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https://issues.apache.org/jira/browse/HADOOP-3130?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12584213#action_12584213
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Devaraj Das commented on HADOOP-3130:
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bq. At least, we have to differentiate between the case of failing to fetch one
map output numerous times and the case of failing to
fetch a lot of different map outputs. In the first case, it is better to
re-execute the map.
In the second case, maybe it makes sense to consider to fail the reducer.
That's how the model works in case of maps - if too many reducers complain
about fetch failure for a particular map, the map is killed. The change here
should be to also consider the host the map ran on otherwise we run into issues
like HADOOP-2175
(https://issues.apache.org/jira/browse/HADOOP-2175?focusedCommentId=12582425#action_12582425).
The problem in the reducers case is this that the numbers are hardcoded and
the decision to kill is totally local. So if a reducer fails to fetch 5 unique
map outputs it kills itself. This should be augmented with your suggestion on
accounting for the shuffle progress.
> Shuffling takes too long to get the last map output.
> ----------------------------------------------------
>
> Key: HADOOP-3130
> URL: https://issues.apache.org/jira/browse/HADOOP-3130
> Project: Hadoop Core
> Issue Type: Bug
> Reporter: Runping Qi
> Attachments: HADOOP-3130.patch, shuffling.log
>
>
> I noticed that towards the end of shufflling, the map output fetcher of the
> reducer backs off too aggressively.
> I attach a fraction of one reduce log of my job.
> Noticed that the last map output was not fetched in 2 minutes.
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