Date: Sat, 12 May 2007 22:04:38 +1000 (EST) From: Ian Smith <smithi@nimnet.asn.au> To: "M. Warner Losh" <imp@bsdimp.com> Cc: tevans.uk@googlemail.com, freebsd-stable@freebsd.org, martin.dieringer@gmx.de Subject: Re: clock problem Message-ID: <Pine.BSF.3.96.1070512210543.24086A-100000@gaia.nimnet.asn.au> In-Reply-To: <20070511.131647.-135505895.imp@bsdimp.com>
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On Fri, 11 May 2007, M. Warner Losh wrote: > : Yes, but Martin already showed it was using the i8254, not TSC; would > : you expect the same effect using powerd with the i8254 clock? It seems > : so, unless it's some problem with est and/or p4tcc under APM (canoworms) > > No. I would not have expected it at all. I would have expected the > i8254 to not be able to provide time much better than a microsecond or > two, but I'd expect time to be relatively stable, modulo the normal > walking due to thermal variation you'd see given the relatively low > quality oscillators that feed it. However, see below. Ok, thanks for that. Matt's post on the various clocks was handy too. > The i8254 time counter has a frequency of about 1.19 MHz, but it wraps > about 18 times a second (or once every ~55ms). I think that if the > clock speed was slow enough, there might be situations where > interrupts are disabled long enough to blow past that 55ms mark, > especially on a 300MHz laptop that might be running at a very slow > clock rate when idle. If it misses the wrap, then you'll see time > slip away. Could well explain Martin's problem; his T42p goes right down to 75MHz (at HZ=1000, if that's relevant) | dev.cpu.0.freq: 1400 | dev.cpu.0.freq_levels: 1400/-1 1225/-1 1200/-1 1050/-1 1000/-1 875/-1 | 800/-1 700/-1 600/-1 525/-1 450/-1 375/-1 300/-1 225/-1 150/-1 75/-1 > Maybe you can experiment with the lower bounds the frequency of the > system can run and keep accurate time. debug.cpufreq.verbose=1 might > be helpful. You can override the lowest setting of powerd by using > the sysctl debug.cpufreq.lowest. Where might I peek to see how/where setting that sysctl acts to modify the dev.cpu.0.freq_levels MIB, since powerd.c just eats what it's fed? </just_curious> TIA, Ian
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