Instructions copied below....
1. As we are going to use a custom kernel, we need to add the linux-phc PPA repository, so just open a terminal and execute:
1 $ sudo add-apt-repository ppa:linux-phc/ppa
2 $ sudo apt-get update
Basically they provide the same kernel as the Ubuntu one but with the patch that is capable for Undervolting.
2. Next we need to install the phc kernel, with headers:
1 $ sudo apt-get install linux-generic-phc linux-headers-generic-phc
Ok, now we need to restart and use the installed phc kernel when booting. After restarting, you can check if you are using the phc kernel by using
1 $ uname -r
If no "phc" appears, then you have to configure grub2 to allow you to chose boot options and select the phc kernel at boot time. This can be done many ways one would be:
(This section was NOT necessary for me.... zcats)
1 sudo update-grub
and count the number of lines that says "Found linux image" (first one is 0) and, when you found the phc one, do:
1 sudo gedit /etc/default/grub
then just change the "GRUB_DEFAULT=0" to the number of the entry you found that has phc. Save the file and do a update-grub command again like in the first step. Now you can reboot into the phc kernel.
3. Great, we are using a phc kernel, now we need to add the actual undervolt module, as the kernel comes only with support for it, not the actual module. First download phc-intel offtree module from here. Right now they have "phc-intel-0.3.2-10 offtree" version. Direct link to the tar.bz2 file here. Unpack the files anywhere and point the terminal to the directory where you extracted the files and do:
1 $ make prepare
2 $ make
3 $ sudo make install
Right now you should have the module installed, check if everything worked by typing:
1 $ cat /sys/devices/system/cpu/cpu0/cpufreq/phc_default_vids
2 40 34 31 27 23 15
If everything worked, you should see some numbers (the ones above are the default ones for my CPU). If not, you may try to restart and check back to see if it works.
4. So right now we are set, the only thing we have to do is... undervolt the CPU.
Now we have to discover the limits of the CPU by lowering it's current voltage one step at a time and set the voltage 4 steps above the critical limit (where the cpu starts to give wrong calculations or the PC simply crashes).
Why four? Well the recommendation was 2 steps above, but with this limit the PC crashed in the same day hours later, so I made it 3 steps, then some days after it crashed again. So I think 4 steps should actually do it. If not, you may increase this value until it suits you.
Ok great now how to find the actual critical limit? There exists a script, created by the community, but it's old and it does not work for multi core CPU's and also in my particular case it sets the voltage directly below this critical limit so the PC crashes instantly.
I updated this script and made it work on multi core CPU's (hopefully it will work for you too), so you just have to download the script, then give it execution permissions and execute it and follow on-screen indications see http://openmindedbrain.info/wp-conte...ndervolt.bash:
1 $ chmod +x intel-phc-undervolt.bash
2 $ ./intel-phc-undervolt.bash
Oh and make sure you close as many applications as possible when executing the script as the system may actually turn instable (it is intended actually) just to prevent any data loss. The script treats all CPU's cores with the same voltages, so if one of them is weaker than the other (does not support such a aggressive undervolting) is this the critical value that the script uses for all of the others, even if they could possibly use a even lower value.
5. After you finished with the script, now you should have a "phc_tweaked_vids" file, containing the acceptable steps for your CPU. Now you only have to make changes permanent (load them at every boot of the pc) by editing /etc/rc.local file:
1 $ sudo gedit /etc/rc.local
and adding one entry for every core you have, something like:
1 echo "23 20 4 4 4 4" > /sys/devices/system/cpu/cpu0/cpufreq/phc_vids
2 echo "23 20 4 4 4 4" > /sys/devices/system/cpu/cpu1/cpufreq/phc_vids
just before the "exit 0". Note that "23 20 4 4 4 4" are my values, you have to replace the above with your values, that came from the "phc_tweaked_vids" file. So that's about it. Not too easy, not too complicated.
So... is it worth it? I have done some tests to see if the above actually works as said, so first I tested to see if on battery using powertop application and the CPU temperature I could see any differences. None actually, if the CPU is in idle state, there is no actual gain using undervolting, be it battery or temperature.
But when I've done the test under full load (using 2 burnMMX instances, one for every core) and with the CPU set at it's maximum 2.4Ghz I could see a very very noticeable difference.
see website for data of temp and energy (W) saved