<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):

<programlisting>
systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
</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>
