Comment 46 for bug 847001

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.