[ntp:questions] Proposed NTP solution for a network

Hans Jørgen Jakobsen hjj at wheel.dk
Fri Mar 6 11:57:55 UTC 2009


On Fri, 06 Mar 2009 05:44:24 GMT, Unruh wrote:
> Jason <bmwjason at bmwlt.com> writes:
>
>
>
>>A sys admin and I discussed the possible solutions today, and we have a 
>>potential winner, although we can't reach the small 10s of uSec goal, we 
>>can reach several 100s of uSec (and probably less) easily. We are going 
>>to make a proposal to mgmt to increase the number of S1 servers, the 
>>number of GPS receivers, and I'm looking for an Rb clock source for the 
>>main datacenter as well. We are also re-engaging the blade / enclosure 
>>vendor.
>
> It looks, from the situation we have here at my university, that it will
> also depend on the kind of switches you have. We replaced our 10/100
> switches with Gbit switches, and the behaviour of ntp decayed significantly
> ( by factors greater than 2). Ie, whereas before I was getting 10s of usec
> as the "accuracy" of my clocks, it is not creaping toward 100usec. We have
> tried to figure out what the problem is, but have not succeeded. These Gbit
> switches seem to have, or trigger in the ethernet cards, additional and
> assymetric delays of the order of 100-200usec. (assymentric delays are of
> course the worst, since they directly affect the errors in ntp). Now If you
> run a gps PPS to each machine you could do far better, that seems not to be
> an option for you. (I run the PPS on the parallel port, but your blades are
> unlikely to have those-- in fact they are getting rarer even on standalone
> boxes).
>
> Ie, test your switches as well. 

Interrupt coalescing in ethernet card will also be a bad thing for ntp jitter.
/hjj




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