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https://issues.apache.org/jira/browse/MAPREDUCE-4505?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13426827#comment-13426827
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Karthik Kambatla commented on MAPREDUCE-4505:
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Given the overheads involved, it might not be a bad idea to skip the combiner
for more than just a single value. The threshold for running the combiner can
be number of values or a cumulative size of the values.
> Create a combiner bypass path for keys with a single value
> ----------------------------------------------------------
>
> Key: MAPREDUCE-4505
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-4505
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: performance, task
> Reporter: Owen O'Malley
> Assignee: Arun C Murthy
>
> It would help optimize a lot of cases where there aren't a lot of replicated
> keys if the framework would bypass the deserialize/combiner/serialize step
> for keys that only have a single value.
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