docs: document balancing cores and max-jobs
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Graham Christensen
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<title>Advanced Topics</title>
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<xi:include href="distributed-builds.xml" />
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<xi:include href="cores-vs-jobs.xml" />
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<xi:include href="diff-hook.xml" />
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</part>
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121
doc/manual/advanced-topics/cores-vs-jobs.xml
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121
doc/manual/advanced-topics/cores-vs-jobs.xml
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<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="chap-tuning-cores-and-jobs">
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<title>Tuning Cores and Jobs</title>
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<para>Nix has two relevant settings with regards to how your CPU cores
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will be utilized: <xref linkend="conf-cores" /> and
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<xref linkend="conf-max-jobs" />. This chapter will talk about what
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they are, how they interact, and their configuration trade-offs.</para>
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<variablelist>
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<varlistentry>
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<term><xref linkend="conf-max-jobs" /></term>
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<listitem><para>
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Dictates how many separate derivations will be built at the same
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time. If you set this to zero, the local machine will do no
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builds. Nix will still substitute from binary caches, and build
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remotely if remote builders are configured.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><xref linkend="conf-cores" /></term>
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<listitem><para>
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Suggests how many cores each derivation should use. Similar to
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<command>make -j</command>.
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</para></listitem>
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</varlistentry>
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</variablelist>
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<para>The <xref linkend="conf-cores" /> setting determines the value of
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<envar>NIX_BUILD_CORES</envar>. <envar>NIX_BUILD_CORES</envar> is equal
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to <xref linkend="conf-cores" />, unless <xref linkend="conf-cores" />
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equals <literal>0</literal>, in which case <envar>NIX_BUILD_CORES</envar>
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will be the total number of cores in the system.</para>
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<para>The total number of consumed cores is a simple multiplication,
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<xref linkend="conf-cores" /> * <envar>NIX_BUILD_CORES</envar>.</para>
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<para>The balance on how to set these two independent variables depends
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upon each builder's workload and hardware. Here are a few example
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scenarios on a machine with 24 cores:</para>
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<table>
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<caption>Balancing 24 Build Cores</caption>
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<thead>
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<tr>
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<th><xref linkend="conf-max-jobs" /></th>
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<th><xref linkend="conf-cores" /></th>
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<th><envar>NIX_BUILD_CORES</envar></th>
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<th>Maximum Processes</th>
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<th>Result</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>1</td>
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<td>24</td>
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<td>24</td>
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<td>24</td>
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<td>
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One derivation will be built at a time, each one can use 24
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cores. Undersold if a job can’t use 24 cores.
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</td>
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</tr>
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<tr>
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<td>4</td>
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<td>6</td>
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<td>6</td>
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<td>24</td>
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<td>
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Four derivations will be built at once, each given access to
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six cores.
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</td>
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</tr>
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<tr>
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<td>12</td>
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<td>6</td>
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<td>6</td>
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<td>72</td>
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<td>
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12 derivations will be built at once, each given access to six
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cores. This configuration is over-sold. If all 12 derivations
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being built simultaneously try to use all six cores, the
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machine's performance will be degraded due to extensive context
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switching between the 12 builds.
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</td>
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</tr>
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<tr>
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<td>24</td>
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<td>1</td>
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<td>1</td>
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<td>24</td>
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<td>
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24 derivations can build at the same time, each using a single
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core. Never oversold, but derivations which require many cores
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will be very slow to compile.
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</td>
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</tr>
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<tr>
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<td>24</td>
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<td>0</td>
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<td>24</td>
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<td>576</td>
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<td>
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24 derivations can build at the same time, each using all the
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available cores of the machine. Very likely to be oversold,
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and very likely to suffer context switches.
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</td>
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</tr>
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</tbody>
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</table>
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<para>It is up to the derivations' build script to respect
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host's requested cores-per-build by following the value of the
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<envar>NIX_BUILD_CORES</envar> environment variable.</para>
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</chapter>
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@ -238,8 +238,9 @@ false</literal>.</para>
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linkend='opt-cores'>--cores</option> command line switch and
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defaults to <literal>1</literal>. The value <literal>0</literal>
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means that the builder should use all available CPU cores in the
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system.</para></listitem>
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system.</para>
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<para>See also <xref linkend="chap-tuning-cores-and-jobs" />.</para></listitem>
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</varlistentry>
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<varlistentry xml:id="conf-diff-hook"><term><literal>diff-hook</literal></term>
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@ -498,7 +499,10 @@ builtins.fetchurl {
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regardless). It can be
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overridden using the <option
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linkend='opt-max-jobs'>--max-jobs</option> (<option>-j</option>)
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command line switch.</para></listitem>
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command line switch.</para>
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<para>See also <xref linkend="chap-tuning-cores-and-jobs" />.</para>
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</listitem>
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</varlistentry>
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<varlistentry xml:id="conf-max-silent-time"><term><literal>max-silent-time</literal></term>
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