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Sunday, July 12, 2009

wmiirc and Disks

(From the not-useful-unless-you-use-Linux-like-I-do department.)
So, I've been using wmii as the window manager on my laptop.Other than changing its default meta key to be the winkey (I should really do something about that branding), setting it to launch uxterms, configuring it to show CPU and battery info, and setting up pidgin to change a tag color on new messages, I haven't had to change its default settings at all.
That may seem like a lot, but in the world of "nonstandard" window managers, well, it isn't.Programmers (the probable main consumers of -- and face-slappingly-obviously main producers of -- such WMs) are picky people.Indeed, the fact that I didn't have to edit the source code to do any of this is right civilized!(Unlike switching the meta key in, say, Fluxbox.)
However, I did manage to do something stupid when performing that second-to-last tweak.First some background:wmii has a status line at the bottom of the screen.In the wmiirc script there is a function -- status() -- whose output is pasted into the lower right at some frequency.To display battery information you simply create a program (likely a shell script) to echo the information you'd like displayed and have status() call it.I learned as much from this forum post, which also includes a handy script to produce exactly the sort of info I wanted to display.Thus, I adapted this script to my needs and it has been quietly working ever since.
But my hard drive has been being accessed every five seconds (even when the computer is apparently idle) ever since I set that script up.Turns out that here-documents in bash create temp files.Creating temp files is disk activity.Disk activity makes kjournald decide to write a journal every five seconds.Which is probably not good for the disk, definitely not good for battery life, and quite annoying.Here'my version with that problem fixed:
#based on http://bbs.archlinux.org/viewtopic.php?id=1809

rate=`grep 'present rate' /proc/acpi/battery/BAT0/state | tr -d -c 0-9`
current=`grep 'remaining capacity' /proc/acpi/battery/BAT0/state | tr -d -c 0-9`
total=`grep 'last full capacity' /proc/acpi/battery/BAT0/info | tr -d -c 0-9`
percent=`echo "100 * $current / $total" | bc`

speed=`grep -m1 -o 'cpu MHz.*\.' /proc/cpuinfo | tr -d -c '\n0-9'`

if grep -q discharging /proc/acpi/battery/BAT0/state
 minutes=`echo "60 * $current / $rate" | bc`
 echo $speed $minutes $percent'%'
 echo $speed $percent'%'
By the way, should you find yourself in a similar pickle, echo '1' > /proc/sys/vm/block_dump.It will produce kernel messages (viewable by dmesg -- you probably want to turn off your system logger while doing this) which will let you know who is causing blocks to need writeback.