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From: Michael Smith <msmith@freebsd.org>
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Date: Sat, 30 Nov 2002 01:00:02 -0800
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Subject: [acpi-jp 2008] Re: ACPI Questions
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On Friday, November 29, 2002, at 07:41 PM, Mitsuru IWASAKI wrote:

>> How long does it take to change the CPU speed after changing
>> hw.acpi.cpu.*?
>
> Immediately :)
>
> % sysctl hw.acpi.cpu
> hw.acpi.cpu.max_speed: 8
> hw.acpi.cpu.current_speed: 8
> hw.acpi.cpu.performance_speed: 8
> hw.acpi.cpu.economy_speed: 4
>
> Here, what you can change is only performance_speed and economy_speed.
> max_speed and current_speed are read-only.
> Please refer /sys/dev/acpica/acpi_cpu.c.

Note that on a desktop system, the profile is always "performance", so
to change the current speed, change the 'performance' speed.

>> Are there really 8 levels of cpu performance?  I was under the
>> impression that the PIII Mobile really only had two...  Is there any
>> easy way to find the current CPU speed, voltage or bus speed?
>
> I want to know that too...

The CPU speed feature that we currently support is actually usually
implemented in the chipset, and it works by cutting clock cycles to
the CPU (by injecting wait states).

The actual number of states that the system supports is determined by
reading values out of the ACPI registers; 8 (3 bits) is the normal
number of states to be expected with Intel's chipsets.

Note that Andy @ Intel very correctly pointed out that it makes
more sense to run the CPU at full speed and then shut the system
down properly when idle, rather than run the CPU at part speed and
leave all the peripherals and chipset powered on all the time.

FreeBSD doesn't do that yet, AFAIK.

  = Mike

