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

["Hyukjin Kwon (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SPARK-25103?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Hyukjin Kwon resolved SPARK-25103.
----------------------------------
    Resolution: Incomplete

> 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: 2.0.1, 2.1.0, 2.2.0, 2.3.0
>            Reporter: Eyal Farago
>            Priority: Major
>              Labels: bulk-closed
>
> 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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