红帽全球峰会25.4. Overview of systemd hierarchy for cgroups
On the backend, the systemd system and service manager uses the slice, the scope, and the service units to organize and structure processes in the control groups. You can further modify this hierarchy by creating custom unit files or using the systemctl command. Also, systemd automatically mounts hierarchies for important kernel resource controllers at the /sys/fs/cgroup/ directory.
For resource control, you can use the following three systemd unit types:
- Service
A process or a group of processes, which
systemdstarted according to a unit configuration file.Services encapsulate the specified processes so that they can be started and stopped as one set. Services are named in the following way:
<name>.service- Scope
A group of externally created processes. Scopes encapsulate processes that are started and stopped by the arbitrary processes through the
fork()function and then registered bysystemdat runtime. For example, user sessions, containers, and virtual machines are treated as scopes. Scopes are named as follows:<name>.scope- Slice
A group of hierarchically organized units. Slices organize a hierarchy in which scopes and services are placed.
The actual processes are contained in scopes or in services. Every name of a slice unit corresponds to the path to a location in the hierarchy.
The dash (
-) character acts as a separator of the path components to a slice from the-.sliceroot slice. In the following example:<parent-name>.sliceparent-name.sliceis a sub-slice ofparent.slice, which is a sub-slice of the-.sliceroot slice.parent-name.slicecan have its own sub-slice namedparent-name-name2.slice, and so on.
The service, the scope, and the slice units directly map to objects in the control group hierarchy. When these units are activated, they map directly to control group paths built from the unit names.
The following is an abbreviated example of a control group hierarchy:
Control group /:
-.slice
├─user.slice
│ ├─user-42.slice
│ │ ├─session-c1.scope
│ │ │ ├─ 967 gdm-session-worker [pam/gdm-launch-environment]
│ │ │ ├─1035 /usr/libexec/gdm-x-session gnome-session --autostart /usr/share/gdm/greeter/autostart
│ │ │ ├─1054 /usr/libexec/Xorg vt1 -displayfd 3 -auth /run/user/42/gdm/Xauthority -background none -noreset -keeptty -verbose 3
│ │ │ ├─1212 /usr/libexec/gnome-session-binary --autostart /usr/share/gdm/greeter/autostart
│ │ │ ├─1369 /usr/bin/gnome-shell
│ │ │ ├─1732 ibus-daemon --xim --panel disable
│ │ │ ├─1752 /usr/libexec/ibus-dconf
│ │ │ ├─1762 /usr/libexec/ibus-x11 --kill-daemon
│ │ │ ├─1912 /usr/libexec/gsd-xsettings
│ │ │ ├─1917 /usr/libexec/gsd-a11y-settings
│ │ │ ├─1920 /usr/libexec/gsd-clipboard
…
├─init.scope
│ └─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 18
└─system.slice
├─rngd.service
│ └─800 /sbin/rngd -f
├─systemd-udevd.service
│ └─659 /usr/lib/systemd/systemd-udevd
├─chronyd.service
│ └─823 /usr/sbin/chronyd
├─auditd.service
│ ├─761 /sbin/auditd
│ └─763 /usr/sbin/sedispatch
├─accounts-daemon.service
│ └─876 /usr/libexec/accounts-daemon
├─example.service
│ ├─ 929 /bin/bash /home/jdoe/example.sh
│ └─4902 sleep 1
…This example shows that services and scopes contain processes and are placed in slices that do not contain processes of their own.
'%20d='M201.5%20305.5c-13.8%200-24.9-11.1-24.9-24.6%200-13.8%2011.1-24.9%2024.9-24.9%2013.6%200%2024.6%2011.1%2024.6%2024.9%200%2013.6-11.1%2024.6-24.6%2024.6zM504%20256c0%20137-111%20248-248%20248S8%20393%208%20256%20119%208%20256%208s248%20111%20248%20248zm-132.3-41.2c-9.4%200-17.7%203.9-23.8%2010-22.4-15.5-52.6-25.5-86.1-26.6l17.4-78.3%2055.4%2012.5c0%2013.6%2011.1%2024.6%2024.6%2024.6%2013.8%200%2024.9-11.3%2024.9-24.9s-11.1-24.9-24.9-24.9c-9.7%200-18%205.8-22.1%2013.8l-61.2-13.6c-3-.8-6.1%201.4-6.9%204.4l-19.1%2086.4c-33.2%201.4-63.1%2011.3-85.5%2026.8-6.1-6.4-14.7-10.2-24.1-10.2-34.9%200-46.3%2046.9-14.4%2062.8-1.1%205-1.7%2010.2-1.7%2015.5%200%2052.6%2059.2%2095.2%20132%2095.2%2073.1%200%20132.3-42.6%20132.3-95.2%200-5.3-.6-10.8-1.9-15.8%2031.3-16%2019.8-62.5-14.9-62.5zM302.8%20331c-18.2%2018.2-76.1%2017.9-93.6%200-2.2-2.2-6.1-2.2-8.3%200-2.5%202.5-2.5%206.4%200%208.6%2022.8%2022.8%2087.3%2022.8%20110.2%200%202.5-2.2%202.5-6.1%200-8.6-2.2-2.2-6.1-2.2-8.3%200zm7.7-75c-13.6%200-24.6%2011.1-24.6%2024.9%200%2013.6%2011.1%2024.6%2024.6%2024.6%2013.8%200%2024.9-11.1%2024.9-24.6%200-13.8-11-24.9-24.9-24.9z'/%3e%3c/svg%3e){kind=small}