Powersaving on a Thinkpad t480s running (Arch) Linux

#linux #powerusage #battery #mobile

Recently I tuned my t480s to enable more power-saving features and settings. I used the following tools:

Powertop

Powertop is used to auto-tune several power-saving settings. Creating a systemd-service at /etc/systemd/system/powertop.service allows us to automatically run the auto-tune command on the system startup:

[Unit]
Description=Powertop tunings

[Service]
Type=oneshot
ExecStart=/usr/bin/powertop --auto-tune

[Install]
WantedBy=multi-user.target

Don't forget to enable and run the service via:

sudo systemctl enable powertop.service --now

Intel-Undervolt

Intel-undervolt can be used to reduce the voltage used by several hardware components on intel based systems. The configuration is located at /etc/intel-undervolt.conf. I edited the file to include the following settings.

Warning, YOU MUST NOT JUST COPY THOSE! Using values not suitable for your system can lead to voiding your warranty, or making your OS unstable. This can really f*ck up your system. On my Lenovo t480s with an i5-8250u everything works as expected.

undervolt 0 'CPU' -125
undervolt 1 'GPU' -80
undervolt 2 'CPU Cache' -100
undervolt 3 'System Agent' -80
undervolt 4 'Analog I/O' 0

Again, don't forget to enable the corresponding systemd service:

sudo systemctl enable intel-undervolt --now

cpupower and x86_energy_perf_policy

Utilizing powertop and intel-undervolt as described is a one-time, zero interaction configuration. Additionally, I wanted to have the possibility to modify the power consumption of my Thinkpad manually.
I created a small bash program called power.sh to change some CPU related settings, based on a supplied command-line argument:

#!/bin/bash

setPowerSave() {
  sudo x86_energy_perf_policy --all 'balance-power'
  sudo cpupower frequency-set --max 1600MHz
}

setPerformance() {
  sudo x86_energy_perf_policy --all 'balance-performance'
  sudo cpupower frequency-set --max 3400MHz
}

setSilent() {
  sudo x86_energy_perf_policy --all 'balance-power'
  sudo cpupower frequency-set --max 2400MHz
}

case $1 in

  bat)
    setPowerSave
    ;;

  sil)
    setSilent
    ;;

  ac)
    setPerformance
    ;;

  *)
    echo "The argument $1 is not supported"
    ;;
esac

The script can process three different command-line arguments:

Argument value: bat

Reduces the max frequency to 1600MHz and sets the performance hint to balance-power.
Useful when the battery is key.
Argument value: sil

Reduces the max frequency to 2400MHz and sets the performance hint to balance_performance. Keeps my system mostly silent, as the fan usually never turns on.
Argument value: ac

Sets the max frequency to 3400MHz (the design maximum on my i5) and sets the performance hint to balance_performance. Useful when developing or running anything more performance intensive.

For convenience, I add an alias to my .bashrc to be able to call my script via the shorthand pwr

alias pwr="sh ~./power.sh"

Now I can execute pwr bat to force my system into a custom power-save mode or pwr ac when I want more performance.

Testing the changes

I used two commands to test my changes.

Firstly, to test the power in watt being drawn from the battery when the device is off AC reading power_now can be used:

cat /sys/class/power_supply/BAT0/power_now

Alternatively, btm can automatically read the file for you when called with the --battery option like this:

btm -g --hide_time -c --battery

Secondly, intel-undervolt brings its own tool to monitor the power usage of the SOC, temperatures, and frequencies of the different cores:

intel-undervolt measure

Btw. I do not use TLP, as I had repeated problems with it, higher power usage, hot system especially when on AC, and no turbo mode are just some of them. And yes, I checked my config, reinstalled, and so on. As I am really happy with my current solution, I do not regret installing TLP.

Happy coding!

Update 01.08.2020:
Removed the part that undervolting could damage hardware. This is not correct as pointed out by Lukáš Hozda in the comments. Thanks!