On Friday, June 8, 2012 2:00:33 AM UTC+10, unruh wrote: > On 2012-06-07, Julien Ridoux <[email protected]> wrote: > > On Thursday, June 7, 2012 2:26:54 AM UTC+10, unruh wrote: > >> On 2012-06-06, Julien Ridoux <[email protected]> wrote: > >> > On Wednesday, June 6, 2012 4:41:59 PM UTC+10, unruh wrote: > >> >> On 2012-06-06, Harlan Stenn <[email protected]> wrote: > >> > Dear unruh (Dr. William Unruh?), > >> > > > > >> > >> > > >> > I would encourage you to read the scientific papers we published. I am > >> > confident you will see the level of hype being much lower. The > >> > description of the algorithm you are requesting can be found in several > >> > papers. Repeating myself, the main one being : ""Robust Synchronization > >> > of Absolute and Difference Clocks over Networks". It does describe the > >> > algorithm in more details. It may not deliver all the details you are > >> > after (or be slightly outdated), but the code publicly available is also > >> > there for you to read. It is the ultimate reference after all. > >> > > >> > >> It would be nice to have a link to that publication, in particular not > >> not one where you have to buy it. > >> And asking people to read the code is a cop out. I have tried hard to > >> read the ntpd code for example, and the chrony code. While the latter is > >> slightly (only sightly) better than the former, both are a real pain to > >> figure out what is going on. Reading code is always a case of far fr too > >> many trees for to see the forest. > >> > >> For others, I have found a copy at > >> http://www.cubinlab.ee.unimelb.edu.au/~darryl/Publications/synch_ToN.pdf > > > > Please note that the page http://www.synclab.org/docs/ I mentioned before > > has links to all publications but the most recent. Please click the "Full > > text" link to access the one of interest. I have updated most links for > > convenience and missing ones will be added with the next update to the > > website. > > > >> > It was not my intention to describe the entire algorithm in my previous > >> > message (it has been done in publicly available papers), but instead > >> > give some pointers and general answers to some of the questions raised > >> > earlier. My message did not intend to sparkle a heated discussion but > >> > instead try to provide honest answers to this group. > >> > >> I am very confused by your graph comparing ntpd to radclock. The ntp has > >> huge oscillations while from what I have read it is critically damped > >> while what I see is a pretty high Q (of the order of 10 or so) in the > >> graphs. Things I have not seen in my looks at ntpd. (Ntpd does have > >> problems which I have spent time pointing out in the comparison with > >> chrony) but oscillations like that I have not seen. Is this really ntpd > >> or some stripped down testbed? > > > > I am assuming you are talking about figure 13 in the paper mentioned above. > > The version of ntpd used in this plot is the stock standard version shipped > > with FreeBSD 6.1 (this data set has been captured in late 2006). The > > machine used is a Pentium 3 @600MHz (now long dead), a DELL Optiplex GX1 > > with an onboard 3Com 100 Mbps NIC. In this experiment, ntpd is configured > > as a broadcast client only listening to a stratum-1 server on the LAN. > > > > The broadcast configuration may be a cause for the large oscillations? I > > suppose ntpd experts may have an opinion on this. > > > > Another paper of ours (more recent) shows more comparisons of radclock > > agains ntpd under a variety of scenarios: > > http://www.synclab.org/pubs/radclock_2012_TON.pdf > > In particular, figures 8 to 11 show ntpd performance that may resemble what > > you have observed: no erratic behaviour on the LAN if the polling period is > > small enough. > > > > I would be interested in looking at a performance comparison of ntpd vs. > > chrony if you have one accessible. I will also try to start a comparison of > > chrony vs. radclock. It could be valuable to compare our findings. I have a > > fair few things on my plate these days, and this may get delayed a fair bit. > > There is some on my page www.theory.physics.ubc.ca/chrony/chrony.html > (note that there have been changes recently so that the time tracking of > all the machines right now is a mess). > > Miroslav Lichvar has also done extensive testing of ntpd vs chrony using > a > synthetic testbed (ie, the data is manufactured data but the algorithm > is the same so that the system can be run at far faster than real time). > I do not have the web references right now, but if you look back over > the > past year or so in this newsgroup for posts from him you will find the > comparisons. > > Note, it would be really good if you persuaded your newsposting program > to put in line breaks, instead of sending out huge long lines of text. > Some newsreaders simply throw away the ends of lines that do not fit on > one line, making reading difficult. > > The polling behaviour of ntpd should make no difference to the > "oscillations" since the time scale of the feedback loop is adjusted to > the polling period so as always to keep it roughly critically damped > (modulo the feedback problems introduced by the highly non-linear clock > filtering algorithm) > .
Thank for the link to the performance page and the reference to Miroslav Lichvar. I will come back to you once I have run some performance comparison using chrony. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
