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https://issues.apache.org/jira/browse/MAPREDUCE-4813?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13504635#comment-13504635
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Jason Lowe commented on MAPREDUCE-4813:
---------------------------------------

MAPREDUCE-4815 only addresses FileOutputCommitter and friends, but the 
committer is arbitrary user code.  It could be doing all sorts of things 
including connecting to databases, etc.  So I still think we need this, 
although the priority of it is reduced given how many things are built from 
FileOutputCommitter.
                
> AM timing out during job commit
> -------------------------------
>
>                 Key: MAPREDUCE-4813
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-4813
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: applicationmaster
>    Affects Versions: 0.23.3, 2.0.1-alpha
>            Reporter: Jason Lowe
>            Assignee: Jason Lowe
>            Priority: Critical
>         Attachments: MAPREDUCE-4813.patch
>
>
> The AM calls the output committer's {{commitJob}} method synchronously during 
> JobImpl state transitions, which means the JobImpl write lock is held the 
> entire time the job is being committed.  Holding the write lock prevents the 
> RM allocator thread from heartbeating to the RM.  Therefore if committing the 
> job takes too long (e.g.: the job has tons of files to commit and/or the 
> namenode is bogged down) then the AM appears to be unresponsive to the RM and 
> the RM kills the AM attempt.

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