title: mpirun with slurm : how to run multiple processes on a single CPU
body: <p>I would like to write slurm batches (sbatch) to run several mpi applications. Thus I would like to be able to run something like that </p>
  
  <pre><code>salloc --nodes=1 mpirun -n 6 hostname 
  </code></pre>
  
  <p>But I get this message : </p>
  
  <blockquote>
    <p>There are not enough slots available in the system to satisfy the 6 slots
    that were requested by the application:
     hostname</p>
    
    <p>Either request fewer slots for your application, or make more slots available for use.</p>
  </blockquote>
  
  <p>The node has actually 4 CPUs. I therefore looking for something allowing more task per CPU but I cannot find the proper option.  I know that mpi alone is able to run several processes when physical resources are missing. I think the problem is on the slurm side. 
  Do you have any suggestions/comments?</p>