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https://issues.apache.org/jira/browse/MAPREDUCE-3721?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13193432#comment-13193432
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Arun C Murthy commented on MAPREDUCE-3721:
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+1 lgtm, good catch!
> Race in shuffle can cause it to hang
> ------------------------------------
>
> Key: MAPREDUCE-3721
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-3721
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mrv2
> Affects Versions: 0.23.0
> Reporter: Siddharth Seth
> Assignee: Siddharth Seth
> Priority: Blocker
> Attachments: MR-3721.txt
>
>
> If all current {{Fetcher}}s complete while an in-memory merge is in progress
> - shuffle could hang.
> Specifically - if the memory freed by an in-memory merge does not bring
> {{MergeManager.usedMemory}} below {{MergeManager.memoryLimit}} and all
> current Fetchers complete before the in-memory merge completes, another
> in-memory merge will not be triggered - and shuffle will hang. (All new
> fetchers are asked to WAIT).
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