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F.5. Booting with systemd

systemd is the first process started by the kernel. It replaces the venerable SysVinit program (also called init) and the newer Upstart init system. systemd coordinates the rest of the boot process and configures the environment for the user.
systemd improves on other init systems with increased parallelization. It starts the process of loading all programs it launches immediately, and manages information between interdependent programs as they load. By dissociating programs and their means of communication, each program is able to load without waiting for unrelated or even dependent programs to load first.
The Boot Process