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https://issues.apache.org/jira/browse/HADOOP-4666?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12650249#action_12650249
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Matei Zaharia commented on HADOOP-4666:
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Thanks Johan for the quick commit.
> Launch reduces only after a few maps have run in the Fair Scheduler
> -------------------------------------------------------------------
>
> Key: HADOOP-4666
> URL: https://issues.apache.org/jira/browse/HADOOP-4666
> Project: Hadoop Core
> Issue Type: Improvement
> Components: contrib/fair-share
> Reporter: Matei Zaharia
> Assignee: Matei Zaharia
> Fix For: 0.20.0
>
> Attachments: fairscheduler-reduce-launch-condition.patch
>
>
> It makes no sense to schedule reduces for a job before its maps have started
> running. As an initial fix, we will wait until a certain percent have run
> (likely 10%). In the future it would be good to choose the time to wait based
> on amount of map output data as well - launching reducers that will mostly be
> idle is not helpful. Average amount of map output bytes per mapper is easy to
> compute using counters in JobInProgress.
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