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
7<title>Control Groups</title>
8
9<para>To keep track of the processes in a running system, systemd uses
10<emphasis>control groups</emphasis> (cgroups). A control group is a
11set of processes used to allocate resources such as CPU, memory or I/O
12bandwidth. There can be multiple control group hierarchies, allowing
13each kind of resource to be managed independently.</para>
14
15<para>The command <command>systemd-cgls</command> lists all control
16groups in the <literal>systemd</literal> hierarchy, which is what
17systemd uses to keep track of the processes belonging to each service
18or user session:
19
20<screen>
21$ systemd-cgls
22├─user
23│ └─eelco
24│ └─c1
25│ ├─ 2567 -:0
26│ ├─ 2682 kdeinit4: kdeinit4 Running...
27│ ├─ <replaceable>...</replaceable>
28│ └─10851 sh -c less -R
29└─system
30 ├─httpd.service
31 │ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
32 │ └─<replaceable>...</replaceable>
33 ├─dhcpcd.service
34 │ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
35 └─ <replaceable>...</replaceable>
36</screen>
37
38Similarly, <command>systemd-cgls cpu</command> shows the cgroups in
39the CPU hierarchy, which allows per-cgroup CPU scheduling priorities.
40By default, every systemd service gets its own CPU cgroup, while all
41user sessions are in the top-level CPU cgroup. This ensures, for
42instance, that a thousand run-away processes in the
43<literal>httpd.service</literal> cgroup cannot starve the CPU for one
44process in the <literal>postgresql.service</literal> cgroup. (By
45contrast, it they were in the same cgroup, then the PostgreSQL process
46would get 1/1001 of the cgroup’s CPU time.) You can limit a service’s
47CPU share in <filename>configuration.nix</filename>:
48
49<programlisting>
50systemd.services.httpd.serviceConfig.CPUShares = 512;
51</programlisting>
52
53By default, every cgroup has 1024 CPU shares, so this will halve the
54CPU allocation of the <literal>httpd.service</literal> cgroup.</para>
55
56<para>There also is a <literal>memory</literal> hierarchy that
57controls memory allocation limits; by default, all processes are in
58the top-level cgroup, so any service or session can exhaust all
59available memory. Per-cgroup memory limits can be specified in
60<filename>configuration.nix</filename>; for instance, to limit
61<literal>httpd.service</literal> to 512 MiB of RAM (excluding swap):
62
63<programlisting>
64systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
65</programlisting>
66
67</para>
68
69<para>The command <command>systemd-cgtop</command> shows a
70continuously updated list of all cgroups with their CPU and memory
71usage.</para>
72
73</chapter>