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https://issues.apache.org/jira/browse/HADOOP-4683?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12650475#action_12650475
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Jothi Padmanabhan commented on HADOOP-4683:
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Discussed offline with Owen. While the sleep itself is one part of the problem,
the more compelling case for having a separate thread would be the case where
the JobTracker is slow and the shuffle scheduler is stuck because the actual
call to getMapCompletionEvents is taking a long time to return.
> Move the call to getMapCompletionEvents in
> ReduceTask.ReduceCopier.fetchOutputs to a separate thread
> ----------------------------------------------------------------------------------------------------
>
> Key: HADOOP-4683
> URL: https://issues.apache.org/jira/browse/HADOOP-4683
> Project: Hadoop Core
> Issue Type: Bug
> Components: mapred
> Reporter: Devaraj Das
> Assignee: Jothi Padmanabhan
> Fix For: 0.20.0
>
>
> The method ReduceTask.ReduceCopier.fetchOutputs makes a call to
> getMapCompletionEvents every iteration of the loop. This should be moved out
> to a separate thread. This might slow down the shuffle scheduler in some
> cases since there is a sleep inside the getMapCompletionEvents method.
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