Ubuntu, CPU Scaling, Battery life and You

CPU Scaling is a feature built into most modern (mobile) CPUs that allows them to scale up or down in how fast they run and how much energy they suck down based on demand. If you have a fairly modern mobile computer there’s a very good chance that your CPU(s) can handle frequency scaling.

Why should you care? Well, you can control this to tell you computer how much power and how fast it should allow it’s CPU(s) to operate. This can save some energy and thus battery life at the expense of a little performance – which is great for extending the use time of your laptop when it’s unplugged.

Can your CPU(s) handle scaling? There’s an easy way to find out. Open up a terminal session (Applications -> Accessories ->Terminal) and type or paste the following into it:

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

On my machine I get back

1667000 1333000 1000000

Those are in Hertz, so my machine is capable of 1.66Ghz, 1.33Ghz and 1.00 Ghz.

Now that you know your CPU(s) can handle scaling, let’s see what modes are available. In the terminal, type or paste:

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

Again, I get:

powersave ondemand userspace conservative performance

Powersave will keep the CPU constantly at the lowest frequency. Ondemand will set the CPU at the lowest frequency (in my case, 1.00Ghz) until use of the CPU increases, then it will automatically bump it up to the highest frequency (with me, 1.66Ghz). Userspace means that a different program will be used to control the CPU’s scaling. Conservative is where your CPU will go up as needed, starting at the lowest frequency, and then bumping up to the next available until it maxes out. Performance simply sets the CPU(s) at the highest available frequency and keeps it there.

The lower your frequency, the less power you use. So, if you’re bent on extending your battery life to the max, you’d want to keep your CPU(s) at their lowest frequency – but you’ll do this at the expense of computing power. In my case my 1.66Ghz processors would effectively be 1.00Ghz processors.

Now, how do you actively control this? It’s fairly easy. Right click on an empty space in your taskbar (where your applets and such things as Applications, Places and System are located) and choose “Add to panel”. From there, find the CPU Frequency Scaling Monitor. Double click on this and it will appear in your taskbar. Right click on it and choose Properties and you can set various options like have it show your CPU frequency as a frequency (i.e. 1.33Ghz) or as a percentage. If you have multiple CPU’s or a dual/quad core machine you can also choose which CPU to monitor.

To configure this applet to actually allow you to control how your CPU(s) scale, you’ll have to had back to the terminal.

Type this:

sudo dpkg-reconfigure gnome-applets

This will throw up a nifty blue screen asking you to say Yes. Do so. Then it will ask if you want to install cpufreq-selector with SUID root. Say yes. Once you’ve done this, go back to your CPU Frequency Scaling Monitor in your taskbar and left click it. You should now be presented with a bunch of options from which you can choose the one you want. You can also directly set the frequency at which your CPU(s) will run at, which can be handy if you want to scale up or down for a short bit and then manually change it again.

As you can see, I’ve got mine set to Ondemand, allowing the frequency to scale up through three different settings (1 GHz, 1.33 GHz or 1.66 GHz) as needed.

While this will take effect immediately, it will only be in effect until you reboot at which time your default settings will come back. To change the default head back into your terminal and type:

gconf-editor

From there head to apps -> gnome-power-manager -> cpufreq. Find the settings policy_ac and policy_battery and change them to whichever setting you want for the default.

For those with multiple cores or processors who happen to be a bit needy in the info department (like myself) you can add an applet for each CPU. Just add as many applets as you have CPUs and then right click on them, choose Preferences and use the drop down to choose which CPU that particular applet is monitoring.

Now you know a lot more about CPU Frequency Scaling then you may have when you started reading this article and you know how to set it on your computer.