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68 lines
2.9 KiB
XML
68 lines
2.9 KiB
XML
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<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" 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
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set of processes used to allocate resources such as CPU, memory or
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I/O bandwidth. There can be multiple control group hierarchies,
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allowing each kind of resource to be managed independently.
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</para>
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<para>
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The command <literal>systemd-cgls</literal> lists all control groups
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in the <literal>systemd</literal> hierarchy, which is what systemd
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uses to keep track of the processes belonging to each service or
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user session:
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</para>
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<programlisting>
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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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│ ├─ ...
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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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│ └─...
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├─dhcpcd.service
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│ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
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└─ ...
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</programlisting>
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<para>
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Similarly, <literal>systemd-cgls cpu</literal> shows the cgroups in
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the CPU hierarchy, which allows per-cgroup CPU scheduling
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priorities. By default, every systemd service gets its own CPU
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cgroup, while all user sessions are in the top-level CPU cgroup.
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This ensures, for instance, that a thousand run-away processes in
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the <literal>httpd.service</literal> cgroup cannot starve the CPU
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for one process in the <literal>postgresql.service</literal> cgroup.
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(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 <literal>configuration.nix</literal>:
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</para>
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<programlisting language="bash">
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systemd.services.httpd.serviceConfig.CPUShares = 512;
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</programlisting>
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<para>
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By default, every cgroup has 1024 CPU shares, so this will halve the
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CPU 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
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memory allocation limits; by default, all processes are in the
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top-level cgroup, so any service or session can exhaust all
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available memory. Per-cgroup memory limits can be specified in
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<literal>configuration.nix</literal>; for instance, to limit
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<literal>httpd.service</literal> to 512 MiB of RAM (excluding swap):
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</para>
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<programlisting language="bash">
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systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
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</programlisting>
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<para>
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The command <literal>systemd-cgtop</literal> shows a continuously
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updated list of all cgroups with their CPU and memory usage.
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</para>
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</chapter>
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