Adding some more information. Running the following as root in a gnome
terminal:
SYSFILE=/sys/class/backlight/intel_backlight/brightness
OLD_VAL=$(cat $SYSFILE)
NEW_VAL=$((OLD_VAL + 100))
T0=$(date +%N)
time echo $NEW_VAL | tee $SYSFILE
T1=$(date +%N)
echo "Total time (millis): $(((T1 - T0)/1000))"
Gives the following:
real 0m0.002s
user 0m0.002s
sys 0m0.001s
Total time (millis): 3502
In other words, the write to sysfs returns in 2ms (and indeed,
brightness changes instantly), but it takes 3502ms for control to get
back to the prompt.
During that time, the system seems to hang, but keypresses are
collected: they are all echoed to the terminal right after the command
returns. So the question then is: where are the 3 seconds spent?
I'm happy to experiment if someone has instructions.
** Also affects: gnome-shell (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/847001
Title:
Adjusting display brightness is very slow on several Dell laptops
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