[Pool] Offset monitoring chart

Hal Murray hmurray at megapathdsl.net
Fri Feb 28 05:39:42 UTC 2014


philip at gladstonefamily.net said:
> I sometimes get little red dots on my offset monitoring chart on the  -2000
> ms line. Are these placeholder for lost packets or are they really  an
> indication of really bad time?

What are you monitoring?  loopstats? peerstats?

> If my server reports 'unsynchronized' @ stratum 16 (along with bad  time),
> then could that trigger the -2000 ms marker?

The unsynchronized/stratum-16 means that your server doesn't know the time.  
That happens at startup and when it looses contact with all the servers from 
your config file.

(I'm assuming you are interested in the case when the server has been running 
correctly for a while rather than the reboot or restart-ntpd case.)

What sort of setup do you have?  Local GPS clock?  Slow connection to the 
internet?

Are all the red dots at exactly 2 seconds offset?

I've seen cases where GPS devices will be off by a second when they are 
recovering from fading out because they don't have a good-enough signal.  
Usually ntpd will filter those out, but you can see them if you look at 
clockstats or peerstats.

If you have a slow internet connection, you can get some really "interesting" 
delays.  The buzzword is bufferbloat.  If you "know" the clock on the other 
end is accurate, you can use the data from rawstats to plot the network 
transit times in each direction.  With my slow DSL line, I've seen many 
delays of over 3.5 seconds.  They happen when I'm downloading a bunch of web 
stuff.  That turns into offsets of 1.75 seconds.

If you look at enough data from rawstats, you will sometimes find cases where 
the round trip time was many seconds.  I assume some link someplace is 
flapping and some router is holding packets that it should drop.  I've seen 
delays of over 30 seconds.




-- 
These are my opinions.  I hate spam.





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