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From "Carlo Curino (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-4585) Checkpoint shuffle aggregation as map output
Date Mon, 27 Aug 2012 19:20:08 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-4585?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13442638#comment-13442638
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Carlo Curino commented on MAPREDUCE-4585:
-----------------------------------------

The image posted shows an example of a run where checkpointing-restart helps "hiding" part
of the shuffling costs, by allowing (two in this example) reducers to take turns over the
only available slot, and thus getting to the point in which all maps have completed with most
of the shuffling done. The basic behavior would have the reducer one sitting on the slot until
all maps complete, and thus reduce 2 paying the full cost of shuffling "after" map5 completed
and a new slot is available. We believe this should help for non-ideal schedules (multiple
waves of maps/reduces which are not perfectly aligned) and to handle skew.
 
                
> Checkpoint shuffle aggregation as map output
> --------------------------------------------
>
>                 Key: MAPREDUCE-4585
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-4585
>             Project: Hadoop Map/Reduce
>          Issue Type: New Feature
>          Components: task
>            Reporter: Chris Douglas
>         Attachments: shufflecheckpoint.pdf
>
>
> Map output collected during the shuffle can be spilled and written as a composite of
map outputs. Particularly if the job employs a combiner, this checkpoint can provide fault
tolerance and improve job throughput by aggregating intermediate output. The latter is especially
helpful for jobs with multiple waves of reduces.

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