cpufreq hint

Ken Moffat ken at kenmoffat.uklinux.net
Tue Jul 12 08:59:20 PDT 2005

 Dynamically changing cpu frequency.

 das eine Mal als Tragödie, das andere Mal als Farce
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AUTHOR: Ken Moffat <ken at kenmoffat.uklinux.net>
DATE: 2005-06-15
SYNOPSIS: cpu frequency scaling using the kernel or powernowd.
DESCRIPTION: With processors that support it (e.g. recent athlon64,
many laptop processors), you can reduce the processor frequency, to
reduce wasted energy or conserve battery power.

PREREQUISITES: 2.6.9 or later kernel, the bootscripts target LFS-6.1



 Frequency scaling is an important part of increasing the battery life
of notebooks,  but it also has a place in reducing power consumption,
and thus waste heat, on desktops and servers.  Most CPUs that support
it will have at least two valid frequency/voltage settings, e.g. my
athlon64 Winchester 3200 (2.0 GHz) can run at approx 1.0 GHz, 1.8 GHz,
and 2.0 GHz.  Give the ondemand governor a heavy load and the speed
quickly ramps up.  After the load has fallen back, the speed will
first fall to 1.8 GHz and then some seconds later to 1.0 GHz.

 The kernel began by offering 'powersave' (slow) and 'performance' 
(fast) drivers, with a 'userspace' alternative to allow a daemon to
dynamically change the frequency.  As of 2.6.9, an 'ondemand' driver
was made available.  Other demand-based governors, such as 
'conservative', have been introduced in later kernels.

 Meanwhile, the powernowd daemon was developed, see 
http://www.deater.net/john/powernowd.html - this will dynamically
change the frequency as the load goes up and down.

 It would seem that the 'ondemand' driver makes powernowd redundant,
but some CPUs (like the ppc 7447) apparently have a very high 
transition latency which causes the ondemand governor to decline to
manage them - on my 7447A the powersave and performance drivers
('governors') work, but trying to install 'ondemand' fails.  Equally,
on a laptop powernowd will probably be a little quicker at dropping
the speed, and can therefore give a little more battery life.

 On x86 and x86_64 it appears that your bios has to support frequency
scaling (typically labelled Cool'n'Quiet on boards with AMD CPUs) -
after upgrading to a new mobo/cpu, I lost the ability to use both the
'ondemand' governor and powernowd until I configured the bios properly.

Kernel Configuration

On the Processor menu, select CPU Frequency scaling

Set the Default CPUFreq governor to userspace (the default is often

Select the governors you wish to use, typically 'performance', 
'powersave', and try 'ondemand'.  Feel free to select any other
available governors.  I assume most people will want to build these
governors in, rather than compiling them as modules.

Select the correct cpu-freq hardware driver (on a mac there is nothing
further to select, on x86, choose from POWERNOW_K7, POWERNOW_K8,
SPEEDSTEP_CENTRINO, and similar options).


After you've booted the new kernel, you can test which of the kernel
governors work.  As root, echo the governor name to 
/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor - presumably,
people with SMP boxes need to try this for each CPU.  Then test the
status of the command, e.g. with 'echo $?' - if the governor loaded,
the status will be 0 and you can use that governor, otherwise the
status will be 1.

You can view the current CPU frequency in /proc/cpuinfo.  To test the
ondemand or other variable-speed governors, run something "expensive"
(e.g. untar the gcc .bz2, or do a big compile) and repeatedly check
the spead in the cpuinfo file.  You should find that it ramps up
shortly after the load starts, and falls back a few seconds after the
load drops. [ BUT, if you are in an xterm on a kernel > 2.6.12-rc5,
see the ignore_nice line in my bootscript below. ]

Bootscript for kernel cpufreq

If one or more of the demand-based governors works for you, you're
almost done, you just need to ensure the appropriate governor is
loaded after you boot.  For my desktop box (athlon64) I use the
following :

cat >>/etc/rc.d/init.d/cpufreq << EOF

. /etc/sysconfig/rc
. ${rc_functions}

case "${1}" in
		boot_mesg "Enabling ondemand cpu frequency"
		echo ondemand \
		boot_mesg "ignoring niceness for ondemand"
		echo 1 \
		boot_mesg "Enabling powersave cpu frequency"
    		echo powersave \
		boot_mesg "Enabling performance cpu frequency"
		echo performance \
		echo "Usage: ${0} {start|demand|powersave|performance}"
		exit 1

chmod 754 /etc/rc.d/init.d/cpufreq

ln -s ../init.d/cpufreq /etc/rc.d/rcsysinit.d/S45cpufreq

 Note that this allows me to set powersave or performance if I ever wish
to, and that I just install this in rcsysinit.d so there is no 'stop'

 The 'ignore_nice' ensures the old behaviour of ondemand for kernels
newer than 2.6.12-rc5 : without this, anything I start in an xterm has a
niceness of 10 and the kernel doesn't allow it to influence the cpu


 This is based on 0.96.  Untar it, you'll notice the Makefile is
described as -very- simple.  Unfortunately, it installs into /usr/sbin
- on a laptop, you probably want to bring it up as part of the boot
process, so I think it belongs in /sbin.  The easy way to do that is

 make && install -m 755 powernowd /sbin

 There is an example powernowd.init in the package, which could be
adapted for the bootscript, using variations of "dyn" (dynamic), "low",
"high" - I go with something a little simple - dynamic frequency
control at all times:

Bootscript for powernowd

cat >>/etc/rc.d/init.d/powernowd << EOF

. /etc/sysconfig/rc
. ${rc_functions}

case "${1}" in
		boot_mesg "Starting powernowd"
		loadproc /sbin/powernowd
		boot_mesg "Starting powernowd"
		statusproc /sbin/powernowd
		boot_mesg "Starting powernowd"
		killproc /sbin/powernowd
		echo "Usage: ${0} {start|status|stop}"
		exit 1

chmod 754 /etc/rc.d/init.d/powernowd

ln -s ../init.d/powernowd /etc/rc.d/rcsysinit.d/S45powernowd
ln -s ../init.d/powernowd /etc/rc.d/rc0.d/K45powernowd
ln -s ../init.d/powernowd /etc/rc.d/rc6.d/K45powernowd


Barry Shilliday for bringing the scaling_governors to my attention
in an article in PC Plus magazine.

Eric Piel for explaining how to restore the old behaviour for the
ondemand governor.


2005-07-12	First version.

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