Linux 4.1 = +50% power efficiency (when idle)

Increased battery efficiency

Increased battery efficiency

On my laptop, I’m running Fedora 22 which was shipped initially with a Linux 4.0 kernel. It was difficult to get 4h of battery life (3h30 was usually enough to deplete the battery down to 5%). Recently, the kernel was changed to 4.1 and because after 5h working on my laptop I got notified that I still had 10% power I got curious,

Therefore with a fully charged battery, I booted with Fedora 22 Linux kernel 4.0.4-301. I used powertop to measure the battery power usage in Watt in graphical (ex-init 5 level) and multi-user target (ex-init 3 level). I then rebooted using Linux kernel 4.1.3-201 and did the same measurement. I waited each time that the system settled down and that successive measurements where constant. Nothing was running, WiFi was ON and connected (Link Quality=64/70), screen brightness at 30%, Graphical target is using Gnome 3.

SystemD Target (~init level) Linux 4.0 (in W) Linux 4.1 (in W) Progress
Multi-User (init 3) 12,8 8,66 -32%
Graphical (init 5) 13,1 8,51 -35%

Wow! That’s great. And the estimated battery power is now up from 4h to 6h30 with WiFi ON. But with light browsing usage and some Arduino development, I got a bit more than 5h15 without requiring a wall power connection!! That close to 50% more battery life than with earlier kernels.

Where does this come from? I don’t know. There seems to have been a few pull requests about power management for kernel 4,1 but none stroke me as relevant for such a huge improvement, Matthew Garrett has proposed a patch to improve dramatically the power efficiency of Intel’s Haswell and Broadwell CPUs (and I happen to have an Haswell one), so that could have been that patch, but I did not find it in the kernel 4.1 changelog, so I doubt it was yet implemented. So I really don’t know what made change in the kernel bring such an improvement. (note: I’m running Fedora 22 and without software update, just by selecting kernel 4.0 or 4.1 at boot, I can see the difference in power consumption. So this is really a kernel-side improvement).

Did you also witness improvement when switching to Linux kernel 4.1? Let me know using any social media means!

Note to self: telinit is now deprecated in favour of systemd targets. Runlevel 3 can be reached by invoking sudo systemctl isolate multi-user.target and the switch to the “runlevel 5” can be triggered using sudo systemctl isolate graphical.target.

Picture credits: Picture was created by me using elements from the KDE project. The original materials were licensed under GNU LGPLv3, and the picture is also provided under this license terms and conditions.