One thing we want to do for the next versions of GNOME and Fedora is to improve battery performance. Your laptop may well be advertised by the manufacturer to have “up to 10 hours of battery life” or some such claim. You probably don’t get anywhere near this.

Let’s put out some rough numbers here to give an overall sense of scale for the problem. For a modern ultrabook:

The battery is 50 watt-hours (Wh) – it can power a load of 50W for an hour or a load of 5W for 10 hours.

The baseline idle consumption of the system – this is RAM refresh, the power consumption of peripherals in power-saving mode, etc, is 5W.

The screen and keyboard backlights, if both turned on to 100%, draw 5W.

The CPU/GPU can sustain about 15W – this is a thermal limit, so it can draw more for short bursts, but over time it will be throttled to an average.

All other peripherals (Wifi, bluetooth, touchpad, etc.) can use about 5W of power combined when not in power-saving mode.

So the power draw of the system can range from about 5W (the manufacturer’s 10 hours) to 30W (1 hour 40 minutes). If you have such an ultrabook, how is it doing right now? I’d guess it’s using about 15W unless you pay a lot of attention to power usage. Some of the things that might be going wrong:

Your keyboard/screen backlights are likely higher than is needed.

Some of the devices on your system don’t have power-saving turned on properly.

You likely have some background CPU activity (webpage ads, for example).

Of course, if you are running a compilation in the background, you want your CPU to be using that full 15W of power to get it done as soon as possible – in this case, your battery isn’t going to last very long. But a lot of usage is closer to idle.

Measuring power usage

I’ve made assertions above about power used by different things. But how did I actually measure that? powertop is the state of the art for measuring power usage and tweaking it on Linux. It will show you a figure for current battery discharge rate, but it bounces around by several watts; partly because powertop’s own data collection loads the system. The effect of a kernel option is usually much smaller than that. One of the larger effects I discovered on my laptop was that turning USB autosuspend for the touchscreen saves about 150mW. When you tweak a tunable in powertop, without a way to measure power usage more accurately, it’s hard to know whether any observed differences are real.

To support figuring out what is going on with power, I wrote gnome-battery-bench. What it does is pretty simple – it plays back recorded sequences of events in a loop and monitors battery charge to estimate power usage. Because battery usage is being averaged over minutes, a lot of the noise is averaged out. And the event sequences can be changed to exercise different usage patterns by the user.

The above screenshot shows gnome-battery-bench running a “Light Duty” benchmark that combines scrolling around in a Wikipedia page and typing in gedit. Instantaneous usage bounces around a lot from the activity and from random noise, but after a few cycles the averaged power and estimated battery lifetime converge. The corresponding idle power usage is about 5.5W, so we see then know that we’re using about 2.9W from the activity.

gnome-battery-bench is designed as a graphical application because I want to encourage people to explore with it and find out interactively what is using power on their system. And graphing is also useful so that the user can see when something is going wrong with the measurement; sometimes batteries will report data that jumps around. But there’s also a command line version that can be used for automatic scripting of benchmarks.

I decided to use recorded sequences of events for a couple of reasons: first, it’s easy for anybody to create new test sequences – you just run the gnome-battery-bench command line tool in record mode and do what you want to test. Second, playing back event sequences at a low level simulates user interaction very accurately. There is little CPU overhead, and as far as the desktop is concerned it’s exactly like user input. Of course, not everything can be easily tested by simply playing back canned sequences, but our goal here isn’t to test everything, just to be able to test some things that are reasonably representative.

The gnome-battery-bench README file describes in more detail how it works and how to install it on your system.

Next steps

gnome-battery-bench is basically usable as is. The main remaining thing to do with it is to spend some time designing and recording a couple of sequences that better reflect actual usage. The current tests I checked in are basically just placeholders.

On the operating system, we need to make sure that we are actually shipping with as many power-saving options on for peripherals as can be supported. In particular, “SATA link power management” makes a several-watt difference.

Backlight management is another place we can make improvements. Some problems are simply bad defaults. If ambient light sensors are present on the system, using them can be a big win. If not, simply using appropriate defaults is already an improvement.

Beyond that, in GNOME, we can optimize application and system code for efficiency and to not do things unnecessarily often in the background. Eventually I’d like to figure out a way to have power consumption also tracked by perf.gnome.org so we can see how code changes affect our power consumption and avoid regressions.