title: Spark looks "zombie" on EMR cluster
body: <p>We are running a simple spark application that executes multiple count() functions on each column for a given dataframe.
  After few jobs the application seems "stuck", it looks like it still running (there are active tasks on executors), but nothing actually happens (for 3 hours until I kill it).
  In Ganglia we see that all of the nodes are on &lt;5% cpu usage.
  When we execute this application again (without changing anything) it finishes after 10 minutes.</p>
  
  <p>More details:</p>
  
  <ol>
  <li>We run spark on EMR cluster, 20 nodes.</li>
  <li>Dataframe contains ~20M rows.</li>
  </ol>
  
  <p>Any ideas?</p>