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https://issues.apache.org/jira/browse/HADOOP-1338?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12494360
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Doug Cutting commented on HADOOP-1338:
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What's the motivation for more than one wave of reduces? Typically
applications are better served by fewer output files, so that can't be it. Is
it to decrease the granularity of each reduce, so that, in the case of reduce
failures, job latency is reduced?
> Improve the shuffle phase by using the "connection: keep-alive" and doing
> batch transfers of files
> --------------------------------------------------------------------------------------------------
>
> Key: HADOOP-1338
> URL: https://issues.apache.org/jira/browse/HADOOP-1338
> Project: Hadoop
> Issue Type: Improvement
> Components: mapred
> Reporter: Devaraj Das
> Fix For: 0.14.0
>
>
> We should do transfers of map outputs at the granularity of
> *total-bytes-transferred* rather than the current way of transferring a
> single file and then closing the connection to the server. A single
> TaskTracker might have a couple of map output files for a given reduce, and
> we should transfer multiple of them (upto a certain total size) in a single
> connection to the TaskTracker. Using HTTP-1.1's keep-alive connection would
> help since it would keep the connection open for more than one file transfer.
> We should limit the transfers to a certain size so that we don't hold up a
> jetty thread indefinitely (and cause timeouts for other clients).
> Overall, this should give us improved performance.
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