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</style><div class="section" id="launching-only-on-the-local-node">
<h1><span class="section-number">10.5. </span>Launching only on the local node<a class="headerlink" href="#launching-only-on-the-local-node" title="Permalink to this heading"></a></h1>
<p>It is common to develop MPI applications on a single workstation or
laptop, and then move to a larger parallel / HPC environment once the
MPI application is ready.</p>
<p>Open MPI supports running multi-process MPI jobs on a single machine.
In such cases, you can simply avoid listing a hostfile or remote
hosts, and simply list a number of MPI processes to launch. For
example:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>mpirun<span class="w"> </span>-n<span class="w"> </span><span class="m">6</span><span class="w"> </span>mpi-hello-world
Hello<span class="w"> </span>world,<span class="w"> </span>I<span class="w"> </span>am<span class="w"> </span><span class="m">0</span><span class="w"> </span>of<span class="w"> </span><span class="m">6</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>my-laptop<span class="o">))</span>
Hello<span class="w"> </span>world,<span class="w"> </span>I<span class="w"> </span>am<span class="w"> </span><span class="m">1</span><span class="w"> </span>of<span class="w"> </span><span class="m">6</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>my-laptop<span class="o">)</span>
...
Hello<span class="w"> </span>world,<span class="w"> </span>I<span class="w"> </span>am<span class="w"> </span><span class="m">5</span><span class="w"> </span>of<span class="w"> </span><span class="m">6</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>my-laptop<span class="o">)</span>
</pre></div>
</div>
<p>If you do not specify the <code class="docutils literal notranslate"><span class="pre">-n</span></code> option, <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> will default to
launching as many MPI processes as there are processor cores (not
hyperthreads) on the machine.</p>
<div class="section" id="mpi-communication">
<h2><span class="section-number">10.5.1. </span>MPI communication<a class="headerlink" href="#mpi-communication" title="Permalink to this heading"></a></h2>
<p>When running on a single machine, Open MPI will most likely use the
<code class="docutils literal notranslate"><span class="pre">ob1</span></code> PML and the following BTLs for MPI communication between
peers:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">self</span></code>: used for sending and receiving loopback MPI messages
— where the source and destination MPI process are the same.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">sm</span></code>: used for sending and receiving MPI messages where the source
and destination MPI processes can share memory (e.g., via SYSV or
POSIX shared memory mechanisms).</p></li>
</ul>
</div>
<div class="section" id="shared-memory-mpi-communication">
<h2><span class="section-number">10.5.2. </span>Shared memory MPI communication<a class="headerlink" href="#shared-memory-mpi-communication" title="Permalink to this heading"></a></h2>
<div class="admonition error">
<p class="admonition-title">Error</p>
<p>TODO This should really be moved to the networking section.</p>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">sm</span></code> BTL supports two modes of shared memory communication:</p>
<ol class="arabic">
<li><p><strong>Two-copy:</strong> Otherwise known as “copy-in / copy-out”, this mode is
where the sender copies data into shared memory and the receiver
copies the data out.</p>
<p>This mechanism is always available.</p>
</li>
<li><p><strong>Single copy:</strong> In this mode, the sender or receiver makes a
single copy of the message data from the source buffer in one
process to the destination buffer in another process. Open MPI
supports three flavors of shared memory single-copy transfers:</p>
<ul>
<li><p><a class="reference external" href="https://knem.gitlabpages.inria.fr/">Linux KNEM</a>. This is a
standalone Linux kernel module, made specifically for HPC and MPI
libraries to enable high-performance single-copy message
transfers.</p>
<p>Open MPI must be able to find the KNEM header files in order to
build support for KNEM.</p>
</li>
<li><p><a class="reference external" href="https://github.com/hjelmn/xpmem">Linux XPMEM</a>. Similar to
KNEM, this is a standalone Linux kernel module, made specifically
for HPC and MPI libraries to enable high-performance single-copy
message transfers. It is derived from the Cray XPMEM system.</p>
<p>Open MPI must be able to find the XPMEM header files in order to
build support for XPMEM.</p>
</li>
<li><p>Linux Cross-Memory Attach (CMA). This mechanism is built-in to
modern versions of the Linux kernel. Although more performance
than the two-copy shared memory transfer mechanism, CMA is the
lowest performance of the single-copy mechanisms. However, CMA
is likely the most widely available because it is enabled by
default in several modern Linux distributions.</p>
<p>Open MPI must be built on a Linux system with a recent enough
Glibc and kernel version in order to build support for Linux CMA.</p>
</li>
</ul>
</li>
</ol>
<p>Which mechanism is used at run time depends both on how Open MPI was
built and how your system is configured. You can check to see which
single-copy mechanisms Open MPI was built with via two mechanisms:</p>
<ol class="arabic">
<li><p>At the end of running <code class="docutils literal notranslate"><span class="pre">configure</span></code>, Open MPI emits a list of
transports for which it found relevant header files and libraries
such that it will be able to build support for them. You might see
lines like this, for example:</p>
<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>Shared memory/copy in+copy out: yes
Shared memory/Linux CMA: yes
Shared memory/Linux KNEM: no
Shared memory/XPMEM: no
</pre></div>
</div>
<p>The above output indicates that Open MPI will be built with 2-copy
(as mentioned above, 2-copy is <em>always</em> available) and with Linux
CMA support. KNEM and XPMEM support will <em>not</em> be built.</p>
</li>
<li><p>After Open MPI is installed, the <code class="docutils literal notranslate"><span class="pre">ompi_info</span></code> command can show
which <code class="docutils literal notranslate"><span class="pre">smsc</span></code> (shared memory single copy) components are
available:</p>
<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>shell$ ompi_info | grep smsc
MCA smsc: cma (MCA v2.1.0, API v1.0.0, Component v5.1.0)
</pre></div>
</div>
<p>This Open MPI installation only supports the Linux CMA single-copy
mechanism.</p>
</li>
</ol>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>As implied by the SMSC component names, none of them are
supported on macOS. macOS users will use the two-copy mechanism.</p>
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