title: Spark: how long does it keep RDD in cache
body: <p>For example, I cached a number of RDDs in memory. </p>
  
  <p>Then I leave the application for a few days or more. </p>
  
  <p>And then I try to access cached RDDs.</p>
  
  <p><strong>Will they still be in memory?</strong> </p>
  
  <p>Or Spark will clean unused cached RDDs after some period of time. </p>
  
  <p>Please help!  </p>