title: When specifying local[n1,n2,n3] for spark master, what are the three parameters?
body: <p>For launching spark, I have seen:</p>
  
  <pre><code>--master local[n1,n2,n3] 
  </code></pre>
  
  <p>where n1, n2, and n3 are integers.</p>
  
  <p>What do these refer to ?</p>