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<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" 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 <literal>systemd-cgls</literal> 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:
</para>
<programlisting>
$ systemd-cgls
├─user
│ └─eelco
│ └─c1
│ ├─ 2567 -:0
│ ├─ 2682 kdeinit4: kdeinit4 Running...
│ ├─ ...
│ └─10851 sh -c less -R
└─system
├─httpd.service
│ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
│ └─...
├─dhcpcd.service
│ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
└─ ...
</programlisting>
<para>
Similarly, <literal>systemd-cgls cpu</literal> 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 <literal>configuration.nix</literal>:
</para>
<programlisting language="bash">
systemd.services.httpd.serviceConfig.CPUShares = 512;
</programlisting>
<para>
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
<literal>configuration.nix</literal>; for instance, to limit
<literal>httpd.service</literal> to 512 MiB of RAM (excluding swap):
</para>
<programlisting language="bash">
systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
</programlisting>
<para>
The command <literal>systemd-cgtop</literal> shows a continuously
updated list of all cgroups with their CPU and memory usage.
</para>
</chapter>
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