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Suspend out-of-focus Firefox to save battery from useless CPU usage

November 29th, 2014 Leave a comment Go to comments

My Firefox (or rather Iceweasel) is prone to constantly spinning and eating about 50-70% CPU on average when should be supposed to just sit idle. I tried to find the root cause, but Javascript profiler sees nothing and other forays didn’t end up with much (I discovered that spinning progress wheel when some tab is forever loading amounts to about 20% CPU, though). Not sure if always, but sometimes it spins within WebGLImageConverter::run() (no callgraph, sorry).

So in the end I decided to treat the symptoms instead. Cleaning my CPU fan helped with the noise, but the main problem is that running firefox brings my battery life to about 1/2 to 1/3. So one obvious solution would be to just stop the damn process when I don’t use it. I typically don’t do background downloads while on battery (or otherwise), so that means I want it stopped when the window is inactive – not in focus. This is surprisingly exotic idea, apparently, and not easy to do in most window managers.

I even tried switching to awesome or i3 window managers which should make this easy, but I’m psychologically not up to that; I think I’m too conservative, but I decided not to stick with that. I use the MATE desktop environment with marco window manager. Perhaps switching to sawfish would be a good option, but in the end I just decided to write a shell script that will periodically assess the situation and suspend or resume firefox as needed. Of course this introduces extra wakeups and ambient CPU load, but when powertop reports that my GoogleTalkPlugin (running at all times for whatever reason) wakes up 150 times per second, the powersaving situation on Linux is still too messy – so who cares?

Here goes the script, in the hope that it will be useful for someone else too. Run it in a terminal or backgrounded in your ~/.xprofile, it will stop the firefox process when out of focus for more than 10s and on battery, and resume it within a second when switching back. In practice, I found these timings completely acceptable so far, and didn’t notice any ill effects of constant STOP/CONT either.

firefox-suspender.sh:

#!/bin/bash
#
# firefox-suspender: Periodically check whether firefox is out of focus
# and STOP it in that case after a time delay; if in focus but stopped,
# send SIGCONT.
#
# (c) Petr Baudis <pasky@ucw.cz>  2014
# MIT licence if this is even copyrightable
 
loop_delay=1    # [s]
stop_delay=10   # [s]
 
last_in_focus=$(date +%s)
firefoxpid=
state=running
 
while true; do
  sleep $loop_delay
 
  # Get active window id
  window=$(xprop -root _NET_ACTIVE_WINDOW)
  window=${window#*# }
  # What kind of window is it?
  class=$(xprop -id "$window" WM_CLASS)
  # echo Active window $window, class $class
 
  if [[ "$class" =~ Navigator ]]; then
    # Firefox!  We know it is running.  Make sure we
    # have its pid and update the last seen date.
    # If we stopped it, resume again.
    if [ "$state" = stopped ]; then
      echo "$(date)  Resuming firefox @ $firefoxpid"
      if kill -CONT $firefoxpid; then
        state=running
      else
        firefoxpid=
      fi
    fi
    last_in_focus=$(date +%s)
    if [ -z "$firefoxpid" ]; then
      firefoxpid=$(pidof iceweasel)
    fi
    if [ -z "$firefoxpid" ]; then
      firefoxpid=$(pidof firefox)
    fi
 
    continue
  fi
 
  # Not Firefox!  If it's running, we are on battery and
  # it's been long enough, stop it now.
  if [ "$state" != running ]; then
    continue
  fi
 
  read battery </sys/class/power_supply/BAT0/status
  if [ "$battery" != Discharging ]; then
    continue
  fi
 
  if [ $(($(date +%s) - last_in_focus)) -ge $stop_delay ]; then
    echo "$(date)  Stopping firefox @ $firefoxpid"
    if ! kill -STOP $firefoxpid; then
      firefoxpid=
    fi
    state=stopped
  fi
done

An improved version of this script: https://github.com/mkoura/browser-suspender

Categories: linux Tags: , , , ,
  1. pl
    November 29th, 2014 at 02:49 | #1

    Oh, thanks for sharing! This improves my ‘workflow’ alot, before I’ve (pkill -CONT or pkill -STOP) manually. Going to try it out tomorrow.

  2. Martin Vidner
    December 3rd, 2014 at 17:22 | #2

    I am using Tamefox for this: https://github.com/lmacken/tamefox

  3. András Korn
    April 3rd, 2017 at 12:10 | #3

    I also wrote a similar script that even handles multiple concurrent Firefox instances (only unsuspending the one you use and even suspending the others if you switch between them); it also tries to avoid suspending background downloads (but you have to keep the downloads window open).

    It’s at https://gist.github.com/akorn/a31581c30dcecfa6dc1c113f9f63930b if anyone’s interested.

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