cgroups (aka control groups) is a Linux kernel feature to limit, police and account the resource usage of certain processes (actually process groups). Compared to other approaches like the 'nice' command or /etc/security/limits.conf, cgroups are infinitely more flexible.
Control groups can be used in multiple ways:
- create and manage them on the fly using tools like cgcreate, cgexec, cgclassify etc
- the "rules engine daemon", to automatically move certain users/groups/commands to groups (/etc/cgrules.conf and /etc/rc.d/cgred)
- through other software such as Linux Containers (LXC) virtualization
Unfortunately this feature is often underappreciated due to lack of easy "how-to" style documentation. This is an attempt of fixing the problem. :)
Note: I'm only learning to use cgroups as I type this, so do not take everything I say here as pure gold.
First, install the utilities for managing cgroups; you need the libcgroup package from AUR. makepkg and install it as usual.
Next, you need to define where to mount the cgroup controller virtual file systems. Let's start with the 'cpu' and 'memory' controllers:
Template:File Then run the following to create these directories and mount the controller file systems:
Now when you list the controller directory, you should see some files (cgroup tunables) in it:
ls -l /mnt/cgroups/memory
You might also want to run this at boot:
One of the powers of croups is that you can create "ad-hoc" groups on the fly. In fact, you can even grant the privileges to create custom groups to regular users. Run this as root (replace $USER with your user name):
sudo cgcreate -a $USER -g memory,cpu:me
That's it! Now all the tunables in the group "me" are writable by your user:
$ ls -l /mnt/cgroups/memory/me 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. Let's say that, as a normal user, you want to run a bash shell under a new subgroup called 'foo':
cgcreate -g memory,cpu:me/foo cgexec -g memory,cpu:me/foo bash
There we go! Just to make sure:
$ cat /proc/self/cgroup 11:memory:/me/foo 6:cpu:/me/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 > /mnt/cgroups/memory/me/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 > /mnt/cgroups/cpu/me/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:me/foo `pidof bash` $ cat /proc/13244/cgroup 11:memory:/me/foo 6:cpu:/me/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 "me" and "me/foo" group definitions would look like this:
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
For commands and configuration files, see relevant man pages, e.g. man cgcreate or man cgrules.conf