[ntp:hackers] clustered polling times

Tim Shoppa shoppa at trailing-edge.com
Sun Feb 1 10:51:12 UTC 2009


Dave Hart <davehart at gmail.com> wrote:
> ntpd has a tendency to have peer poll times clustered together, which is
> particularly noticable with maxpoll 12, as seen in this extract of the
> central columns of ntpq -c peers output:
>
>  st t when poll
> ================
>   2 u 1353 2048
>   2 u 1375 2048
>   2 u  964 2048
>   2 u 1312 2048
>   2 u 1278 2048
>   2 u 1259 2048
>   2 u 1317 2048
>   2 u  957 2048
>   2 u 1331 2048
>
> One cluster around 960 seconds ago, the other around 1300.

I've noticed this too, but I always thought it was the case that
the ntpd algorithms do not optimally deal with large numbers of peers.
Maybe the flaws you and I see are like you or me pointing out how
someone who eats 20 times a day doesn't always eat a balanced meal
every single time. If you set maxpoll to 12, isn't it true that 
by definition you must be satisfied with getting a good time
once every 2048 seconds?

The peer selection algorithms work really really well for throwing
out falsetickers when there are a small number of peers. Once you
have e.g. 9 peers the peer selection algorithm can afford to throw
out multiple good ones and still have plenty left over. So using
ntpd as a check on multiple servers and paying attention to the
falseticker column can be very misleading.

Tim.


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