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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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