[ltp] Question on APM vs ACPI

[Brian D. Ropers-Huilman]

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Nathan Kurz said the following on 2004-12-07 21:38:
| On Tue, Dec 07, 2004 at 07:50:59AM +0100, Bob Alexander wrote:
|>2) ACPI is much better on CPU throttling (ala SpeedStep) when running on
|>battery
|
| I'm trying to understand this myself.  It seems that throttling is
| only possible under ACPI, but that frequency scaling is independent of
| APM/ACPI.  It also seems that scaling has a much larger effect than
| throttling on power consumption.  My impression is that SpeedStep is
| the combination of these two.  Is this accurate?

What do you mean by throttling? As I understand it, throttling is merely
dynamically changing CPU frequency, which is really just frequency
scaling with automatic controls. cpudyn is described as, "A daemon to
control laptop power consumption via cpufreq and disk standby." cpufreq
says, "Daemon to adjust CPU speed for power saving." speedfreq is a
"daemon to control the CPU speed in 2.6 kernels"

I happen to use cpuspeedy by hand:

ibis root # cat /proc/cpuinfo |grep MHz
model name      : Intel(R) Pentium(R) M processor 1700MHz
cpu MHz         : 1694.868
ibis root # cpuspeedy -s
cpuspeedy: CPU speed is 1.70 GHz (100%)
ibis root # cpuspeedy min
cpuspeedy: CPU speed is now 600 Mhz (35%)
ibis root # cat /proc/cpuinfo |grep MHz
model name      : Intel(R) Pentium(R) M processor 1700MHz
cpu MHz         : 598.189
ibis root # cpuspeedy -s
cpuspeedy: CPU speed is 600 Mhz (35%)
ibis root # cpuspeedy max
cpuspeedy: CPU speed is now 1.70 GHz (100%)
ibis root # cat /proc/cpuinfo |grep MHz
model name      : Intel(R) Pentium(R) M processor 1700MHz
cpu MHz         : 1694.868
ibis root # cpuspeedy -s
cpuspeedy: CPU speed is 1.70 GHz (100%)
ibis root #


I simply have not looked into cpufreqd or cpudyn enough to know what
they are doing or how.

Now, it is SpeedStep that is enabling this, but SpeedStep is simply the
_ability_ to change frequency on-the-fly, not a program in and of
itself. I have the speedstep_centrino module running:

ibis root # modinfo speedstep_centrino
author:         Jeremy Fitzhardinge <jeremy@goop.org>
description:    Enhanced SpeedStep driver for Intel Pentium M processors.
license:        GPL
vermagic:       2.6.9-gentoo-r4 preempt PENTIUM4 gcc-3.3
depends:

and this is what allows cpuspeedy do to its thing. By the way, I have
used both APM and ACPI with limited success but have used cpuspeedy,
by-hand, under both with complete success.

For what it's worth, I appear to get better battery life out of APM.
- --
Brian D. Ropers-Huilman  .:. Asst. Director .:.  HPC and Computation
Center for Computation & Technology (CCT)        bropers@cct.lsu.edu
Johnston Hall, Rm. 350                           +1 225.578.3272 (V)
Louisiana State University                       +1 225.578.5362 (F)
Baton Rouge, LA 70803-1900  USA              http://www.cct.lsu.edu/
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