1. cgroups.txt
https://www.kernel.org/doc/Documentation/cgroup-v1/cgroups.txt
2. Original Design Document
http://0pointer.de/blog/projects/systemd.html
3. Interface Portability and Stability Promise
https://systemd.io/PORTABILITY_AND_STABILITY/
4. Container Interface
https://systemd.io/CONTAINER_INTERFACE
5. initrd Interface
https://systemd.io/INITRD_INTERFACE/
6. XDG Base Directory specification
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
7. Known Environment Variables
https://systemd.io/ENVIRONMENT
8. If run inside a Linux container these arguments may be passed
as command line arguments to systemd itself, next to any of
the command line options listed in the Options section above.
If run outside of Linux containers, these arguments are
parsed from /proc/cmdline instead.
9. unified cgroup hierarchy
https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html
10. legacy cgroup hierarchy
https://www.kernel.org/doc/Documentation/cgroup-v1/
11. systemd Homepage
https://www.freedesktop.org/wiki/Software/systemd/
