Undervolt your notebook CPU for longer battery life

“Undervolting is the practice of reducing the supply voltage of a computer’s CPU. There are mainly three reasons to do so:

1. Reduce power consumption
2. Reduce heat generation
3. Reduce fan noise

1. This howto depends on the kernel module acpi-cpufreq to control your cpu. To find out if you’re using it:

lsmod | grep acpi_cpufreq

2. You need to get a modified version of your acpi_cpufreq module, one with PHC (processor hardware control) support build in. There are several ways to get this module.

a) You can download a patch or a precompiled module on the official forum and official website and compile it yourself.

You can also download the precompiled modules from the authors’ subversion repository, however these are not updated regularly:

svn co http://phcpatches.googlecode.com/svn/trunk/acpi-cpufreq phcpatches/cpufreq
svn co http://phctool.googlecode.com/svn/trunk/ phctool

Note: To find out what kernel you have open a terminal and type:

uname -r

Mini-Howto for compilation (from Ares Drake thread)

extract the tar from /usr/src to /home/”your-username”/”kernelversion”
download latest patch to /home/”your-username”/”kernelversion”
copy /boot/config-$(uname -r).config to /home/”your-username”/”kernelversion”

cd /home/”your-username”/”kernelversion”
patch -p1 < linux-phc*.patch
make oldconfig
make prepare
make scripts
make M=./arch/x86/kernel/cpu/cpufreq

The finished module should be in /home/”your-username”/”kernelversion”/arch/x86/kernel/cpu/cpufreq
Proceed as if you had downloaded this module with copying it to the right place.  After all is done you can savely remove the folder /home/”your-username”/”kernelversion” and uninstall linux-source to save harddisk space.

REMEMBER: First backup your old module (basically copies it to a new name ending with .old)

sudo cp /lib/modules/`uname -r`/kernel/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.ko /lib/modules/`uname -r`/kernel/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.ko.old

b) Then copy the downloaded file to the right place:

sudo cp acpi-cpufreq.ko /lib/modules/$(uname -r)/kernel/arch/x86/kernel/cpu/cpufreq

So once you know, download the right kernel module from Ares Drake thread at Ubuntuforums.net

Kernel modules are also copiled by the THC crew and available on the main download site as well as on the official forum.

You have to redo the entire Step 2 every time you update your kernel, so you might want to save your downloaded module file, as it might work again after a kernel upgrade, i.e. the version -16 worked with -14 and -15 as well, but not with -17.

3. Reboot. If the module is installed correctly, then look here for phc related files

/sys/devices/system/cpu/cpu0/cpufreq/

One should be like this one (values would be of course different):

12:38 10:30 8:24 6:18

In order to minimize any possibility of data corruption I advise you to run this script from Ubuntu Safe Mode / Recovery mode (choose this at boot menu, then root terminal when prompted). Here you will have a terminal only environment, which also lacks any cpu management module. Remember you are in a root environment, here you may also do system wide damages! Be careful with the commands you type in!

First you have to load acpi_cpufreq:

# modprobe acpi_cpufreq

If you haven’t granted execution to the script before do it now

# chmod u+x linux-phc-optimize.bash

cat /sys/devices/system/cpu/cpu0/cpufreq/phc_controls

Then run the scripts the terminal with

The value before the: stands for the frequency, the later for the voltage.

4. Okay, now we need to find the lowest save voltages. We will use this optimizer script.
Download it then right-click the files, open the permissions tab, and and down at the bottom check the box that says “allow executing the file…”.

It needs cpuburn installed, so you might want to install it in advance with synaptic. If you haven’t done so yet

$ sudo ./linux-phc-optimize.bash

# apt-get install cpuburn

What this scipt does is: It stresses your cpu while lowering the cpu voltage step by step unil your system crashes. Then the script knows what is too low and will stay safely above. The script needs to run once for every frequency step your cpu is capable of, usually around 4-6 times. It will tell you, just run it so often until it tells you finished.

In case you have a dual core CPU it is advisable to run two instances of cpuburn,

first run

cpuburn &

then run the script (which will run the other instance)

NOTICE:
This script will crash your system. Several times. This is normal and intended. No harm to your hardware is to be expected. However in case you have open files and / or mounted filesystems data loss or corruption might occur. So be advised to unmount as many partitions as possible and open as few files as necessary while doing this. After a reset the fsck utility will check your partitions for data corruption so be strongly advised to let that check run!

All this is not going to occour if you run the script from the recovery mode

Second, the script stresses only one cpu core. In case you have a dual core, open a terminal and type “burnMMX” to run a second thread to stress the second core while doing this.

When you’re finished with the script, you will have a text file with the new and optimized frequency : voltage pairs.

5. To use these optimized values, you will need to echo them to /sys/devices/system/cpu/cpu0/cpufreq/phc_controls. An easy way to do it automatically every reboot is to add a line like this:

to your /etc/rc.local file. Of course you have to replace the values with your own. Any second core should automatically use the same voltages.

6. Check if it works:

cat /sys/devices/system/cpu/cpu0/cpufreq/phc_controls