title: -cpu-set in openmpi doesn't work as it suppose to be
body: <p>I have 4 hosts to run an openmpi application <code>wrf.exe</code>. There are 48 cores(24 per socket,2 socket per node) on each host.</p>
  <p>I wanna run 40 processes to handle the application. Each node will run 10 processes on 10 cores (0,2,4,6,8,10,12,14,16,18).</p>
  <p>Here are what I tried but I didn't get the mpi manner as I suppose  to be.</p>
  <ol>
  <li><p><code>mpirun --hostfile=4hosts --cpu-set 0,2,4,6,8,10,12,14,16,18 ...</code>. 10 cores run at 100% util of each node. But over 100 of processes <code>wrf.exe</code> run on each node.</p>
  </li>
  <li><p><code>mpirun --hostfile=4hosts -np 40 --cpu-set 0,2,4,6,8,10,12,14,16,18 ...</code>. 40 processes of wrf.exe runs on the host where I start the application. Only 10 cores on the host run at 100% util.</p>
  </li>
  <li></li>
  </ol>
  <p><code>mpirun --hostfile=4hosts -np 40 -N 10 --cpu-set 0,2,4,6,8,10,12,14,16,18 ...</code></p>
  <p><code>mpirun --hostfile=4hosts -N 10 --cpu-set 0,2,4,6,8,10,12,14,16,18 ...</code></p>
  <p><code>mpirun --hostfile=4hosts ---map-by ppr:10:node  --cpu-set 0,2,4,6,8,10,12,14,16,18 ...</code></p>
  <p>All got errors:</p>
  <pre><code>Conflicting directives for mapping policy are causing the policy
  to be redefined:
  
    New policy:   RANK_FILE
    Prior policy:  UNKNOWN
  
  Please check that only one policy is defined.
  </code></pre>
  <p>There are four ip in my hostfile which indicates the hosts I would like to run my application. I'm using openmpi on centos7.6.</p>
  <p>How to use -cpu-set in mpirun to run total 40 process on 4 node, 10 process on specified cores on each node?</p>