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https://issues.apache.org/jira/browse/MAPREDUCE-4358?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13397825#comment-13397825
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Robert Joseph Evans commented on MAPREDUCE-4358:
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Isn't that what mapreduce.job.reduce.slowstart.completedmaps is for? If you
don't want any reducers to run until all of the maps have finished then you
want to set it to 1.0, not the 0.05 that is the default. I think part of the
issue is that the default value for
mapreduce.job.reduce.slowstart.completedmaps is still set for when map and
reduce slots were completely separate. Perhaps this config does not make since
any more now that reduce tasks can block map tasks from running. Or perhaps we
need another config so that the AM will not fill more than X% of the queue with
reduces until all map tasks have completed.
We have taken the rout of setting the slowstart to 1.0 even on our 1.0.2
clusters because it improves the cluster utilization and we have not seen much
of a hit to the end to end time of our jobs.
> Reducers are assigned containers before all maps are assigned containers
> ------------------------------------------------------------------------
>
> Key: MAPREDUCE-4358
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-4358
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mrv2
> Affects Versions: 2.0.0-alpha
> Reporter: Ahmed Radwan
>
> Reducers start to get containers before all maps are. We have seen this issue
> and it is problematic since if there is no avaialable resources for the
> remaining maps, the job will just stall where reducers are waiting for
> mappers which are unable to start because there is no containers available.
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