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https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12617773#action_12617773
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Alejandro Abdelnur commented on HADOOP-3702:
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On the first bullet item, Finally got the {{Stringifier}} thing, Thanks.
On the second bullet item, the serialized chain element {{JobConf}} (A) is
deserialized, a new {{JobConf}} (B) with the task's {{JobConf}} as parent is
created (to ensure all Hadoop runtime values are avail), the values of A are
applied to B to ensure the config values of the chain element have precedence
when the chain mapper/reducer gets its conf, B.
On the third bullet item, Yes, you are right. 2 reasons, one is to keep code
simpler, the second is that if a MultithreadedMapRunner is used a single
pipeline will not work as it is not thread safe.
On the fourth bullet item, this will go away as I'm using {{Stringifier}} in
the next version of patch.
> add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
> -------------------------------------------------------------------------
>
> Key: HADOOP-3702
> URL: https://issues.apache.org/jira/browse/HADOOP-3702
> Project: Hadoop Core
> Issue Type: New Feature
> Components: mapred
> Environment: all
> Reporter: Alejandro Abdelnur
> Assignee: Alejandro Abdelnur
> Priority: Minor
> Attachments: patch3702.txt, patch3702.txt, patch3702.txt,
> patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt,
> patch3702.txt
>
>
> On the same input, we usually need to run multiple Maps one after the other
> without no Reduce. We also have to run multiple Maps after the Reduce.
> If all pre-Reduce Maps are chained together and run as a single Map a
> significant amount of Disk I/O will be avoided.
> Similarly all post-Reduce Maps can be chained together and run in the Reduce
> phase after the Reduce.
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