On Tue, 2009-03-03 at 21:19 +0100, Jesper Krogh wrote: > john stultz wrote: > > On Tue, 2009-03-03 at 07:04 +0100, Jesper Krogh wrote: > >> john stultz wrote: > >>> On Mon, 2009-03-02 at 10:53 +0100, Jesper Krogh wrote: > >>>> john stultz wrote: > >>>>> Ok, so it seems ntp hasn't really had a chance to settle down, its only > >>>>> made a 10ppm adjustment so far. NTPd will stop corrections at ~ > >>>>> +/-500ppm, so you're not at that bound yet, where things would be really > >>>>> broken. > >>>>> > >>>>> If the affected kernel isn't resetting in the logs anymore, I'd be > >>>>> interested in what the new ppm value is. > >>>> After 20 hours.. its still resetting. > >>>> Mar 2 10:43:24 quad12 ntpd[4416]: synchronized to 10.194.133.12, stratum 4 > >>>> Mar 2 10:50:37 quad12 ntpd[4416]: time reset -1.103654 s > >>> So what's the "ntpdc -c kerninfo" output now? > >> Mar 3 06:41:10 quad12 ntpd[4416]: time reset -0.813957 s > >> Mar 3 06:45:20 quad12 ntpd[4416]: synchronized to LOCAL(0), stratum 13 > >> Mar 3 06:45:36 quad12 ntpd[4416]: synchronized to 10.194.133.12, stratum 4 > >> Mar 3 06:51:57 quad12 ntpd[4416]: synchronized to 10.194.133.13, stratum 4 > >> Mar 3 07:00:29 quad12 ntpd[4416]: time reset -0.783390 s > >> jk@quad12:~$ ntpdc -c kerninfo > >> pll offset: 0 s > >> pll frequency: -28.691 ppm > > > > > > This is baffling. You've only gone from -34.754ppm to -28.691ppm in over > > a day? And you're still not syncing? If the calibration was so bad that > > NTP couldn't sync, I'd expect the freq value to hit +/-500ppm before it > > gave up. This just doesn't follow my expectations. > > It's resetting.. without deep knowledge about ntp, doesnt that mean > "start over again"? I believe it hits +/-500ppm No, the "time reset" message means that when the offset is larger then .125sec (the slew boundary), NTPd has corrected it by calling settimeofday instead of slewing the clock. Here's some background about how NTP and the kernel interact: Every time NTPd calls adjtimex(), its provides the current offset from the tracked ntp server. The kernel takes this offset and applies a temporary correction factor to the clocksource frequency to converge that offset. It also takes the provided offset, dampens it, and then uses the result to adjust the frequency value. Once the freq value hits the max adjustment value (+/- 500ppm), then NTP will start throwing error messages and give up. The part that is so odd with your data, is that the freq value isn't changing very much. After a time reset, I'd expect to see adjustments in the 100us, then multiple ms, and only once we get above 100ms to see another time reset. All the while, these adjustment values should be tweaking the freq value, causing the clocks to converge. The case I can think of that could cause this, is if the drift is somehow jumping above the slew boundary before NTPd actually makes any adjtimex calls, so we end up with minimal correction to the freq value, but that still doesn't completely vibe with the data. > > Could you provide: > > /usr/sbin/ntpdc -c version > > $ ntpdc -c version > ntpdc 4.2.4p4@1.1520-o Tue Jan 6 15:51:00 UTC 2009 (1) > > > Do you see the same behavior if you drop all but one server (including > > the local clock: 127.127.1.0)? > > > > You might also add "minpoll 4 maxpoll 4" to the server line to speed up > > testing. > > Will try those option while debugging. > > > Actually, if you could, I'd be interested if you could send your > > ntp.conf > > http://krogh.cc/~jesper/ntp.conf Cool, I see you're collecting stats already. Depending on the results of the tests above I may want to check those out as well. > But this seems to be a "regression". Since 2.6.27.19 doesn't misbehave. > Same NTP, same configuration, same hardware. only change is the kernel > version. Or am I missing some parameter here? > > Would it make sense to try to bisect it? Well, I suspect you'll just bisect it to the fast-pit TSC calibration causing a different correction freq to be needed for synchronization. The odd part is that the userland NTPd isn't behaving as I'd expect if the TSC calibration was really so bad that NTP couldn't handle it. Bisection may be something worth trying just to verify or disprove that theory, so if you have the time, it would be interesting to see. But if the theory is true then we're back to the same spot. I guess something to test my idea above (that the drift is bad enough that NTPd isn't making slew adjustments via adjtimex offset) is to remove NTPd from the init.d startup. Then after rebooting (into 2.6.29), run the attached python script for 10 minutes or so to get an idea of the ppm drift. Then repeat with 2.6.26. To run: ./drift-test.py It will give some wild ppm numbers, but after a few minutes it should settle down to the "natural drift" of the system. thanks -john