title: Spark: Memory not released after unpersist
body: <p>Very simply I use spark 2.4.3 on a 17 node cluster and I have a Dataset which I persist. At the end, after some calculations/actions, I use unpersist() but the Dataset remains in memory at the end according to the storage tab in Spark UI. Even if I use unpersist(true) the Dataset is still there in the end. Why is this happening?</p>
  <p><a href="https://i.stack.imgur.com/C4tQt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/C4tQt.png" alt="storage" /></a></p>