title: mpirun without options runs a program on one process only
body: <p><a href="https://www.open-mpi.org/doc/v1.10/man1/mpirun.1.php" rel="nofollow">Here</a> I read</p>
  
  <blockquote>
    <p>If no value is provided for the number of copies to execute (i.e.,
    neither the "-np" nor its synonyms are provided on the command line),
    Open MPI will automatically execute a copy of the program on each
    process slot (see below for description of a "process slot")</p>
  </blockquote>
  
  <p>So I would expect</p>
  
  <pre><code>mpirun program
  </code></pre>
  
  <p>to run eight copies of the program (actually a simple hello world), since I have an Intel® Core™ i7-2630QM CPU @ 2.00GHz × 8, but it doesn't: it simply runs a single process.</p>