Andy Melnikov (nponeccop) wrote,
Andy Melnikov


Keeping the First User PID Small

Another thing we can learn from the MacOS boot-up logic is that shell scripts are evil. Shell is fast and shell is slow. It is fast to hack, but slow in execution. The classic sysvinit boot logic is modelled around shell scripts. Whether it is /bin/bash or any other shell (that was written to make shell scripts faster), in the end the approach is doomed to be slow. On my system the scripts in /etc/init.d call grep at least 77 times. awk is called 92 times, cut 23 and sed 74. Every time those commands (and others) are called, a process is spawned, the libraries searched, some start-up stuff like i18n and so on set up and more. And then after seldom doing more than a trivial string operation the process is terminated again. Of course, that has to be incredibly slow. No other language but shell would do something like that. On top of that, shell scripts are also very fragile, and change their behaviour drastically based on environment variables and suchlike, stuff that is hard to oversee and control.

So, let's get rid of shell scripts in the boot process! Before we can do that we need to figure out what they are currently actually used for: well, the big picture is that most of the time, what they do is actually quite boring. Most of the scripting is spent on trivial setup and tear-down of services, and should be rewritten in C, either in separate executables, or moved into the daemons themselves, or simply be done in the init system.

It is not likely that we can get rid of shell scripts during system boot-up entirely anytime soon. Rewriting them in C takes time, in a few case does not really make sense, and sometimes shell scripts are just too handy to do without. But we can certainly make them less prominent.

A good metric for measuring shell script infestation of the boot process is the PID number of the first process you can start after the system is fully booted up. Boot up, log in, open a terminal, and type echo $$. Try that on your Linux system, and then compare the result with MacOS! (Hint, it's something like this: Linux PID 1823; MacOS PID 154, measured on test systems we own.)

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