[jira] [Created] (SPARK-25103) CompletionIterator may delay GC of completed resources

["Eyal Farago (JIRA)" <ji...@apache.org>]

Eyal Farago created SPARK-25103:
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             Summary: CompletionIterator may delay GC of completed resources
                 Key: SPARK-25103
                 URL: https://issues.apache.org/jira/browse/SPARK-25103
             Project: Spark
          Issue Type: Bug
          Components: Spark Core
    Affects Versions: 2.3.0, 2.2.0, 2.1.0, 2.0.1
            Reporter: Eyal Farago


while working on SPARK-22713 , I fund (and partially fixed) a scenario in which an iterator is already exhausted but still holds a reference to some resources that can be GCed at this point.

However, these resources can not be GCed because of this reference.

the specific fix applied in SPARK-22713 was to wrap the iterator with a CompletionIterator that cleans it when exhausted, thing is that it's quite easy to get this wrong by closing over local variables or _this_ reference in the cleanup function itself.

I propose solving this by modifying CompletionIterator to discard references to the wrapped iterator and cleanup function once exhausted.

 
 * a dive into the code showed that most CompletionIterators are eventually used by 
{code:java}
org.apache.spark.scheduler.ShuffleMapTask#runTask{code}
which does:

{code:java}
writer.write(rdd.iterator(partition, context).asInstanceOf[Iterator[_ <: Product2[Any, Any]]]){code}

looking at 
{code:java}
org.apache.spark.shuffle.ShuffleWriter#write{code}
implementations, it seems all of them first exhaust the iterator and then perform some kind of post-processing: i.e. merging spills, sorting, writing partitions files and then concatenating them into a single file... bottom line the Iterator may actually be 'sitting' for some time after being exhausted.

 



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