On 2011-04-18, Mike S <[email protected]> wrote: > 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.
You are confusing measurements with the effects of noise. > > 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. Sure, but that is a different question. What I demonstrated was that with it usec precision ( and yes, computers internal jitter allows usec precision. The jitter is not terribly large in general-- I have measured it.) it would take 1 sec to get to 1PPM. That is a factor of 1000-100000 better than ntpd does. That network jitter would have to be really really horrible to get that bad. And even on a network, I get 20usec ( which means it would take 20 sec to get down to 1PPM). Your "explanation" of time-rate tradeoff is true, but so far off of waht ntpd does that it is also irrelevant. It is also one of the reasons why chrony gets so much faster a convergence after an error-- it operates much closer to that ideal. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
