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<div class="title"><a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> and multi-threading </div> </div>
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<div class="textblock"><h1><a class="anchor" id="TopicMultiThreading_MakingEigenMT"></a>
Make Eigen run in parallel</h1>
<p>Some <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a>'s algorithms can exploit the multiple cores present in your hardware. To this end, it is enough to enable OpenMP on your compiler, for instance: GCC: <code>-fopenmp</code> ICC: <code>-openmp</code> MSVC: check the respective option in the build properties. You can control the number of thread that will be used using either the OpenMP API or Eiegn's API using the following priority: </p>
<div class="fragment"><div class="line">OMP_NUM_THREADS=n ./my_program</div>
<div class="line">omp_set_num_threads(n);</div>
<div class="line"><a class="code" href="namespaceEigen.html#af9cd17c2fe18204239cd11c88c120b50">Eigen::setNbThreads</a>(n);</div>
</div><!-- fragment --><p> Unless setNbThreads has been called, <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> uses the number of threads specified by OpenMP. You can restore this bahavior by calling</p>
<div class="fragment"><div class="line"><a class="code" href="namespaceEigen.html#af9cd17c2fe18204239cd11c88c120b50">setNbThreads</a>(0); </div>
</div><!-- fragment --><p> You can query the number of threads that will be used with: </p>
<div class="fragment"><div class="line">n = <a class="code" href="namespaceEigen.html#a9aca97d83e21b91a04ec079360dfffeb">Eigen::nbThreads</a>(n);</div>
</div><!-- fragment --><p> You can disable <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a>'s multi threading at compile time by defining the EIGEN_DONT_PARALLELIZE preprocessor token.</p>
<p>Currently, the following algorithms can make use of multi-threading: general matrix - matrix products <a class="el" href="classEigen_1_1PartialPivLU.html" title="LU decomposition of a matrix with partial pivoting, and related features. ">PartialPivLU</a></p>
<h1><a class="anchor" id="TopicMultiThreading_UsingEigenWithMT"></a>
Using Eigen in a multi-threaded application</h1>
<p>In the case your own application is multithreaded, and multiple threads make calls to <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a>, then you have to initialize <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> by calling the following routine <b>before</b> creating the threads: </p>
<div class="fragment"><div class="line"><span class="preprocessor">#include <Eigen/Core></span></div>
<div class="line"></div>
<div class="line"><span class="keywordtype">int</span> main(<span class="keywordtype">int</span> argc, <span class="keywordtype">char</span>** argv)</div>
<div class="line">{</div>
<div class="line"> <a class="code" href="namespaceEigen.html#a820c0e0460934cc17eb6dacbad54a9f5">Eigen::initParallel</a>();</div>
<div class="line"> </div>
<div class="line"> ...</div>
<div class="line">}</div>
</div><!-- fragment --><p>In the case your application is parallelized with OpenMP, you might want to disable <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a>'s own parallization as detailed in the previous section. </p>
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