title: What is scheduler delay in spark UI's event timeline
body: <p>I am using YARN environment to run spark programs,
  with option <code>--master yarn-cluster</code>.</p>
  
  <p>When I open a spark application's application master, I saw a lot of <code>Scheduler Delay</code> in a stage. Some of them are even more than 10 minutes. I wonder what are they and why it takes so long?</p>
  
  <p>Update:
  Usually operations like aggregateByKey take much more time (i.e. scheduler delay) before executors really start doing tasks. Why is it?</p>