Andrew Or created SPARK-3493:
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             Summary: Unrolling behavior is too aggressive in dropping blocks
                 Key: SPARK-3493
                 URL: https://issues.apache.org/jira/browse/SPARK-3493
             Project: Spark
          Issue Type: Improvement
          Components: Block Manager
    Affects Versions: 1.1.0
            Reporter: Andrew Or
            Assignee: Andrew Or
            Priority: Critical


When unrolling a new block, we (almost) always call `ensureFreeSpace` with a 
fixed fraction (spark.storage.unrollFraction) of the block manager's total 
memory minus a small delta. This means we're essentially reserving this chunk 
of memory in advance because we allocate all of this space even if the new 
block is very small.

We need to call `ensureFreeSpace` incrementally and more cautiously to avoid 
unnecessarily dropping blocks where possible.

Thanks to [~tdas] for originally reporting this.





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