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+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+.. _init-manager:
+
+Selecting an Initialization Manager
+***********************************
+
+By default, the Yocto Project uses :wikipedia:`SysVinit <Init#SysV-style>` as
+the initialization manager. There is also support for BusyBox init, a simpler
+implementation, as well as support for :wikipedia:`systemd <Systemd>`, which
+is a full replacement for init with parallel starting of services, reduced
+shell overhead, increased security and resource limits for services, and other
+features that are used by many distributions.
+
+Within the system, SysVinit and BusyBox init treat system components as
+services. These services are maintained as shell scripts stored in the
+``/etc/init.d/`` directory.
+
+SysVinit is more elaborate than BusyBox init and organizes services in
+different run levels. This organization is maintained by putting links
+to the services in the ``/etc/rcN.d/`` directories, where `N/` is one
+of the following options: "S", "0", "1", "2", "3", "4", "5", or "6".
+
+.. note::
+
+   Each runlevel has a dependency on the previous runlevel. This
+   dependency allows the services to work properly.
+
+Both SysVinit and BusyBox init are configured through the ``/etc/inittab``
+file, with a very similar syntax, though of course BusyBox init features
+are more limited.
+
+In comparison, systemd treats components as units. Using units is a
+broader concept as compared to using a service. A unit includes several
+different types of entities. ``Service`` is one of the types of entities.
+The runlevel concept in SysVinit corresponds to the concept of a target
+in systemd, where target is also a type of supported unit.
+
+In systems with SysVinit or BusyBox init, services load sequentially (i.e. one
+by one) during init and parallelization is not supported. With systemd, services
+start in parallel. This method can have an impact on the startup performance
+of a given service, though systemd will also provide more services by default,
+therefore increasing the total system boot time. systemd also substantially
+increases system size because of its multiple components and the extra
+dependencies it pulls.
+
+On the contrary, BusyBox init is the simplest and the lightest solution and
+also comes with BusyBox mdev as device manager, a lighter replacement to
+:wikipedia:`udev <Udev>`, which SysVinit and systemd both use.
+
+The ":ref:`device-manager`" chapter has more details about device managers.
+
+Using SysVinit with udev
+=========================
+
+SysVinit with the udev device manager corresponds to the
+default setting in Poky. This corresponds to setting::
+
+   INIT_MANAGER = "sysvinit"
+
+Using BusyBox init with BusyBox mdev
+====================================
+
+BusyBox init with BusyBox mdev is the simplest and lightest solution
+for small root filesystems. All you need is BusyBox, which most systems
+have anyway::
+
+   INIT_MANAGER = "mdev-busybox"
+
+Using systemd
+=============
+
+The last option is to use systemd together with the udev device
+manager. This is the most powerful and versatile solution, especially
+for more complex systems::
+
+   INIT_MANAGER = "systemd"
+
+This will enable systemd and remove sysvinit components from the image.
+See :yocto_git:`meta/conf/distro/include/init-manager-systemd.inc
+</poky/tree/meta/conf/distro/include/init-manager-systemd.inc>` for exact
+details on what this does.
+
+Controling systemd from the target command line
+-----------------------------------------------
+
+Here is a quick reference for controling systemd from the command line on the
+target. Instead of opening and sometimes modifying files, most interaction
+happens through the ``systemctl`` and ``journalctl`` commands:
+
+-  ``systemctl status``: show the status of all services
+-  ``systemctl status <service>``: show the status of one service
+-  ``systemctl [start|stop] <service>``: start or stop a service
+-  ``systemctl [enable|disable] <service>``: enable or disable a service at boot time
+-  ``systemctl list-units``: list all available units
+-  ``journalctl -a``: show all logs for all services
+-  ``journalctl -f``: show only the last log entries, and keep printing updates as they arrive
+-  ``journalctl -u``: show only logs from a particular service
+
+Using systemd-journald without a traditional syslog daemon
+----------------------------------------------------------
+
+Counter-intuitively, ``systemd-journald`` is not a syslog runtime or provider,
+and the proper way to use ``systemd-journald`` as your sole logging mechanism is to
+effectively disable syslog entirely by setting these variables in your distribution
+configuration file::
+
+   VIRTUAL-RUNTIME_syslog = ""
+   VIRTUAL-RUNTIME_base-utils-syslog = ""
+
+Doing so will prevent ``rsyslog`` / ``busybox-syslog`` from being pulled in by
+default, leaving only ``systemd-journald``.
+
+Summary
+-------
+
+The Yocto Project supports three different initialization managers, offering
+increasing levels of complexity and functionality:
+
+.. list-table::
+   :widths: 40 20 20 20
+   :header-rows: 1
+
+   * -
+     - BusyBox init
+     - SysVinit
+     - systemd
+   * - Size
+     - Small
+     - Small
+     - Big [#footnote-systemd-size]_
+   * - Complexity
+     - Small
+     - Medium
+     - High
+   * - Support for boot profiles
+     - No
+     - Yes ("runlevels")
+     - Yes ("targets")
+   * - Services defined as
+     - Shell scripts
+     - Shell scripts
+     - Description files
+   * - Starting services in parallel
+     - No
+     - No
+     - Yes
+   * - Setting service resource limits
+     - No
+     - No
+     - Yes
+   * - Support service isolation
+     - No
+     - No
+     - Yes
+   * - Integrated logging
+     - No
+     - No
+     - Yes
+
+.. [#footnote-systemd-size] Using systemd increases the ``core-image-minimal``
+   image size by 160\% for ``qemux86-64`` on Mickledore (4.2), compared to SysVinit.