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<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>
<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>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>For more information about using shared memory MPI
communication, see the <a class="reference internal" href="../tuning-apps/networking/shared-memory.html"><span class="doc">Shared Memory</span></a> page.</p>
</div>
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