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https://issues.apache.org/jira/browse/MAPREDUCE-6957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16166894#comment-16166894
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Jooseong Kim commented on MAPREDUCE-6957:
-----------------------------------------
Thank you for the quick review!
> shuffle hangs after a node manager connection timeout
> -----------------------------------------------------
>
> Key: MAPREDUCE-6957
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-6957
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mrv2
> Reporter: Jooseong Kim
> Assignee: Jooseong Kim
> Fix For: 2.9.0, 3.0.0-beta1, 2.7.5, 2.8.3
>
> Attachments: MAPREDUCE-6957.001.patch, MAPREDUCE-6957.002.patch,
> MAPREDUCE-6957.003.patch
>
>
> After a connection failure from the reducer to the node manager, shuffles
> started to hang with the following message:
> {code}
> org.apache.hadoop.mapreduce.task.reduce.Fetcher: fetcher#1 - MergeManager
> returned status WAIT ...
> {code}
> There are two problems that leads to the hang.
> Problem 1.
> When a reducer has an issue connecting to the node manager, copyFromHost may
> call putBackKnownMapOutput on the same task attempt multiple times.
> There are two call sites of putBackKnownMapOutput in copyFromHost since
> MAPREDUCE-6303:
> 1. In the finally block of copyFromHost
> 2. In the catch block of openShuffleUrl.
> When openShuffleUrl fails to connect from the catch block in copyFromHost, it
> returns null.
> By the time openShuffleUrl returns null, putBackKnownMapOutput would have
> been called already for all remaining map outputs.
> However, the finally block calls putBackKnownMapOutput one more time on the
> map outputs.
> Problem 2. Problem 1 causes a leak in MergeManager.
> The problem occurs when multiple fetchers get the same set of map attempt
> outputs to fetch.
> Different fetchers reserves memory from MergeManager in Fetcher.copyMapOutput
> for the same map outputs.
> When the fetch succeeds, only the first map output gets committed through
> ShuffleSchedulerImpl.copySucceeded -> InMemoryMapOutput.commit, because
> commit() is gated by !finishedMaps[mapIndex].
> This may lead to a condition where usedMemory > memoryLimit, while
> commitMemory < mergeThreshold.
> This gets the MergeManager into a deadlock where a merge is never triggered
> while MergeManager cannot reserve additional space for map outputs.
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