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https://issues.apache.org/jira/browse/HADOOP-1338?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12649282#action_12649282
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Matei Zaharia commented on HADOOP-1338:
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Ah that makes sense. Perhaps the max data fetched at a time should be
smaller (1-10 MB), but I think there are important jobs with less than 1 KB
of output per map, such as counting jobs or filtering jobs. Definitely for
jobs with a lot of data, it doesn't make sense to degrade the performance of
the shuffle.
> 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 Core
> Issue Type: Improvement
> Components: mapred
> Reporter: Devaraj Das
>
> 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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