this post was submitted on 20 Aug 2023
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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One of the big issues with process monitoring, in the general sense, is how PID 1 checks on processes
The cgroups usage lets them make use of a very powerful Linux-specific feature. Some competitors such as Upstart tried to use ptrace for this, but that causes services to run slower
“Is a process running” I think is a harder question than you realize. systemd also offers the ability to ask “is a process running correctly” through watchdogs, and “is a process using too much memory” or “is a process using too much CPU” and offer corrective action if they are
The systemd.target issues I mention are related to different design goals. Systemd tries to start as many services as possible at once, but we need some services up within 1 second, and the rest can take longer
One option I offered was a modification to systemd so that targets could handle Before/After during our design, but the maintenance of porting it over for each update versus using OpenRC was decided to be too much effort