Here are the code and the measurements.
partman/lib/base.sh currently uses statements such as:
template=$(cat $dir/question)
As reported before, with the long default
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
then current busybox sh using both fork() and execve() runs
cat /dev/null
for 1000 iterations in 22.2 seconds on unmodified armel-eabi NSLU2
(133MHz armv5te.)
Changing to the the modified short PATH=/bin , then 1000 iterations
take 21.5 seconds.
Changing tryexec()/shell/ash.c to use only fork() and no execve()
[verified using strace]:
-----
struct BB_applet const *app = (struct BB_applet const
*)find_applet_by_name(cmd);
int argc = 0;
{
char **p;
for (p = argv; *p; ++p)
++argc;
}
if (app) {
__environ = envp;
exit(app->main(argc, argv));
}
-----
runs 1000 iterations of the builtin applet
cat /dev/null
in 12.7 seconds.
The syntax for busybox sh to replace
template=$(cat $dir/question)
is
read template < $dir/question
Then 1000 iterations of
read line < /dev/null
takes 0.66 seconds. [Thus fork() is almost as slow as execve()
when measured in the busybox sh environment.]
So, avoiding both fork+execve is faster by an order of magnitude
than using fork and avoiding only execve. Avoiding both fork+exec
also bounds the required testing, because only partman is affected,
and not everything else that uses busybox sh.
Some d-i developer probably could edit partman/lib/base.sh to use 'read'
instead of 'cat', test it, and report the results in less than a day.
I got lost in debian-installer-20080522/build/README.
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