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<section id="sec-title-slide"><h1 class="title">Combining MPI and OpenMP</h1><h2 class="author">Dr Carey Pridgeon</h2><p class="date">Created: 2022-02-03 Thu 13:42</p>
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<section id="slide-sec-">
<h2 id="org7404b29">Combining MPI and OpenMP</h2>
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<ul>
<li>While <b>MPI</b> could be used as a parallelism library, it isn't a particularly
efficient one.</li>
<li><b>OpenMP</b> was designed intentional for exactly this purpose, whereas <b>MPI</b> was
designed primarily for use on distributed computing systems. It just happened
that a consequence of this was it could do paralelism as well.</li>
<li>Thus far with <b>MPI</b> we've looked at how to move data over a cluster and build
our own data structures. However, the nodes are rarely, if ever single core
machines, so we need to make use of a parallelism library to take advantage of
the nodes properly.</li>
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<ul>
<li>On this module I've only covered <b>OpenMP</b> in any detail, but any parallelism
library would work in combination with <b>MPI</b>.</li>
<li>In the commercial world <b>OneTBB</b>, Intels Threading Building Blocks (TBB),
is gaining popularity. It is more advanced, feature wise than <b>OpenMP</b>.</li>
<li>It removes some of the things I consider to be unnecessary now, such as
barrier control, and provides fine grain parallelism, thus reducing wasted
computational time.</li>
<li>This in turn saves energy, as more can be done using less power.</li>
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<li>There's no inherent advantage, in terms of speed alone, between <b>OpenMP</b> and
<b>TBB</b>, as they both falter at the same number of cores due to the inter core
communication problem, starting to suffer at <b>14 to 16</b> cores, slowly
worsening, then topping out at <b>around 32</b> cores, at which point the computer
is spending more time managing the code than running the problem it's trying
to solve.</li>
<li>There are various counts I've found for the exact number of cores, but the
problem remains the same, they're both limited, just as threaded programming
is limited by mutex lock queues, and seriel programming is limited by its very
nature.</li>
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<ul>
<li>To integrate any Parallelism library, or a Concurrency library if you
wanted to doesn't take anything like the detail you might think.</li>
<li>To continue, open the source code in
<b>code/distributed/demonstration_code/OpenMP_and_MPI</b> and we'll go through this
example program together.</li>
</ul>
</section>
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<h3 id="org5e5a902">Obligatory XKCD</h3>
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<p><img src="img/multi.png" alt="multi.png" />
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<ul>
<li>Copyright: Randall Munroe</li>
<li>Mirrored to avoid bandwidth stealing</li>
<li>This Lecture is Licensed under <a href="https://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International</a></li>
<li>By Dr Carey Pridgeon Coventry University, UK, 2021</li>
</ul>
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