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<span id="label-quickstart-launching-apps"></span><h1><span class="section-number">10.1. </span>Quick start: Launching MPI applications<a class="headerlink" href="#quick-start-launching-mpi-applications" title="Permalink to this heading"></a></h1>
<p>Although this section skips many details, it offers examples that will
probably work in many environments.</p>
<div class="admonition caution">
<p class="admonition-title">Caution</p>
<p>Note that this section is a “Quick start” — it does
not attempt to be comprehensive or describe how to build Open MPI
in all supported environments. The examples below may therefore
not work exactly as shown in your environment.</p>
<p>Please consult the other sections in this chapter for more details,
if necessary.</p>
</div>
<div class="section" id="using-mpirun-to-launch-applications">
<h2><span class="section-number">10.1.1. </span>Using <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> to launch applications<a class="headerlink" href="#using-mpirun-to-launch-applications" title="Permalink to this heading"></a></h2>
<p>Open MPI supports both <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> and
<a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpiexec"><span class="std std-ref">mpiexec(1)</span></a> (they are exactly equivalent) to
launch MPI applications. For example:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>mpirun<span class="w"> </span>-n<span class="w"> </span><span class="m">2</span><span class="w"> </span>mpi-hello-world
<span class="c1"># or</span>
shell$<span class="w"> </span>mpiexec<span class="w"> </span>-n<span class="w"> </span><span class="m">2</span><span class="w"> </span>mpi-hello-world
<span class="c1"># or</span>
shell$<span class="w"> </span>mpiexec<span class="w"> </span>-n<span class="w"> </span><span class="m">1</span><span class="w"> </span>mpi-hello-world<span class="w"> </span>:<span class="w"> </span>-n<span class="w"> </span><span class="m">1</span><span class="w"> </span>mpi-hello-world
</pre></div>
</div>
<p>are all equivalent. For simplicity, the rest of this documentation
will simply refer to <code class="docutils literal notranslate"><span class="pre">mpirun</span></code>.</p>
<div class="section" id="other-mpirun-options">
<h3><span class="section-number">10.1.1.1. </span>Other <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> options<a class="headerlink" href="#other-mpirun-options" title="Permalink to this heading"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">mpirun</span></code> supports the <code class="docutils literal notranslate"><span class="pre">--help</span></code> option which provides a usage
message and a summary of the options that it supports. It should be
considered the definitive list of what options are provided.</p>
<p>Several notable options are:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">--hostfile</span></code>: Specify a hostfile for launchers (such as the
<code class="docutils literal notranslate"><span class="pre">rsh</span></code> launcher) that need to be told on which hosts to start
parallel applications. Note that for compatibility with other MPI
implementations, <em>–machinefile</em> is a synonym for <code class="docutils literal notranslate"><span class="pre">--hostfile</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">--host</span></code>: Specify a host or list of hosts to run on (see
<a class="reference internal" href="scheduling.html#running-scheduling-host-option-label"><span class="std std-ref">this FAQ entry for more details</span></a>).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">-n</span></code>: Indicate the number of processes to start.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">--mca</span></code>: Set MCA parameters (see <a class="reference internal" href="../mca.html#label-running-setting-mca-param-values"><span class="std std-ref">how to set MCA params</span></a> for more details).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">--wdir</span> <span class="pre">DIRECTORY</span></code>: Set the working directory of the started
applications. If not supplied, the current working directory is
assumed (or <code class="docutils literal notranslate"><span class="pre">$HOME</span></code>, if the current working directory does not
exist on all nodes).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">-x</span> <span class="pre">ENV_VARIABLE_NAME</span></code>: The name of an environment variable to
export to the parallel application. The <code class="docutils literal notranslate"><span class="pre">-x</span></code> option can be
specified multiple times to export multiple environment variables to
the parallel application.</p></li>
</ul>
<p>Note that the <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> command supports a
<em>large</em> number of options. Be sure to see the <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> man page for much more information.</p>
</div>
</div>
<div class="section" id="launching-on-a-single-host">
<h2><span class="section-number">10.1.2. </span>Launching on a single host<a class="headerlink" href="#launching-on-a-single-host" title="Permalink to this heading"></a></h2>
<p>It is common to develop MPI applications on a single laptop or
workstation. In such simple “single program, multiple data (SPMD)” cases,
use <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> and
specify how many MPI processes you want to launch via the <code class="docutils literal notranslate"><span class="pre">-n</span></code>
option:</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>This starts a six-process parallel application, running six copies
of the executable named <code class="docutils literal notranslate"><span class="pre">mpi-hello-world</span></code>.</p>
