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https://issues.apache.org/jira/browse/HADOOP-2399?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12605167#action_12605167
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Owen O'Malley commented on HADOOP-2399:
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This Jira was marked as an incompatible change because it did change the
semantics. However, without this change there was an allocation (and later
garbage collection) for every key and value passed to the reduce, which had
measurable performance costs.
> Input key and value to combiner and reducer should be reused
> ------------------------------------------------------------
>
> Key: HADOOP-2399
> URL: https://issues.apache.org/jira/browse/HADOOP-2399
> Project: Hadoop Core
> Issue Type: Bug
> Components: mapred
> Affects Versions: 0.15.1
> Reporter: Owen O'Malley
> Assignee: Owen O'Malley
> Fix For: 0.17.0
>
> Attachments: 2399-3.patch, 2399-4.patch
>
>
> Currently, the input key and value are recreated on every iteration for input
> to the combiner and reducer. It would speed up the system substantially if we
> reused the keys and values. The down side of doing it, is that it may break
> applications that count on holding references to previous keys and values,
> but I think it is worth doing.
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