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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id="sec-cgroups">
<title>Control Groups</title>
<para>To keep track of the processes in a running system, systemd uses
<emphasis>control groups</emphasis> (cgroups). A control group is a
set of processes used to allocate resources such as CPU, memory or I/O
bandwidth. There can be multiple control group hierarchies, allowing
each kind of resource to be managed independently.</para>
<para>The command <command>systemd-cgls</command> lists all control
groups in the <literal>systemd</literal> hierarchy, which is what
systemd uses to keep track of the processes belonging to each service
or user session:
<screen>
$ systemd-cgls
├─user
│ └─eelco
│ └─c1
│ ├─ 2567 -:0
│ ├─ 2682 kdeinit4: kdeinit4 Running...
│ ├─ <replaceable>...</replaceable>
│ └─10851 sh -c less -R
└─system
├─httpd.service
│ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
│ └─<replaceable>...</replaceable>
├─dhcpcd.service
│ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
└─ <replaceable>...</replaceable>
</screen>
Similarly, <command>systemd-cgls cpu</command> shows the cgroups in
the CPU hierarchy, which allows per-cgroup CPU scheduling priorities.
By default, every systemd service gets its own CPU cgroup, while all
user sessions are in the top-level CPU cgroup. This ensures, for
instance, that a thousand run-away processes in the
<literal>httpd.service</literal> cgroup cannot starve the CPU for one
process in the <literal>postgresql.service</literal> cgroup. (By
contrast, it they were in the same cgroup, then the PostgreSQL process
would get 1/1001 of the cgroup’s CPU time.) You can limit a service’s
CPU share in <filename>configuration.nix</filename>:
<programlisting>
systemd.services.httpd.serviceConfig.CPUShares = 512;
</programlisting>
By default, every cgroup has 1024 CPU shares, so this will halve the
CPU allocation of the <literal>httpd.service</literal> cgroup.</para>
<para>There also is a <literal>memory</literal> hierarchy that
controls memory allocation limits; by default, all processes are in
the top-level cgroup, so any service or session can exhaust all
available memory. Per-cgroup memory limits can be specified in
<filename>configuration.nix</filename>; for instance, to limit
<literal>httpd.service</literal> to 512 MiB of RAM (excluding swap)
and 640 MiB of RAM (including swap):
<programlisting>
systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
systemd.services.httpd.serviceConfig.ControlGroupAttribute = [ "memory.memsw.limit_in_bytes 640M" ];
</programlisting>
</para>
<para>The command <command>systemd-cgtop</command> shows a
continuously updated list of all cgroups with their CPU and memory
usage.</para>
</chapter>
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