title: $time meaning when using parallel processing on a scientific cluster?
body: <p>I'm running my finite-difference program on a scientific cluster at my school. The program uses openmpi to parallelize the code. </p>
  
  <p>When time the program in serial I get: </p>
  
  <pre><code>real    78m40.592s
  user    78m34.920s
  sys     0m0.999s
  </code></pre>
  
  <p>When I run it with 8 mpi processors I get:</p>
  
  <pre><code>real    12m45.929s
  user    101m9.271s
  sys     0m29.735s
  </code></pre>
  
  <p>When I run it with 16 mpi processors I get: </p>
  
  <pre><code>real    4m46.936s
  user    37m30.000s
  sys     0m1.150s
  </code></pre>
  
  <p>So my question is: if the user time is the total CPU time, then why are the user times so different from each other for different numbers of processors?</p>
  
  <p>Thanks,</p>
  
  <p>Anthony G. </p>