How to check which MCA parameters are used in OpenMPI?
<p>In the <a href="https://github.com/open-mpi/ompi-release" rel="nofollow">OpenMPI</a> codebase, each module has multiple variants. When calling <code>mpirun</code>, you can select the modules from the <a href="https://www.open-mpi.org/faq/?category=tuning#mca-def" rel="nofollow">Modular Component Architecture (MCA)</a> that you would like to use. The options include...</p>

<ul>
<li>collective algorithms (coll): basic, tuned, inter, cuda, ml, sm, ...</li>
<li>byte-transfer layer (btl): openib, tcp, ...</li>
<li>point-to-point management layer (pml): cm, ob1, ...</li>
<li>matching transport layer (mtl): mxm, psm, ...</li>
</ul>

<p>You can specify your choice of MCA components like this:</p>

<p><code>mpirun  --mca btl self,openib --mca pml ob1 -np $nProcs ./myprogram</code></p>

<hr>

<p><strong>My questions:</strong></p>

<ol>
<li>If I leave some MCA parameters unspecified, what are the defaults?</li>
<li>Is there a verbose mode that will print all of the MCA components that are being used? (I tried adding <code>-v</code> to my <code>mpirun</code> command, and it didn't print anything extra.) </li>
</ol>