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doc/overlays.xml: add information on BLAS/LAPACK switching

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Matthew Bauer 2020-04-20 15:49:56 -05:00
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doc/using/overlays.xml
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@ -137,4 +137,80 @@ self: super:
Overlays are similar to other methods for customizing Nixpkgs, in particular the <literal>packageOverrides</literal> attribute described in <xref linkend="sec-modify-via-packageOverrides"/>. Indeed, <literal>packageOverrides</literal> acts as an overlay with only the <varname>super</varname> argument. It is therefore appropriate for basic use, but overlays are more powerful and easier to distribute.
</para>
</section>
<section xml:id="sec-overlays-alternatives">
<title>Using overlays to configure alternatives</title>
<para>
Certain software has different implementations of the same
interface. Other distributions have functionality to switch
between these. For example, Debian provides <link
xlink:href="https://wiki.debian.org/DebianAlternatives">DebianAlternatives</link>.
Nixpkgs has what we call <literal>alternatives</literal>, which
are configured through overlays.
</para>
<section xml:id="sec-overlays-alternatives-blas-lapack">
<title>BLAS/LAPACK</title>
<para>
In Nixpkgs, we have multiple implementations of the BLAS/LAPACK
numerical linear algebra interfaces. They are:
</para>
<itemizedlist>
<listitem>
<para>
<link xlink:href="https://www.openblas.net/">OpenBLAS</link>
</para>
<para>
The Nixpkgs attribute is <literal>openblas</literal> for
ILP64 and <literal>openblasCompat</literal> for LP64. This
is the default.
</para>
</listitem>
<listitem>
<para>
<link xlink:href="http://www.netlib.org/lapack/">LAPACK
reference</link> (also provides BLAS)
</para>
<para>
The Nixpkgs attribute is <literal>lapack-reference</literal>.
</para>
</listitem>
<listitem>
<para>
<link
xlink:href="https://software.intel.com/en-us/mkl">Intel
MKL</link> (only works on x86 architecture)
</para>
<para>
The Nixpkgs attribute is <literal>mkl</literal>.
</para>
</listitem>
</itemizedlist>
<para>
Introduced in <link
xlink:href="https://github.com/NixOS/nixpkgs/pull/83888">PR
#83888</link>, we are able to override the ‘blas’ and ‘lapack’
packages to use different implementations, through the
‘blasProvider’ and ‘lapackProvider’ argument. This can be used
to select a different provider. For example, an overlay can be
created that looks like:
</para>
<programlisting>
self: super:
{
blas = super.blas.override {
blasProvider = self.mkl;
}
lapack = super.lapack.override {
lapackProvider = self.mkl;
}
}
</programlisting>
<para>
This overlay uses Intel’s MKL library for both BLAS and LAPACK
interfaces. Note that the same can be accomplished at runtime
using <literal>LD_PRELOAD</literal> of libblas.so.3 and
liblapack.so.3.
</para>
</section>
</section>
</chapter>