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From "Devaraj Das (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-1043) Optimize the shuffle phase (increase the parallelism)
Date Tue, 27 Feb 2007 05:39:05 GMT
Optimize the shuffle phase (increase the parallelism)
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                 Key: HADOOP-1043
                 URL: https://issues.apache.org/jira/browse/HADOOP-1043
             Project: Hadoop
          Issue Type: Improvement
          Components: mapred
            Reporter: Devaraj Das
         Assigned To: Devaraj Das


In the current shuffle code, only one map output location node is accessed from any Reduce
at any given point of time. For example, if a particular node, say machine1.foo.com ran 300
maps, the reducer would fetch just one output from there at a time. machine1.foo.com will
be inserted into a Set datastructure (uniqueHosts) and until it gets removed from there, no
other map output will be fetched from that machine. The fact that only one map output is fetched
at a time from any particular host seems fine, but the logic for removing a node from uniqueHosts
is such that there could be a lot of delay before a node gets deleted from the Set datastructure
(even after the map output has been fetched from that node). This probably leads to suboptimal
performance since it reduces the parallelism in fetching.

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