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https://issues.apache.org/jira/browse/MAPREDUCE-4358?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13398757#comment-13398757
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Ahmed Radwan commented on MAPREDUCE-4358:
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@Robert, I think mapreduce.job.reduce.slowstart.completedmaps is related but
different. The issue here is not to wait for a % of mappers to totally complete
before start allocating containers to reducers, but the issue is to prevent
reducers from occupying containers while these containers are still needed by
mappers.
@Jason, watching headroom and preempting reducers should be sufficient to
address this issue, but this doesn't seem to work in our case. It is using Fifo.
@Harsh, MAPREDUCE-4228 seems to address a bug with the behavior of
mapreduce.job.reduce.slowstart.completedmaps, which as I mentioned above is
different.
@Sharad, Yes we are seeing this in a customer cluster.
> 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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