On Mon, Mar 28, 2016 at 04:22:59AM +1100, Bruce Evans wrote: > On Sun, 27 Mar 2016, Konstantin Belousov wrote: > > We could even trust CPU report about its frequency as the last resort. > > From what I saw, the calibrated RTC frequency is very close to the Err, that should have been TSC, and not RTC.
> > reported frequency, and if runtime recalibration based on ntp client > > feedback is implemented, all would be good. > > phk axed my RTC calibration code (actually i8254 and TSC calibration code > starting from the RTC). It worked perfectly except it didn't adjust for > the time to read the hardware. The problem with it is that the RTC is no > more or less accurate than the other timers. It should be, but isn't in > practice. Normally it it is controlled by different hardware (crystal?) > with different temperature characteristics and different exposure to > temperature changes by being physically separate. The i8254 is supposed > to run at precisely 1193182 Hz but usually has an inaccuracy of 10-50 Hz. > The RTC seconds update is supposed to occur at precisely 1 Hz but usually > has an unrelated inaccuracy of similar magnitude. Calibrating these clocks > relative to each other just gives the relative difference in their > inaccuracy. The difference can be measured very accurately at any time. > It varies later with temperature. I never finished code to track the > termperature changes and calibrate adjustments based on the temperature. > It is simpler to start with a hard-coded i8254 frequency of 1193182 and > adjust everything later. ntpd handles adjustment of a single timer well > enough. This works because the nominal frequency of 1193182 is so standard > that inaccuracies in it are closer to 10 ppm than the 1000 ppm that would > break ntpd. > > Bruce _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
