Date: Sun, 27 Mar 2016 20:32:51 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Bruce Evans <brde@optusnet.com.au> Cc: John Baldwin <jhb@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org, "'rstone@freebsd.org'" <rstone@freebsd.org> Subject: Re: svn commit: r297039 - head/sys/x86/x86 Message-ID: <20160327173251.GB1741@kib.kiev.ua> In-Reply-To: <20160328033624.N850@besplex.bde.org> References: <20160324090917.GC1741@kib.kiev.ua> <20160325010649.H898@besplex.bde.org> <20160324162447.GD1741@kib.kiev.ua> <20160325060901.N2059@besplex.bde.org> <20160325084902.GH1741@kib.kiev.ua> <20160326021219.X911@besplex.bde.org> <20160326174210.GU1741@kib.kiev.ua> <20160327051601.S2318@besplex.bde.org> <20160327130706.GA1741@kib.kiev.ua> <20160328033624.N850@besplex.bde.org>
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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
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