<p>If you do not specify the <code class="docutils literal notranslate"><span class="pre">-n</span></code> option, <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> will default to launching as many MPI processes as
there are processor cores (not hyperthreads) on the machine.</p>
</div>
<div class="section" id="launching-in-a-non-scheduled-environments-via-ssh">
<h2><span class="section-number">10.1.3. </span>Launching in a non-scheduled environments (via <code class="docutils literal notranslate"><span class="pre">ssh</span></code>)<a class="headerlink" href="#launching-in-a-non-scheduled-environments-via-ssh" title="Permalink to this heading"></a></h2>
<p>In general, Open MPI requires the following to launch and run MPI
applications:</p>
<ol class="arabic simple">
<li><p>You must be able to login to remote nodes non-interactively (e.g.,
without entering a password or passphrase).</p></li>
<li><p>Open MPI’s executables must be findable (e.g., in your <code class="docutils literal notranslate"><span class="pre">PATH</span></code>).</p></li>
<li><p>Open MPI’s libraries must be findable (e.g., in your
<code class="docutils literal notranslate"><span class="pre">LD_LIBRARY_PATH</span></code>).</p></li>
</ol>
<p><a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> accepts a <code class="docutils literal notranslate"><span class="pre">--hostfile</span></code> option (and its
synonym, the <code class="docutils literal notranslate"><span class="pre">--machinefile</span></code> option) to specify a hostfile containing one
hostname per line:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>cat<span class="w"> </span>my-hostfile.txt
node1.example.com
node2.example.com
node3.example.com<span class="w"> </span><span class="nv">slots</span><span class="o">=</span><span class="m">2</span>
node4.example.com<span class="w"> </span><span class="nv">slots</span><span class="o">=</span><span class="m">10</span>
</pre></div>
</div>
<p>The optional <code class="docutils literal notranslate"><span class="pre">slots</span></code> attribute tells Open MPI the <em>maximum</em> number
of processes that can be allocated to that node. If <code class="docutils literal notranslate"><span class="pre">slots</span></code> is not
provided, Open MPI — by default — uses the number of
processor cores (not hyperthreads) on that node.</p>
<p>Assuming that each of the 4 nodes in <cite>my-hostfile.txt</cite> have 16 cores:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>mpirun<span class="w"> </span>--hostfile<span class="w"> </span>my-hostfile.txt<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">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node1.example.com<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">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node1.example.com<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">15</span><span class="w"> </span>of<span class="w"> </span><span class="m">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node1.example.com<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">16</span><span class="w"> </span>of<span class="w"> </span><span class="m">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node2.example.com<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">17</span><span class="w"> </span>of<span class="w"> </span><span class="m">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node2.example.com<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">31</span><span class="w"> </span>of<span class="w"> </span><span class="m">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node2.example.com<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">32</span><span class="w"> </span>of<span class="w"> </span><span class="m">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node3.example.com<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">33</span><span class="w"> </span>of<span class="w"> </span><span class="m">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node3.example.com<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">34</span><span class="w"> </span>of<span class="w"> </span><span class="m">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node4.example.com<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">43</span><span class="w"> </span>of<span class="w"> </span><span class="m">44</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node4.example.com<span class="o">)</span>
</pre></div>
</div>
<p>You can see the breakdown of how many processes Open MPI launched on
each node:</p>
<ul class="simple">
<li><p>node1: 16, because no <code class="docutils literal notranslate"><span class="pre">slots</span></code> was specified</p></li>
<li><p>node2: 16, because no <code class="docutils literal notranslate"><span class="pre">slots</span></code> was specified</p></li>
<li><p>node3: 2, because <code class="docutils literal notranslate"><span class="pre">slots=2</span></code> was specified</p></li>
<li><p>node2: 10, because <code class="docutils literal notranslate"><span class="pre">slots=10</span></code> was specified</p></li>
</ul>
<p>Note, however, that not all environments require a hostfile. For
example, Open MPI will automatically detect when it is running in
batch / scheduled environments (such as Slurm, PBS/Torque, SGE,
LoadLeveler), and will use host information provided by those systems.</p>
<p>Also note that if using a launcher that requires a hostfile and no
hostfile is specified, all processes are launched on the local host.</p>
</div>
<div class="section" id="launching-in-scheduled-environments">
<h2><span class="section-number">10.1.4. </span>Launching in scheduled environments<a class="headerlink" href="#launching-in-scheduled-environments" title="Permalink to this heading"></a></h2>
<p>In scheduled environments (e.g., in a Slurm job, or PBS/Pro, or LSF,
or any other schedule), the user tells the scheduler how many MPI
processes to launch, and the scheduler decides which hosts to use.
