ping from laptop C:\Users\xeno>ping 192.168.1.105 -n 100 Pinging 192.168.1.105 with 32 bytes of data: Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=5ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Reply from 192.168.1.105: bytes=32 time=4ms TTL=128 Reply from 192.168.1.105: bytes=32 time=3ms TTL=128 Ping statistics for 192.168.1.105: Packets: Sent = 100, Received = 100, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 3ms, Maximum = 5ms, Average = 3ms On Fri, Nov 24, 2017 at 3:35 AM Neil Davies wrote: > On 24 Nov 2017, at 09:20, Hal Murray wrote: > > > neil.davies@pnsol.com said: > > There are a few more issues - the relative drift between the two clocks > can be as high as 200ppm, though typically 50-75ppm is what we observe, but > this drift is monotonic. > > > 200 ppm seems pretty high, but not off scale. If ntpd is running and not > getting confused by long queuing delays, it should correct the drift to > well > under 1 ppm. If you turn on loopstats, you can graph it. > > > I’m saying that is the maximum rate of drift between two clocks even > when they are under NTP control. As you say below the clock rates > are not completely stable they are temperature dependent. > When we did this with the guys at CERN we could > correlate the results with the workload (see below for references). > > We’ve got ~1M experiments using this approach across various networks, > the numbers are what we are seeing in practice. > > The caveat is that, after a while (i.e several 100s) the clock drift can > make > a significant difference (i.e a few ms) in the one-way delay estimation. > > > If you are blasting the network and adding long queuing delays, ntpd can > easily get confused. > > There is another quirk to keep in mind. The temperature coefficient of > the > crystal is ballpark of 1 ppm per C. Things can change significantly if an > idle system starts flinging lots of bits around. > > > Also NTP can make changes at one (or both) ends - they show up as distinct > direction changes in the drift. > > > I'm not sure what you mean by "direction change". I'd expect a graph of > the > time offset vs time to be linear and the slope would have a sharp change > if > ntpd changed it's "drift" correction and/or maybe a rounded bend as a > system > warmed up. > > > Don’t forget you are measuring the difference in the rates between two NTP > clocks, > hence the change when one of the NTP systems decides to change the drift > rate > the relative rate can change direction. > > > ---------- > > Are you happy with whatever you are doing? Should we try to set things > up > > so ntpd works well enough? How close would you like the times to be? … > > > Yep, we’re very happy - we don’t care that there is a linear clock drift > (we > can correct for that) and the step changes are infrequent and can be > eliminated > from the long term analysis. > > You might find §4.4 (esp §4.4.5) and §5.6 in > https://cds.cern.ch/record/1504817/files/CERN-THESIS-2013-004.pdf > interesting. It illustrates these sort of issues. > > > > > > -- > These are my opinions. I hate spam. > > > > > _______________________________________________ > Bloat mailing list > Bloat@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/bloat > -- Caleb Cushing http://xenoterracide.com