[ntp:questions] Visualization of clock control
Hi, I wrote a tool to visualize the data generated by the clknetsim simulator and I thought some of you might find it interesting. The goal was to show how a clock is controlled by NTP client and at the same time see its offset from true time and the NTP measurements (the actual offset and delay seen by the client). Here are some example runs of the tool captured to animated gifs: http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_10us.gif http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_100us.gif http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_1000us.gif The simulations were done with a clock wandering at 1 ppb/s, 10/100/1000us network jitter with exponential distribution and the NTP clients were configured to use 64s polling interval. The white line is the reference clock. The red line is the clock controlled by ntp and green is chrony. The blue lines are the NTP measurements made by chrony. Both clients were getting the same data, but the polling intervals were not exactly the same so the frequency changes in the red line don't match exactly with the blue lines. The tool is included in the clknetsim git as visclocks.py. It also has a game mode, where you control the frequency and phase of the clock by mouse and you can try to beat the other clients. :) -- Miroslav Lichvar ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Visualization of clock control
On 4 Jan, 2012, at 22:54 , Miroslav Lichvar wrote: > Here are some example runs of the tool captured to animated gifs: > http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_10us.gif > http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_100us.gif > http://mlichvar.fedorapeople.org/clknetsim/chrony_ntp/vis/visclocks_1000us.gif > > The simulations were done with a clock wandering at 1 ppb/s, > 10/100/1000us network jitter with exponential distribution and the NTP > clients were configured to use 64s polling interval. That's pretty neat. I think, however, that the clock wander of 1 ppb/s is about an order of magnitude too large for real life, at least for machines kept in an air conditioned room (and the behavior of clocks in machines subject to environmental variations probably can't be modeled by "wander" at all). My measurements against precise hardware tended towards a value of 1ppb/10s, which is also consistent with the 10^-8/1000s which sometimes shows up on Allan variance plots (I think there's a square root relationship in there if the wander is a truly random walk). The other difficulty with respect to real life may be modeling network jitter as exponential, since I believe the probability distribution for network delays is heavy-tailed (i.e. with extreme values way over-represented; this is a problem when using statistics which assume the underlying error distribution is gaussian). I don't know how to fix that, though. Dennis Ferguson ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Visualization of clock control
On Thu, Jan 05, 2012 at 11:40:25AM +0800, Dennis Ferguson wrote: > On 4 Jan, 2012, at 22:54 , Miroslav Lichvar wrote: > > The simulations were done with a clock wandering at 1 ppb/s, > > 10/100/1000us network jitter with exponential distribution and the NTP > > clients were configured to use 64s polling interval. > > That's pretty neat. I think, however, that the clock wander of 1 ppb/s > is about an order of magnitude too large for real life, at least for machines > kept in an air conditioned room (and the behavior of clocks in machines > subject to environmental variations probably can't be modeled by "wander" at > all). My measurements against precise hardware tended towards a value of > 1ppb/10s, which is also consistent with the 10^-8/1000s which sometimes shows > up on Allan variance plots (I think there's a square root relationship in > there > if the wander is a truly random walk). I think the 1ppb/10s random walk wander corresponds to ~0.32ppb/1s. The +0.5 slope in the variance plot intersecting 10^-8 at 1000s would be ~0.6ppb/s wander. I tried to model some thermal effects by adding a sine, triangle or pulse wave to the clock frequency, but it seemed to me the effect it had on the overall RMS time error was similar to just increasing the wander. So instead of three or more parameters of the clock I set only one. Sometimes I use even 10ppb/s wander, to simulate a machine with varying CPU load and I think the results are not that different from what I see on my desktop. BTW, the simulator can be configured to read the clock frequency from a file. If you have real data from a PPS refclock, you can use that and see at what random walk wander will ntpd give similar results. > The other difficulty with respect to real life may be modeling network jitter > as exponential, since I believe the probability distribution for network > delays > is heavy-tailed (i.e. with extreme values way over-represented; this is a > problem > when using statistics which assume the underlying error distribution is > gaussian). > I don't know how to fix that, though. I'd definitely be interested in a better model for network delays. I guess we could try to make a collection of the ntp rawstats logs from various network environments and see how the distribution looks like. -- Miroslav Lichvar ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Visualization of clock control
Hal Murray wrote: In article <20120105125942.GA15654@localhost>, Miroslav Lichvar writes: The other difficulty with respect to real life may be modeling network jitter as exponential, since I believe the probability distribution for network delays is heavy-tailed (i.e. with extreme values way over-represented; this is a problem when using statistics which assume the underlying error distribution is gaussian). I don't know how to fix that, though. I'd definitely be interested in a better model for network delays. I guess we could try to make a collection of the ntp rawstats logs from various network environments and see how the distribution looks like. My 2 cents... I have a 384K DSL line. I'd say it runs in one of 3 modes. 1) Mostly idle. There is a little background traffic: NTP, fetchmail checks and occasional fetches. yum checks at 3 AM... 2) Big download. Occasionally, I download something like a CD. That puts a consistent load on the download direction. The queueing delays go up to ballpark of 1 second. ntpd doesn't like that. After a while it sometimes to a bogus offset. huffpuff helps, but a big download can take a long long time. 3) Flurry of web downloads. If I'm browsing the web, I occasionally hit a page with lots of pictures and such. Mozilla opens lots of TCP connections. The round trip time goes over 3.5 seconds. I don't think any of that fits an exponential tail. I've got lots of log files. Maybe I should make a histogram... I'm on 2Mbit adsl with maximum upload of 288 kbit. Actual uploads were topping out at only about 130kbit until I enabled altq traffic shaping with a cap at about 250kbit. I now hit 250kbit on upload. Certain traffic also gets priority including ntp. Apart from when my servers are being hammered by abuse this has worked very well. David ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Visualization of clock control
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On Thu, 12 Jan 2012 04:37:41 -0600, Hal Murray wrote: > 3) Flurry of web downloads. If I'm browsing the web, I occasionally > hit > a page with lots of pictures and such. Mozilla opens lots of TCP > connections. The round trip time goes over 3.5 seconds. Sounds like you have a buffer bloat issue. http://queue.acm.org/detail.cfm?id=2076798 and related articles from google for buffer bloat. -BEGIN PGP SIGNATURE- Version: GnuPG v2.0.14 (GNU/Linux) iEYEARECAAYFAk8SSiUACgkQL6j7milTFsGaYACfXDgLHo91HHRBaRMRVIxnLidZ 0BsAnjiQdOJZi53s/6UHgrGB9AKmvpMu =GK5R -END PGP SIGNATURE- ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
