[ntp:questions] NTPD can take 10 hours to achieve stability

Mike S mikes at flatsurface.com
Mon Apr 18 23:06:53 UTC 2011


At 01:51 PM 4/18/2011, unruh wrote...
>Since you can measure the time to usec, in 1 sec you can measure rate
>offsets of 1PPM and offsets of 1usec.

That's the thing. You can't do that.

It's not a matter of time precision on a single machine. It's a matter 
of comparing times on two or more machines. Network jitter is 
unpredictable, especially if the hosts NTP is syncing with are remote, 
so single readings can't be trusted to the us (a full size Ethernet 
frame @ 1Gbps is ~12 us), or even the ms level. There can also be 
jitter within the host you're syncing to (irq latency, etc.). Using a 
longer time constant averages out the jitter. I'm sure there are papers 
covering the math behind it, somewhere.







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