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<h1><span class="section-number">11.7.3. </span>Reducing wireup time<a class="headerlink" href="#reducing-wireup-time" title="Permalink to this heading"></a></h1>
<p>Open MPI’s run-time uses an <em>out-of-band</em> (OOB) communication
subsystem to pass messages during the launch, initialization, and
termination stages for the job. These messages allow <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> to tell
its daemons what processes to launch, and allow the daemons in turn to
forward stdio to <code class="docutils literal notranslate"><span class="pre">mpirun</span></code>, update <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> on process status, etc.</p>
<p>The OOB uses TCP sockets for its communication, with each daemon
opening a socket back to <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> upon startup. In a large cluster,
this can mean thousands of connections being formed on the node where
<code class="docutils literal notranslate"><span class="pre">mpirun</span></code> resides, and requires that <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> actually process all
these connection requests. <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> defaults to processing
connection requests sequentially — so on large clusters, a
backlog can be created that can cause remote daemons to timeout
waiting for a response.</p>
<p>Fortunately, Open MPI provides an alternative mechanism for processing
connection requests that helps alleviate this problem. Setting the MCA
parameter <code class="docutils literal notranslate"><span class="pre">oob_tcp_listen_mode</span></code> to <code class="docutils literal notranslate"><span class="pre">listen_thread</span></code> causes
<code class="docutils literal notranslate"><span class="pre">mpirun</span></code> to startup a separate thread dedicated to responding to
connection requests. Thus, remote daemons receive a quick response to
their connection request, allowing <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> to deal with the message
as soon as possible.</p>
<div class="admonition error">
<p class="admonition-title">Error</p>
<p>TODO This seems very out of date. We should have content
about PMIx instant on.</p>
</div>
<p>This parameter can be included in the default MCA parameter file,
placed in the user’s environment, or added to the <code class="docutils literal notranslate"><span class="pre">mpirun</span></code> command
line. See <a class="reference internal" href="../../mca.html#label-running-setting-mca-param-values"><span class="std std-ref">this FAQ entry</span></a>
for more details on how to set MCA parameters.</p>
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