title: Default number of executors and cores for spark-shell
body: <p>If I run a spark program in spark shell, is it possible that the program can hog the entire hadoop cluster for hours?</p>
  
  <p>usually there is a setting called num-executors and executor-cores.</p>
  
  <pre><code>spark-shell --driver-memory 10G --executor-memory 15G --executor-cores 8
  </code></pre>
  
  <p>but if they are not specified and I just run "spark-shell"... will it consume the entire cluster? or are there reasonable defaults.</p>