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https://issues.apache.org/jira/browse/MAPREDUCE-4228?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13269671#comment-13269671
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alex gemini commented on MAPREDUCE-4228:
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this is mainly for small jobs,isn't it?the time frame between started map and
completed map job will affect the small job's latency,if that's the case,why
not just remove this parameter,what's the difference if we set it to 0.33 and
1.0?we didn't prevent other reduce to locate,isn't it?
> mapreduce.job.reduce.slowstart.completedmaps is not working properly to delay
> the scheduling of the reduce tasks
> ----------------------------------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-4228
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-4228
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: applicationmaster, mrv2
> Affects Versions: 0.23.1
> Reporter: Jason Lowe
> Assignee: Jason Lowe
>
> If no more map tasks need to be scheduled but not all have completed, the
> ApplicationMaster will start scheduling reducers even if the number of
> completed maps has not met the mapreduce.job.reduce.slowstart.completedmaps
> threshold. For example, if the property is set to 1.0 all maps should
> complete before any reducers are scheduled. However the reducers are
> scheduled as soon as the last map task is assigned to a container. For a job
> with very long-running maps, a cluster with enough capacity to launch all map
> tasks could cause reducers to launch prematurely and waste cluster resources.
> Thanks to Phil Su for discovering this issue.
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