The scheduler then passes both pieces of information (the number of
processes and the hosts to use) to Open MPI.</p>
<p>There are two ways to launch in a scheduled environment. Nominally,
they both achieve the same thing: they launch MPI processes. Them
main user-observable difference between the two methods is that
<a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> has many more features than scheduler
direct launchers.</p>
<div class="section" id="using-open-mpi-s-mpirun-1">
<h3><span class="section-number">10.1.4.1. </span>Using Open MPI’s <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a><a class="headerlink" href="#using-open-mpi-s-mpirun-1" title="Permalink to this heading"></a></h3>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Technically, Open MPI’s <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> is a
thin layer around the PRRTE <code class="docutils literal notranslate"><span class="pre">prun</span></code>. Hence, most of the
functionality described here is really about <code class="docutils literal notranslate"><span class="pre">prun</span></code>. For
simplicity, however, this docmentation will describe
everything in terms of <code class="docutils literal notranslate"><span class="pre">mpirun</span></code>.</p>
</div>
<p>When using the full-featured <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> in a
scheduled environment, there is no need to specify a hostfile or
number of MPI processes to launch. <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a>
will receive this information directly from the scheduler. Hence, if
you want to launch an MPI job that completely “fills” your scheduled
allocation (i.e., one MPI process for each slot in the scheduled
allocation), you can simply:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span><span class="c1"># Write a script that runs your MPI application</span>
shell$<span class="w"> </span>cat<span class="w"> </span>my-slurm-script.sh
<span class="c1">#!/bin/sh</span>
<span class="c1"># There is no need to specify -n or --hostfile because that</span>
<span class="c1"># information will automatically be provided by Slurm.</span>
mpirun<span class="w"> </span>mpi-hello-world
</pre></div>
</div>
<p>You then submit the <code class="docutils literal notranslate"><span class="pre">my-slurm-script.sh</span></code> script to Slurm for
execution:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span><span class="c1"># Use -n to indicate how many MPI processes you want to run.</span>
<span class="c1"># Slurm will pick the specific hosts which will be used.</span>
shell$<span class="w"> </span>sbatch<span class="w"> </span>-n<span class="w"> </span><span class="m">40</span><span class="w"> </span>my-slurm-script.sh
Submitted<span class="w"> </span>batch<span class="w"> </span>job<span class="w"> </span><span class="m">1234</span>
shell$
</pre></div>
</div>
<p>After Slurm job 1234 completes, you can look at the output file to see
what happened:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>cat<span class="w"> </span>slurm-1234.out
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">40</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node37.example.com<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">40</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node37.example.com<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">2</span><span class="w"> </span>of<span class="w"> </span><span class="m">40</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node37.example.com<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">39</span><span class="w"> </span>of<span class="w"> </span><span class="m">40</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node19.example.com<span class="o">)</span>
</pre></div>
</div>
<p>Note that the Slurm scheduler picked the hosts on which the processes
ran.</p>
<p>The above example shows that simply invoking <code class="docutils literal notranslate"><span class="pre">mpirun</span>
<span class="pre">mpi-hello-world</span></code> — with no other CLI options — obtains
the number of processes to run and hosts to use from the scheduler.</p>
<p><a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> has many more features not described in
this Quick Start section. For example, while uncommon in scheduled
environments, you can use <code class="docutils literal notranslate"><span class="pre">-n</span></code> and/or <code class="docutils literal notranslate"><span class="pre">--hostfile</span></code> to launch in
subsets of the overall scheduler allocation. See the <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a> man page for more details.</p>
</div>
<div class="section" id="using-the-scheduler-to-direct-launch-without-mpirun-1">
<h3><span class="section-number">10.1.4.2. </span>Using the scheduler to “direct launch” (without <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a>)<a class="headerlink" href="#using-the-scheduler-to-direct-launch-without-mpirun-1" title="Permalink to this heading"></a></h3>
<p>Some schedulers (such as Slurm) have the ability to “direct launch”
MPI processes without using Open MPI’s <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a>.
For example:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>srun<span class="w"> </span>-n<span class="w"> </span><span class="m">40</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">40</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node14.example.com<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">40</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node14.example.com<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">2</span><span class="w"> </span>of<span class="w"> </span><span class="m">40</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node14.example.com<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">39</span><span class="w"> </span>of<span class="w"> </span><span class="m">40</span><span class="w"> </span><span class="o">(</span>running<span class="w"> </span>on<span class="w"> </span>node203.example.com<span class="o">)</span>
shell$
</pre></div>
</div>
<p>Similar to the prior example, this example launches 40 copies of
<code class="docutils literal notranslate"><span class="pre">mpi-hello-world</span></code>, but it does so via the Slurm <code class="docutils literal notranslate"><span class="pre">srun</span></code> command
without using <a class="reference internal" href="../man-openmpi/man1/mpirun.1.html#man1-mpirun"><span class="std std-ref">mpirun(1)</span></a>.</p>
</div>
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