This is part of the Semicolon&Sons Code Diary - consisting of lessons learned on the job. You're in the unix category.
Last Updated: 2024-04-25
Control Groups, or cgroups are a feature allowing processes to be organized
into multiple hierarchical resources that can be limited and monitored, and
which can stay partitioned even with future children.
With cgroups there are many "root" processes, vs. 1 with init
Traditional resource limits (as implemented by setrlimit()) are (mostly)
per-process. cgroups on the other hand let you enforce limits on entire groups
of processes.
Together with namespaces, they power containers
They are great for babysitting purposes and keeping track of daemons. This is
because cgroup membership is securely inherited by child processes, they
cannot escape. Compare to regular processes which can escape supervision by
parent with double-forking (e.g. CGI script that won't terminate when
grandparent apache is stopped) What's more, in systemd at least, they have
very usable names, being based off files often
$ ps xawf -eo pid,user,cgroup,args
PID USER CGROUP COMMAND
2 root - [kthreadd]
3 root - \_ [ksoftirqd/0]
[...]
4281 root - \_ [flush-8:0]
1 root name=systemd:/systemd-1 /sbin/init
455 root name=systemd:/systemd-1/sysinit.service /sbin/udevd -d
28188 root name=systemd:/systemd-1/sysinit.service \_ /sbin/udevd -d
28191 root name=systemd:/systemd-1/sysinit.service \_ /sbin/udevd -d
1096 dbus name=systemd:/systemd-1/dbus.service /bin/dbus-daemon --system --address=systemd: --nofork --systemd-activation
1131 root name=systemd:/systemd-1/auditd.service auditd
1133 root name=systemd:/systemd-1/auditd.service \_ /sbin/audispd
1135 root name=systemd:/systemd-1/auditd.service \_ /usr/sbin/sedispatch
1171 root name=systemd:/systemd-1/NetworkManager.service /usr/sbin/NetworkManager --no-daemon
4028 root name=systemd:/systemd-1/NetworkManager.service \_ /sbin/dhclient -d -4 -sf /usr/libexec/nm-dhcp-client.action -pf /var/run/dhclient-wlan0.pid -lf /var/lib/dhclient/dhclient-7d32a784-ede9-4cf6-9ee3-60edc0bce5ff-wlan0.lease -
There's a notification system available so that a supervisor process can be
notified when a cgroup runs empty. You can find the cgroups of a process by
reading /proc/$PID/cgroup.