Fan Control Success on IBM T40? (another quick Q)

Nate Lawson nate at
Wed Apr 28 14:24:13 PDT 2004

On Wed, 28 Apr 2004, Kevin Oberman wrote:
> > Date: Wed, 28 Apr 2004 13:15:13 -0700 (PDT)
> > From: Nate Lawson <nate at>
> > Sender: owner-freebsd-acpi at
> >
> > 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.


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