Migrating Arch's initscripts to dash

Question

I recently "discovered" dash and how fast it is as compared to bash. FYI, dash is "a POSIX-compliant shell that is much smaller than bash". That being said, here are some simple benchmarks which compare the speed of bash and dash using a for loop (btw, I'm ran the tests below in bash since 'time' is a bash built-in):
(test.sh w/ output redirection)
for x in `seq 50000`; do
echo "" > /dev/null
done
results for bash:
$ time bash test.sh
real    0m2.143s
user    0m1.740s
sys     0m0.413s
$ time bash test.sh
real    0m2.180s
user    0m1.730s
sys     0m0.450s
$ time bash test.sh
real    0m2.189s
user    0m1.717s
sys     0m0.460s
ave: 2.171 seconds
results for dash:
$ time dash test.sh
real    0m0.741s
user    0m0.350s
sys     0m0.403s
$ time dash test.sh
real    0m0.729s
user    0m0.413s
sys     0m0.327s
$ time dash test.sh
real    0m0.732s
user    0m0.360s
sys     0m0.373s
ave: 0.734 seconds
* In this test, dash is 2.96 times faster than bash!
(test.sh w/o output redirection)
for x in `seq 50000`; do
echo -n ""
done
results for bash:
$ time bash test.sh
real    0m0.845s
user    0m0.833s
sys     0m0.010s
$ time bash test.sh
real    0m0.867s
user    0m0.863s
sys     0m0.007s
$ time bash test.sh
real    0m0.871s
user    0m0.870s
sys     0m0.003s
ave: 0.861 seconds
results for dash:
$ time dash test.sh
real    0m0.167s
user    0m0.177s
sys     0m0.003s
$ time dash test.sh
real    0m0.165s
user    0m0.153s
sys     0m0.020s
$ time dash test.sh
real    0m0.174s
user    0m0.167s
sys     0m0.013s
ave: 0.169 seconds
* In this test, dash is 5.10 times faster than bash! Astonishing!
Now think about this... How much will Arch's boot speed increase if we use dash instead of bash for our initscripts? Of course, there are some caveats like some "bash-specific" or "bash-oriented" (non-POSIX-compliant) shell scripts that won't work in dash.
I don't know if there are already existing efforts to make this happen so I'm bringing this up now. Tell me what you think.
BTW, I also found a discussion about dash and bash in the ubuntu-devel mailing list.
Last edited by djclue917 (2007-11-30 03:40:40)

Answer 1

Out of curiosity (Okay I'm procrastinating doing something else) I did a quick port of /lib/udev/load-modules.sh from bash to dash. This is a known  bottleneck for boot time because it's called several times and is a bit slow.
The big problem is that /etc/rc.conf uses arrays and dash doesn't support them. In the case of load-modules.sh we don't need true arrays but just space-separated values. In a complete move to dash, /etc/rc.conf would be updated to reflect this. But to not break things I just use eval/grep/tr to parse rc.conf. Which is probably not very efficient. (And could lead to bugs)
I tested both with a module I actual use, and a blacklisted module. I saw speed improvements with dash in both cases. Because my testing was not very scientific, I'm posting my code instead of numbers:
load-module.dash
#! /bin/dash
# Implement blacklisting for udev-loaded modules
# Includes module checking
# - Aaron Griffin & Tobias Powalowski for Archlinux
# /etc/rc.conf is only parsable by bash. Fake it for dash.
#. /etc/rc.conf
eval `grep MOD /etc/rc.conf | tr '()' '""'`
[ $# -ne 1 ] && exit 1
if [ -f /proc/cmdline ]; then
for cmd in $(cat /proc/cmdline); do
case $cmd in
*=*) eval $cmd ;;
esac
done
fi
# blacklist framebuffer modules
for x in $(echo /lib/modules/$(uname -r)/kernel/drivers/video/*/*fb*); do
BLACKLIST="$BLACKLIST $(basename $x .ko)"
done
for x in $(echo /lib/modules/$(uname -r)/kernel/drivers/video/*fb*); do
BLACKLIST="$BLACKLIST $(basename $x .ko)"
done
# get the real names from modaliases
i="$(/sbin/modprobe -i --show-depends $1 | sed "s#^insmod /lib.*/\(.*\)\.ko.*#\1#g" | sed 's|-|_|g')"
# get blacklistet modules
k="$(echo $BLACKLIST ${MOD_BLACKLIST} | sed 's|-|_|g')"
j="$(echo $MODULES | sed 's|-|_|g')"
#add disabled MODULES (!) to blacklist - much requested feature
for m in ${j}; do
[ "$m" != "${m#!}" ] && k="${k} ${m#!}"
done
# add disablemodules= from commandline to blacklist
k="${k} $(echo ${disablemodules} | sed 's|-|_|g' | sed 's|,| |g')"
# echo "k = [[ $k ]]" # Confirm that module parsing is working
# echo "m = [[ $m ]]" # Confirm that module parsing is working
if [ "$MOD_AUTOLOAD" = "yes" -o "$MOD_AUTOLOAD" = "YES" ] && [ "$load_modules" != "off" ]; then
if [ "${k}" != "" ]; then
for n in ${i}; do
if echo ${k} | /bin/grep "\<$n\>" 2>&1 >/dev/null; then
/usr/bin/logger -p info \
"udev load-modules: $i is blacklisted"
exit 1
fi
done
fi
/sbin/modprobe $1
else
/usr/bin/logger -p info \
"udev load-modules: autoloading is disabled, not loading $i"
fi
# vim: set et ts=4:
testdash
#!/bin/dash
[ $# -ne 2 ] && exit 1
test_shell=$1
test_mod=$2
modprobe -r $test_mod
for i in `seq 1 100`; do
if [ $test_shell = "dash" ]; then
/bin/dash ./load-modules.dash $test_mod
elif [ $test_shell = "sh" ]; then
/bin/sh ./load-modules.sh $test_mod
elif [ $test_shell = "bash" ]; then
/bin/bash ./load-modules.sh $test_mod
elif [ $test_shell = "shdash" ]; then
/bin/sh ./load-modules.dash $test_mod
elif [ $test_shell = "bashdash" ]; then
/bin/bash ./load-modules.dash $test_mod
fi
modprobe -r slhc
done
Invoke with
time ./testdash dash slhc
Or
time ./testdash sh slhc
You also can test /bin/bash versus /bin/sh (Which i assume is a symlink to bash, but bash handles it differently) or you can test bash or sh with my dash script (Which is slower due to the eval/grep/tr hack)
Someone running bootchart before and after might be interesting.