Cgroups - ArchWiki

# Cgroups

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cgroups (aka control groups) is a Linux kernel feature to limit, police and account the resource usage for a set of processes. Compared to other approaches like the 'nice' 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 used in multiple ways:

- By accessing the `cgroup` filesystem directly.
- Using the `cgm` client (part of the cgmanager package).
- Via tools like `cgcreate`, `cgexec` and `cgclassify` (part of the libcgroupAUR package).
- 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).
- through other software such as Linux Containers (LXC) virtualization, tools like playpen or systemd.

## Installing

Install any of:

- libcgroupAUR - set of standalone tools (`cgcreate`, `cgclassify`, persistence via `cgconfig.conf`).
- cgmanager - simple client (`cgm`) that communicates with its daemon (`cgmanager`) to manage cgroups.
- systemd - for controlling resources of a systemd service.

After installation of the cgmanager package, you need to Start `cgmanager.service` before its `cgm` client can be used.

## Managing Resource Groups with Systemd

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

## Simple usage

### 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 -g memory,cpu:groupname

Alternatively, using `cgmanager` with user ID `1000` and system group root (GID 0):

    # cgm create memory groupname
    # cgm chown  memory groupname 1000 0
    # cgm create cpu    groupname
    # cgm chown  cpu    groupname 1000 0
    # # the above only makes 'cgroup.procs' and 'tasks' writable
    # chown user /sys/fs/cgroup/{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

Likewise, for `cgmanager`:

    $ cgm create memory groupname/foo
    $ cgm create cpu    groupname/foo
    $ bash
    $ cgm movepid memory groupname/foo $pid_of_bash
    $ cgm movepid cpu    groupname/foo $pid_of_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

Alternative invocation with `cgmanager`:

    $ cgm setvalue memory groupname/foo memory.limit_in_bytes 10000000

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

## Useful examples

### Matlab

Matlab does not have any protection against taking all your machine's memory or CPU. Launching a large calculation can thus trash your system. You could put the following in `/etc/cgconfig.conf` to protect from this (where `$USER` is your username):

    /etc/cgconfig.conf

    # Prevent Matlab from taking all memory
    group matlab {
        perm {
            admin {
                uid = $USER;
            }
            task {
                uid = $USER;
            }
        }

        cpuset {
            cpuset.mems="0";
            cpuset.cpus="0-5";
        }
        memory {
    # 5 GiB limit
            memory.limit_in_bytes = 5368709120;
        }
    }

Note: Do not forget to change $USER to the actual username Matlab will be run by.

This cgroup will bind Matlab to cores 0 to 5 (e.g., if you have have 8, Matlab will only see 6) and cap its memory usage to 5 GiB. The "cpu" resource constraint can also be defined to prevent CPU usage, but you may find the "cpuset" constrain to be sufficient.

Launch matlab like this:

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

Make sure to use the right path to the executable.

## Documentation

- For information on controllers and what certain switches and tunables mean, refer to kernel's Documentation/cgroup (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`

Categories:

- Kernel
- Virtualization