Date: Wed, 28 Apr 2004 17:34:46 -0400 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-acpi@FreeBSD.org Cc: Jeffrey Katcher <jmkatcher@yahoo.com> Subject: Re: Fan Control Success on IBM T40? (another quick Q) Message-ID: <200404281734.46945.jhb@FreeBSD.org> In-Reply-To: <20040428142302.T89203@root.org> References: <20040428211826.A02195D07@ptavv.es.net> <20040428142302.T89203@root.org>
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On Wednesday 28 April 2004 05:24 pm, Nate Lawson wrote: > On Wed, 28 Apr 2004, Kevin Oberman wrote: > > > Date: Wed, 28 Apr 2004 13:15:13 -0700 (PDT) > > > From: Nate Lawson <nate@root.org> > > > Sender: owner-freebsd-acpi@freebsd.org > > > > > > I forgot to add -- the reason the clock rate announced at boot time is > > > different is because the test for CPU TSC is done before acpi is > > > enabled. At some point when the SMI is disabled and acpi enabled, the > > > clock rate is switched by your BIOS to the lower rate. You can see > > > this because the clock rate announced by the TSC timecounter will be > > > ~600 mhz while the initial boot clock rate will list ~1600 mhz. > > > > This all makes sense, but it conflicts a bit with my > > observations. (Probably implies something bad about my powers of > > observation.) > > > > I boot and the system (T30) is running at 1.8 GHz. I throttle the CPU. > > Testing clearly shows that the throttling is working. I use the test you > > suggested of calculating an MD5 hash of a big string of zeros. > > > > But, when I "count cycles" to test the CPU speed (code appended), I > > still see 1.8 GHz. > > > > Why don't I see the speed reduced when throttling? I suspect my lack of > > fundamental understanding of the interactions of throttling and the ACPI > > clock. > > This is totally different. You aren't using SpeedStep/performance states, > only throttling. Throttling works by changing the duty cycle but with the > SAME base clock rate. Since the TSC comes from the base rate, your test > reading the TSC will never change. Also, even with SpeedStep, the CPU will "fake" the TSC so that it stays at a constant rate even when the CPU is throttled down to a slower speed. I think that's what the 'ACPI' bit in the cpu_features indicates. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
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