Power consumption with Fedora

drago01 drago01 at gmail.com
Tue Dec 2 21:03:50 UTC 2014

On Tue, Dec 2, 2014 at 5:33 PM, Owen Taylor <otaylor at redhat.com> wrote:
> Hi John,
> Power consumption is definitely an area that Fedora can improve, and an
> area where we've begun to do some investigation.
> Measuring battery life is hard, because when power saving is working,
> battery life depends so much on what the user is doing. Many
> manufacturers don't even provide battery life in their specifications
> or at most, provide a rough figure like "battery lasts up to 10 hours".
> Doing what?
> I've begun writing a test suite that will simulate events of "typical
> user activity" (web browsing, word processing), so that we can at least
> know machine-to-machine or Fedora-version to Fedora-version how well we
> are doing with battery life. Comparing with a different operating system
> is hard, since we typically have no idea how claimed numbers are
> measured.
> The second part of trying to improve power management is getting an
> understanding of what actually uses power. powertop is impressive, but
> can't always be taken at face value. I have some idea that we could
> build up a database (or at least a wiki page or two) with detailed
> information on power usage for a few representative models so that we
> can get an idea about how we need to modify the system software.
> On my laptop (ASUS ux301la), various things surprised me:
>  * The backlight eats a ton of battery - 4W for the display backlight
> and 1W for the keyboard backlight. If you are getting poor battery life,
> turn off the keyboard backlight and turn the screen down. But how do we
> fix this for all users? We used to be much more aggressive about screen
> dimming in GNOME, and it was really annoying. Can we find some happy
> medium?
>  * Although most of the "tunables" in powertop did little that could be
> measured, the "Enable SATA link power management" tunable saves 2W(!) We
> need to consider whether this should be enabled by default, at least on
> battery.

This is like with most of those options a tradeoff between power
consumption and performance. Last time I tested it caused a large hit
on disk throughput but I didn't measure real world workloads. We
should do that for each of those options and decide between:

1) Performance impact is almost non existent -> always enable
2) Impact small but affects some workloads -> only battery
3) Like 2 but affects more workload or a bit more -> only enable when
battery is beyond a threshold like android does with the "power saving
mode" in lollipop
4) It hurts to much its not worth bothering

So we need both performance and power usage measurements and draw and
pick one of the above options for each.

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