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From "Matei Zaharia (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SPARK-3032) Potential bug when running sort-based shuffle with sorting using TimSort
Date Tue, 23 Sep 2014 03:00:35 GMT

    [ https://issues.apache.org/jira/browse/SPARK-3032?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14144243#comment-14144243
] 

Matei Zaharia commented on SPARK-3032:
--------------------------------------

I'm not completely sure that this is because hashCode provides a partial ordering, because
I believe TimSort is supposed to work on partial orderings as well. I believe the problem
is an integer over flow when we subtract key1.hashCode - key2.hashCode. Can you try replacing
the line that returns h1 - h2 in keyComparator with returning Integer.compare(h1, h2)? This
will properly deal with overflow.

Returning h1 - h2 is definitely wrong: for example suppose that h1 = Int.MaxValue and h2 =
Int.MinValue, then h1 - h2 = -1.

Please add a unit test for this case as well.

> Potential bug when running sort-based shuffle with sorting using TimSort
> ------------------------------------------------------------------------
>
>                 Key: SPARK-3032
>                 URL: https://issues.apache.org/jira/browse/SPARK-3032
>             Project: Spark
>          Issue Type: Bug
>          Components: Shuffle
>    Affects Versions: 1.1.0
>            Reporter: Saisai Shao
>            Priority: Critical
>
> When using SparkPerf's aggregate-by-key workload to test sort-based shuffle, data type
for key and value is (String, String), always meet this issue:
> {noformat}
> java.lang.IllegalArgumentException: Comparison method violates its general contract!
>         at org.apache.spark.util.collection.Sorter$SortState.mergeLo(Sorter.java:755)
>         at org.apache.spark.util.collection.Sorter$SortState.mergeAt(Sorter.java:493)
>         at org.apache.spark.util.collection.Sorter$SortState.mergeCollapse(Sorter.java:420)
>         at org.apache.spark.util.collection.Sorter$SortState.access$200(Sorter.java:294)
>         at org.apache.spark.util.collection.Sorter.sort(Sorter.java:128)
>         at org.apache.spark.util.collection.SizeTrackingPairBuffer.destructiveSortedIterator(SizeTrackingPairBuffer.scala:83)
>         at org.apache.spark.util.collection.ExternalSorter.spillToMergeableFile(ExternalSorter.scala:323)
>         at org.apache.spark.util.collection.ExternalSorter.spill(ExternalSorter.scala:271)
>         at org.apache.spark.util.collection.ExternalSorter.maybeSpill(ExternalSorter.scala:249)
>         at org.apache.spark.util.collection.ExternalSorter.insertAll(ExternalSorter.scala:220)
>         at org.apache.spark.shuffle.sort.SortShuffleWriter.write(SortShuffleWriter.scala:85)
>         at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:68)
>         at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:41)
>         at org.apache.spark.scheduler.Task.run(Task.scala:54)
>         at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:199)
>         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
>         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
>         at java.lang.Thread.run(Thread.java:722)
> {noformat}
> Seems the current partitionKeyComparator which use hashcode of String as key comparator
break some sorting contracts. 
> Also I tested using data type Int as key, this is OK to pass the test, since hashcode
of Int is its self. So I think potentially partitionDiff + hashcode of String may break the
sorting contracts.



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