What is control group in linux

cgroups

Control groups (or cgroups as they are commonly known) are a feature provided by the Linux kernel to manage, restrict, and audit groups of processes. Compared to other approaches like the nice(1) command or /etc/security/limits.conf , cgroups are more flexible as they can operate on (sub)sets of processes (possibly with different system users).

Control groups can be accessed with various tools:

• using directives in systemd unit files to specify limits for services and slices;
• by accessing the cgroup filesystem directly;
• via tools like cgcreate , cgexec and cgclassify (part of the libcgroupAUR and libcgroup-gitAUR packages);
• using the «rules engine daemon» to automatically move certain users/groups/commands to groups ( /etc/cgrules.conf and cgconfig.service ) (part of the libcgroupAUR and libcgroup-gitAUR packages); and
• through other software such as Linux Containers (LXC) virtualization.

For Arch Linux, systemd is the preferred and easiest method of invoking and configuring cgroups as it is a part of the default installation.

Installing

Make sure you have one of these packages installed for automated cgroup handling:

• systemd — for controlling resources of a systemd service.
• libcgroupAUR , libcgroup-gitAUR — set of standalone tools ( cgcreate , cgclassify , persistence via cgconfig.conf ).

With systemd

Hierarchy

Current cgroup hierarchy can be seen with systemctl status or systemd-cgls command.

● myarchlinux State: running Jobs: 0 queued Failed: 0 units Since: Wed 2019-12-04 22:16:28 UTC; 1 day 4h ago CGroup: / ├─user.slice │ └─user-1000.slice │ ├─user@1000.service │ │ ├─gnome-shell-wayland.service │ │ │ ├─ 1129 /usr/bin/gnome-shell │ │ ├─gnome-terminal-server.service │ │ │ ├─33519 /usr/lib/gnome-terminal-server │ │ │ ├─37298 fish │ │ │ └─39239 systemctl status │ │ ├─init.scope │ │ │ ├─1066 /usr/lib/systemd/systemd --user │ │ │ └─1067 (sd-pam) │ └─session-2.scope │ ├─1053 gdm-session-worker [pam/gdm-password] │ ├─1078 /usr/bin/gnome-keyring-daemon --daemonize --login │ ├─1082 /usr/lib/gdm-wayland-session /usr/bin/gnome-session │ ├─1086 /usr/lib/gnome-session-binary │ └─3514 /usr/bin/ssh-agent -D -a /run/user/1000/keyring/.ssh ├─init.scope │ └─1 /sbin/init └─system.slice ├─systemd-udevd.service │ └─285 /usr/lib/systemd/systemd-udevd ├─systemd-journald.service │ └─272 /usr/lib/systemd/systemd-journald ├─NetworkManager.service │ └─656 /usr/bin/NetworkManager --no-daemon ├─gdm.service │ └─668 /usr/bin/gdm └─systemd-logind.service └─654 /usr/lib/systemd/systemd-logind

Find cgroup of a process

The cgroup name of a process can be found in /proc/PID/cgroup .

For example, the cgroup of the shell:

0::/user.slice/user-1000.slice/session-3.scope

cgroup resource usage

The systemd-cgtop command can be used to see the resource usage:

Control Group Tasks %CPU Memory Input/s Output/s user.slice 540 152,8 3.3G - - user.slice/user-1000.slice 540 152,8 3.3G - - user.slice/u…000.slice/session-1.scope 425 149,5 3.1G - - system.slice 37 - 215.6M - -

Custom cgroups

systemd.slice(5) systemd unit files can be used to define a custom cgroup configuration. They must be placed in a systemd directory, such as /etc/systemd/system/ . The resource control options that can be assigned are documented in systemd.resource-control(5) .

This is an example slice unit that only allows 30% of one CPU to be used:

Remember to do a daemon-reload to pick up any new or changed .slice files.

As service

Service unit file

Resources can be directly specified in service definition or as a drop-in file:

This example limits the service to 1 gigabyte.

Grouping unit under a slice

Service can be specified what slice to run in:

As root

systemd-run can be used to run a command in a specific slice.

# systemd-run --slice=my.slice command 

—uid=username option can be used to spawn the command as specific user.

# systemd-run --uid=username --slice=my.slice command 

The —shell option can be used to spawn a command shell inside the slice.

As unprivileged user

Unprivileged users can divide the resources provided to them into new cgroups, if some conditions are met.

Cgroups v2 must be utilized for a non-root user to be allowed managing cgroup resources.

Controller types

Not all resources can be controlled by user.

Note: eBPF is technically not a controller but those systemd options implemented using it and only root is allowed to set them.

User delegation

For user to control cpu and io resources, the resources need to be delegated. This can be done with a drop-in file.

For example if your user id is 1000:

/etc/systemd/system/user@1000.service.d/delegate.conf

[Service] Delegate=cpu cpuset io

Reboot and verify that the slice your user session is under has cpu and io controller:

$ cat /sys/fs/cgroup/user.slice/user-1000.slice/cgroup.controllers

cpuset cpu io memory pids

User-defined slices

The user slice files can be placed in ~/.config/systemd/user/ .

