title: MPI - No performance gain when using every available core on the machine
body: <p>I have a C program (acoustic wave solver) that is parallelized with MPI. However, I've been testing the speed up on various numbers of cores and I've noticed something strange. If I use N processes where N is the number of available cores in the machine, then I do not see a performance improvement over the next step down. </p>
  
  <p>So on my 8 core machine then I see speedup from 1 process to 2 processes to 4 processes, but not from 4 to 8. Similarly on my 4 core laptop I see speedup from 1 to 2, but not from 2 to 4. </p>
  
  <p>Any idea what could be causing this?</p>