title: Unable to use all cores with mpirun
body: <p>I'm testing a simple MPI program on my desktop (Ubuntu LTS 16.04/ Intel® Core™ i3-6100U CPU @ 2.30GHz × 4/ gcc 4.8.5 /OpenMPI 3.0.0) and mpirun won't let me use all of the cores on my machine (4). When I run:</p>
  
  <pre><code>$ mpirun -n 4 ./test2
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>--------------------------------------------------------------------------
  There are not enough slots available in the system to satisfy the 4 slots
  that were requested by the application:
    ./test2
  
  Either request fewer slots for your application, or make more slots available
  for use.
  --------------------------------------------------------------------------
  </code></pre>
  
  <p>But if I run with:</p>
  
  <pre><code>$ mpirun -n 2 ./test2
  </code></pre>
  
  <p>everything works fine.</p>
  
  <p>I've seen from other answers that I can check the number of processors with</p>
  
  <pre><code>cat /proc/cpuinfo | grep processor | wc -l
  </code></pre>
  
  <p>and this tells me that I have 4 processors. I'm <strong>not</strong> interested in oversubscribing, I'd just like to be able to use all my processors. Can anyone help?</p>