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<h1><span class="section-number">3.5. </span>General Run-Time Support Notes<a class="headerlink" href="#general-run-time-support-notes" title="Permalink to this heading"></a></h1>
<ul class="simple">
<li><p>The Open MPI installation must be in your <code class="docutils literal notranslate"><span class="pre">PATH</span></code> on all nodes (and
potentially <code class="docutils literal notranslate"><span class="pre">LD_LIBRARY_PATH</span></code> or <code class="docutils literal notranslate"><span class="pre">DYLD_LIBRARY_PATH</span></code>, if
<code class="docutils literal notranslate"><span class="pre">libmpi</span></code>/<code class="docutils literal notranslate"><span class="pre">libshmem</span></code> is a shared library), unless using the
<code class="docutils literal notranslate"><span class="pre">--prefix</span></code> or <code class="docutils literal notranslate"><span class="pre">--enable-mpirun-prefix-by-default</span></code> functionality (see
below).</p></li>
<li><p>Open MPI’s run-time behavior can be customized via Modular Component
Architecture (MCA) parameters (see <a class="reference internal" href="../mca.html#label-running-setting-mca-param-values"><span class="std std-ref">this section</span></a>) for more information on
how to get/set MCA parameter values). Some MCA parameters can be
set in a way that renders Open MPI inoperable. In particular, some
parameters have required options that must be included:</p>
<ul>
<li><p>If specified, the <code class="docutils literal notranslate"><span class="pre">btl</span></code> parameter must include the <code class="docutils literal notranslate"><span class="pre">self</span></code>
component, or Open MPI will not be able to deliver messages to the
same rank as the sender. For example: <code class="docutils literal notranslate"><span class="pre">mpirun</span> <span class="pre">--mca</span> <span class="pre">btl</span> <span class="pre">tcp,self</span>
<span class="pre">...</span></code></p></li>
<li><p>If specified, the <code class="docutils literal notranslate"><span class="pre">btl_tcp_if_exclude</span></code> parameter must include the
loopback device (<code class="docutils literal notranslate"><span class="pre">lo</span></code> on many Linux platforms), or Open MPI will
not be able to route MPI messages using the TCP BTL. For example:
<code class="docutils literal notranslate"><span class="pre">mpirun</span> <span class="pre">--mca</span> <span class="pre">btl_tcp_if_exclude</span> <span class="pre">lo,eth1</span> <span class="pre">...</span></code></p></li>
</ul>
</li>
<li><p>Running on nodes with different endian and/or different datatype
sizes within a single parallel job is supported in this release.
However, Open MPI does not resize data when datatypes differ in size
(for example, sending a 4 byte <code class="docutils literal notranslate"><span class="pre">MPI_DOUBLE</span></code> and receiving an 8 byte
<code class="docutils literal notranslate"><span class="pre">MPI_DOUBLE</span></code> will fail).</p></li>
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