Hi, David-- On May 11, 2010, at 4:50 PM, Russell, David wrote: > The device is a piece of networking equipment and so I doubt that the crystal > is temperature controlled but since it is in a data center the temperature > and power demand is steady.
It's pretty common for system loads to be different during the day and night, and even if this specific machine doesn't have such a change, other machines in the rack, network switches, and so forth produce different levels of heat which mildly change ambient temps, even with good air-conditioning in the datacenter. > If you could see the graph you would see that it seems to oscillate around > -3.05 us/s over a 24 hour timeframe. The graph has a pretty nice looking sine > wave with a 12 hour period. I was expecting that if the "true" drift rate > were -3.05 then NTP would converge upon that value. Are you saying that this > oscillation is typical for non-temperature (cheap) crystals or should the > drift look more random? A 24-hour period to the drift is very typical, yes. 12-hour period is more interesting.... > One new piece of information is that the reported delay is not even close to > the actual delay on the network. For example, a sniffer trace shows that the > actual round trip time is 1.8ms but their NTP process is reporting the delay > as 3.5ms. Are you sure something isn't measuring latency one-way, and the other isn't measuring the round-trip time? 1.8 ms * 2 ~= 3.5 ms > I am not convinced that there isn't a coding error in the implementation and > am trying to differentiate inherent limitations in the device such as normal > clock drift variance, the way that the clock is adjusted etc from a coding > error. There was a coding error in the previous version so it is not too > unlikely that there is another one. You're welcome to request a refund of whatever you've paid for ntpd. :-) > Any thoughts on validating a black box NTP process? Sure: find a more reliable timing source-- cesium or rubidium atomic clock, GPS receiver, WWVB, etc-- and compare that to your ntpd timestamps. However, typical network latency and jitter (which should be less than a microsecond for local gigabit switched traffic, but your latency is somewhat higher) means that it's tough to get time significantly more accurate than around a microsecond, unless you are measuring locally. > I had not realized until I sent out the email that no attachments are > allowed. Is there a way to share a graph? Please put it on a website somewhere, and mail a link to it. Regards, -- -Chuck _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
