66 lines
3.0 KiB
XML
66 lines
3.0 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-cgroups">
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<title>Control Groups</title>
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<para>
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To keep track of the processes in a running system, systemd uses
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<emphasis>control groups</emphasis> (cgroups). A control group is a set of
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processes used to allocate resources such as CPU, memory or I/O bandwidth.
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There can be multiple control group hierarchies, allowing each kind of
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resource to be managed independently.
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</para>
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<para>
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The command <command>systemd-cgls</command> lists all control groups in the
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<literal>systemd</literal> hierarchy, which is what systemd uses to keep
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track of the processes belonging to each service or user session:
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<screen>
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$ systemd-cgls
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├─user
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│ └─eelco
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│ └─c1
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│ ├─ 2567 -:0
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│ ├─ 2682 kdeinit4: kdeinit4 Running...
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│ ├─ <replaceable>...</replaceable>
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│ └─10851 sh -c less -R
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└─system
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├─httpd.service
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│ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
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│ └─<replaceable>...</replaceable>
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├─dhcpcd.service
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│ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
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└─ <replaceable>...</replaceable>
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</screen>
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Similarly, <command>systemd-cgls cpu</command> shows the cgroups in the CPU
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hierarchy, which allows per-cgroup CPU scheduling priorities. By default,
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every systemd service gets its own CPU cgroup, while all user sessions are in
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the top-level CPU cgroup. This ensures, for instance, that a thousand
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run-away processes in the <literal>httpd.service</literal> cgroup cannot
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starve the CPU for one process in the <literal>postgresql.service</literal>
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cgroup. (By contrast, it they were in the same cgroup, then the PostgreSQL
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process would get 1/1001 of the cgroup’s CPU time.) You can limit a
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service’s CPU share in <filename>configuration.nix</filename>:
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<programlisting>
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<link linkend="opt-systemd.services._name_.serviceConfig">systemd.services.httpd.serviceConfig</link>.CPUShares = 512;
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</programlisting>
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By default, every cgroup has 1024 CPU shares, so this will halve the CPU
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allocation of the <literal>httpd.service</literal> cgroup.
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</para>
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<para>
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There also is a <literal>memory</literal> hierarchy that controls memory
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allocation limits; by default, all processes are in the top-level cgroup, so
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any service or session can exhaust all available memory. Per-cgroup memory
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limits can be specified in <filename>configuration.nix</filename>; for
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instance, to limit <literal>httpd.service</literal> to 512 MiB of RAM
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(excluding swap):
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<programlisting>
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<link linkend="opt-systemd.services._name_.serviceConfig">systemd.services.httpd.serviceConfig</link>.MemoryLimit = "512M";
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</programlisting>
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</para>
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<para>
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The command <command>systemd-cgtop</command> shows a continuously updated
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list of all cgroups with their CPU and memory usage.
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</para>
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</chapter>
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