cgroups

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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 package);
  • using the "rules engine daemon" to automatically move certain users/groups/commands to groups (/etc/cgrules.conf and /usr/lib/systemd/system/cgconfig.service) (part of the libcgroupAUR package); and
  • through other software such as Linux Containers (LXC) virtualization.

cgmanager is deprecated and unsupported as it does not work with systemd versions 232 and above.

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

$ systemctl status
● 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:

$ cat /proc/self/cgroup
0::/user.slice/user-1000.slice/session-3.scope

cgroup resource usage

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

$ systemd-cgtop
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:

/etc/systemd/system/my.slice
[Slice]
CPUQuota=30%

Remember to run systemctl 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 Systemd#Drop-in files

[Service]
MemoryMax=1G # Limit service to 1 gigabyte

Grouping unit under a slice

Service can be specified what slice to run in:

[Service]
Slice=my.slice

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.

Switching to cgroups v2

Arch Linux enables both v1 and v2 cgroups. However by default systemd mounts cgroup v1.

Switching to cgroups v2 requires one of the following Kernel parameters at boot time:

  • systemd.unified_cgroup_hierarchy=1 - Systemd will mount /sys/fs/cgroup as cgroup v2
  • cgroup_no_v1="all" - The kernel will disable all v1 cgroup controllers

Alternatively you can mount cgroups v2 manually:

# mount -t cgroup2 none /sys/fs/cgroup

Verify that v2 cgroups have been mounted:

$ ls /sys/fs/cgroup
cgroup.controllers      cgroup.subtree_control  init.scope/      system.slice/
cgroup.max.depth        cgroup.threads          io.cost.model    user.slice/
cgroup.max.descendants  cpu.pressure            io.cost.qos
cgroup.procs            cpuset.cpus.effective   io.pressure
cgroup.stat             cpuset.mems.effective   memory.pressure

If you see something like this, you are still making use of v1 cgroups:

$ ls /sys/fs/cgroup
blkio/    cpu,cpuacct/  freezer/  net_cls@           perf_event/  systemd/
cpu@      cpuset/       hugetlb/  net_cls,net_prio/  pids/        unified/
cpuacct@  devices/      memory/   net_prio@          rdma/

Controller types

Not all resources can be controlled by user.

Controller Can be controlled by user Options
cpu Requires delegation CPUAccounting, CPUWeight, CPUQuota, AllowedCPUs, AllowedMemoryNodes
io Requires delegation IOWeight, IOReadBandwidthMax, IOWriteBandwidthMax, IODeviceLatencyTargetSec
memory Yes MemoryLow, MemoryHigh, MemoryMax, MemorySwapMax
pids Yes TasksMax
rdma No ?
eBPF No IPAddressDeny, DeviceAllow, DevicePolicy
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 by creating a unit overload.

For example if your user id is 1000:

# systemctl edit user@1000.service
[Service]
Delegate=yes

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):

$ cat /proc/self/cgroup
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 won't 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

Note: when using Systemd > = 205 to manage cgroups, you can ignore this file entirely.

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:

/etc/cgconfig.conf 
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;
  }
}
Note:
  • 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 :
 mount {    
   cpuset = /your/path/groupname;
 }

This is equivalent to these shell commands:

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

With the cgroup virtual filesystem

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: this section must be integrated with the following sections (Discuss in Talk:Cgroups#)

Starting with systemd 232, the cgm method described in the next section, this section will instead describe a manual method to limit memory usage.

Create a new cgroup named groupname:

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

Example: set the maximum memory limit to 100MB:

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

Move a process to the cgroup (note: only one PID can be written at a time, repeat this for each process that must be moved):

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

Examples

Matlab

Doing large calculations in MATLAB can crash your system, because Matlab does not have any protection against taking all your machine's memory or CPU. The following examples show a cgroup that constrains Matlab to first 6 CPU cores and 5 GB of memory.

With systemd

~/.config/systemd/user/matlab.slice
[Slice]
AllowedCPUs=0-5
MemoryHigh=6G

Launch Matlab like this (be sure to use the right path):

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

With libcgroup

/etc/cgconfig.conf
group matlab {
    perm {
        admin {
            uid = username;
        }
        task {
            uid = username;
        }
    }

    cpuset {
        cpuset.mems="0";
        cpuset.cpus="0-5";
    }
    memory {
        memory.limit_in_bytes = 5000000000;
    }
}

Change username to the user Matlab is run as.

You can also restrict the CPU share with the cpu constraint.

Launch Matlab like this (be sure to use the right path):

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

Documentation

  • For information on controllers and what certain switches and tunables mean, refer to kernel's documentation v1 or v2 (or install linux-docs and see /usr/src/linux/Documentation/cgroup)
  • A detailed and complete Resource Management Guide can be found in the fedora project documentation.

For commands and configuration files, see relevant man pages, e.g. man cgcreate or man cgrules.conf

See also