine wave patterns.
>>>  The period is usually about 10 hours and the amplitude
>>>   can vary by +-25ms for the 'closer' ones and as much
>>>    as +-100ms for the more distant ones.
>>> ...
>>> Any ideas on what would cause it?
>> 
>> Temperature?
> 
> Plotting with MRTG might show that - I can see peaks in the Winter when the heating turns on in the morning.
> 


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. 

-J




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