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https://issues.apache.org/jira/browse/YARN-8436?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16548057#comment-16548057
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Wilfred Spiegelenburg commented on YARN-8436:
---------------------------------------------
1) fixed
2) The delay is not to make sure that the comparator is called at least once
but to make sure that the sorting has started. The failure will only occur if
and when the sorting is progressed far enough that there is merging and or
inserting elements in a sorted run. The sleep is thus not for a synchronisation
but really a delay for the modifications. The countdown latch would synchronise
the start but that is not what I needed.
Uploading a new patch with the fixed comment
> FSParentQueue: Comparison method violates its general contract
> --------------------------------------------------------------
>
> Key: YARN-8436
> URL: https://issues.apache.org/jira/browse/YARN-8436
> Project: Hadoop YARN
> Issue Type: Bug
> Components: fairscheduler
> Affects Versions: 3.1.0
> Reporter: Wilfred Spiegelenburg
> Assignee: Wilfred Spiegelenburg
> Priority: Minor
> Attachments: YARN-8436.001.patch, YARN-8436.002.patch
>
>
> The ResourceManager can fail while sorting queues if an update comes in:
> {code:java}
> FATAL org.apache.hadoop.yarn.server.resourcemanager.ResourceManager: Error in
> handling event type NODE_UPDATE to the scheduler
> java.lang.IllegalArgumentException: Comparison method violates its general
> contract!
> at java.util.TimSort.mergeLo(TimSort.java:777)
> at java.util.TimSort.mergeAt(TimSort.java:514)
> ...
> at java.util.Collections.sort(Collections.java:175)
> at
> org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FSParentQueue.assignContainer(FSParentQueue.java:223){code}
> The reason it breaks is a change in the sorted object itself.
> This is why it fails:
> * an update from a node comes in as a heartbeat.
> * the update triggers a check to see if we can assign a container on the
> node.
> * walk over the queue hierarchy to find a queue to assign a container to:
> top down.
> * for each parent queue we sort the child queues in {{assignContainer}} to
> decide which queue to descent into.
> * we lock the parent queue when sort to prevent changes, but we do not lock
> the child queues that we are sorting.
> If during this sorting a different node update changes a child queue then we
> allow that. This means that the objects that we are trying to sort now might
> be out of order. That causes the issue with the comparator. The comparator
> itself is not broken.
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