pyrox.dev
1<chapter xmlns="http://docbook.org/ns/docbook"
2 xmlns:xlink="http://www.w3.org/1999/xlink"
3 xmlns:xi="http://www.w3.org/2001/XInclude"
4 version="5.0"
5 xml:id="sec-cgroups">
6 <title>Control Groups</title>
7 <para>
8 To keep track of the processes in a running system, systemd uses
9 <emphasis>control groups</emphasis> (cgroups). A control group is a set of
10 processes used to allocate resources such as CPU, memory or I/O bandwidth.
11 There can be multiple control group hierarchies, allowing each kind of
12 resource to be managed independently.
13 </para>
14 <para>
15 The command <command>systemd-cgls</command> lists all control groups in the
16 <literal>systemd</literal> hierarchy, which is what systemd uses to keep
17 track of the processes belonging to each service or user session:
18<screen>
19$ systemd-cgls
20├─user
21│ └─eelco
22│ └─c1
23│ ├─ 2567 -:0
24│ ├─ 2682 kdeinit4: kdeinit4 Running...
25│ ├─ <replaceable>...</replaceable>
26│ └─10851 sh -c less -R
27└─system
28 ├─httpd.service
29 │ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
30 │ └─<replaceable>...</replaceable>
31 ├─dhcpcd.service
32 │ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
33 └─ <replaceable>...</replaceable>
34</screen>
35 Similarly, <command>systemd-cgls cpu</command> shows the cgroups in the CPU
36 hierarchy, which allows per-cgroup CPU scheduling priorities. By default,
37 every systemd service gets its own CPU cgroup, while all user sessions are in
38 the top-level CPU cgroup. This ensures, for instance, that a thousand
39 run-away processes in the <literal>httpd.service</literal> cgroup cannot
40 starve the CPU for one process in the <literal>postgresql.service</literal>
41 cgroup. (By contrast, it they were in the same cgroup, then the PostgreSQL
42 process would get 1/1001 of the cgroup’s CPU time.) You can limit a
43 service’s CPU share in <filename>configuration.nix</filename>:
44<programlisting>
45<link linkend="opt-systemd.services._name_.serviceConfig">systemd.services.httpd.serviceConfig</link>.CPUShares = 512;
46</programlisting>
47 By default, every cgroup has 1024 CPU shares, so this will halve the CPU
48 allocation of the <literal>httpd.service</literal> cgroup.
49 </para>
50 <para>
51 There also is a <literal>memory</literal> hierarchy that controls memory
52 allocation limits; by default, all processes are in the top-level cgroup, so
53 any service or session can exhaust all available memory. Per-cgroup memory
54 limits can be specified in <filename>configuration.nix</filename>; for
55 instance, to limit <literal>httpd.service</literal> to 512 MiB of RAM
56 (excluding swap):
57<programlisting>
58<link linkend="opt-systemd.services._name_.serviceConfig">systemd.services.httpd.serviceConfig</link>.MemoryLimit = "512M";
59</programlisting>
60 </para>
61 <para>
62 The command <command>systemd-cgtop</command> shows a continuously updated
63 list of all cgroups with their CPU and memory usage.
64 </para>
65</chapter>