To run the command under certain slice:

$ systemd-run --user --slice=my.slice command 

You can also run your login shell inside the slice:

$ systemd-run --user --slice=my.slice --shell

Run-time adjustment

cgroups resources can be adjusted at run-time using systemctl set-property command. Option syntax is the same as in systemd.resource-control(5) .

Warning: The adjustments will be made permanent unless —runtime option is passed. Adjustments are saved at /etc/systemd/system.control/ for system wide options and .config/systemd/user.control/ for user options.

Note: Not all resources changes immediately take effect. For example, changing TaskMax will only take effect on spawning new processes.

For example, cutting off internet access for all user sessions:

$ systemctl set-property user.slice IPAddressDeny=any

With libcgroup

You can enable the cgconfig service with systemd. This allows you to track any errors in cgconfig.conf more easily.

Ad-hoc groups

One of the powers of cgroups is that you can create «ad-hoc» groups on the fly. You can even grant the privileges to create custom groups to regular users. groupname is the cgroup name:

# cgcreate -a user -t user -g memory,cpu:groupname 

Now all the tunables in the group groupname are writable by your user:

$ ls -l /sys/fs/cgroup/memory/groupname

total 0 -rwxrwxr-x 1 user root 0 Sep 25 00:39 cgroup.event_control -rwxrwxr-x 1 user root 0 Sep 25 00:39 cgroup.procs -rwxrwxr-x 1 user root 0 Sep 25 00:39 cpu.rt_period_us -rwxrwxr-x 1 user root 0 Sep 25 00:39 cpu.rt_runtime_us -rwxrwxr-x 1 user root 0 Sep 25 00:39 cpu.shares -rwxrwxr-x 1 user root 0 Sep 25 00:39 notify_on_release -rwxrwxr-x 1 user root 0 Sep 25 00:39 tasks

Cgroups are hierarchical, so you can create as many subgroups as you like. If a normal user wants to run a bash shell under a new subgroup called foo :

$ cgcreate -g memory,cpu:groupname/foo $ cgexec -g memory,cpu:groupname/foo bash 

To make sure (only meaningful for legacy (v1) cgroups):

11:memory:/groupname/foo 6:cpu:/groupname/foo

A new subdirectory was created for this group. To limit the memory usage of all processes in this group to 10 MB, run the following:

$ echo 10000000 > /sys/fs/cgroup/memory/groupname/foo/memory.limit_in_bytes

Note that the memory limit applies to RAM use only — once tasks hit this limit, they will begin to swap. But it will not affect the performance of other processes significantly.

Similarly you can change the CPU priority («shares») of this group. By default all groups have 1024 shares. A group with 100 shares will get a ~10% portion of the CPU time:

$ echo 100 > /sys/fs/cgroup/cpu/groupname/foo/cpu.shares

You can find more tunables or statistics by listing the cgroup directory.

You can also change the cgroup of already running processes. To move all ‘bash’ commands to this group:

$ pidof bash 13244 13266 $ cgclassify -g memory,cpu:groupname/foo `pidof bash` $ cat /proc/13244/cgroup 11:memory:/groupname/foo 6:cpu:/groupname/foo

Persistent group configuration

If you want your cgroups to be created at boot, you can define them in /etc/cgconfig.conf instead. For example, the «groupname» has a permission for $USER and users of group $GROUP to manage limits and add tasks. A subgroup «groupname/foo» group definitions would look like this:

group groupname < perm < # who can manage limits admin < uid = $USER; gid = $GROUP; > # who can add tasks to this group task < uid = $USER; gid = $GROUP; > > # create this group in cpu and memory controllers cpu < >memory < >> group groupname/foo < cpu < cpu.shares = 100; > memory < memory.limit_in_bytes = 10000000; > >

• Comments should begin at the start of a line! The # character for comments must appear as the first character of a line. Else, cgconfigparser will have problem parsing it but will only report cgroup change of group failed as the error, unless you started cgconfig with Systemd
• The permissions section is optional.
• The /sys/fs/cgroup/ hierarchy directory containing all controllers sub-directories is already created and mounted at boot as a virtual file system. This gives the ability to create a new group entry with the $CONTROLLER-NAME command. If for any reason you want to create and mount hierachies in another place, you will then need to write a second entry in /etc/cgconfig.conf following this way :

# mkdir /your/path/groupname # mount -t /your/path -o cpuset groupname /your/path/groupname 

# mkdir /sys/fs/cgroup/memory/groupname 

# echo 100000000 > /sys/fs/cgroup/memory/groupname/memory.limit_in_bytes

# echo pid > /sys/fs/cgroup/memory/groupname/cgroup.procs

~/.config/systemd/user/matlab.slice

[Slice] AllowedCPUs=0-5 MemoryHigh=6G

$ systemd-run --user --slice=matlab.slice /opt/MATLAB/2012b/bin/matlab -desktop

group matlab < perm < admin < uid = username; > task < uid = username; > > cpuset < cpuset.mems="0"; cpuset.cpus="0-5"; >memory < memory.limit_in_bytes = 5000000000; >>

$ cgexec -g memory,cpuset:matlab /opt/MATLAB/2012b/bin/matlab -desktop

systemd.unified_cgroup_hierarchy=0