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From "Todd Lipcon (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MAPREDUCE-3278) 0.20: avoid a busy-loop in ReduceTask scheduling
Date Thu, 27 Oct 2011 04:43:32 GMT
0.20: avoid a busy-loop in ReduceTask scheduling

                 Key: MAPREDUCE-3278
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3278
             Project: Hadoop Map/Reduce
          Issue Type: Improvement
          Components: mrv1, performance, task
    Affects Versions:
            Reporter: Todd Lipcon
            Assignee: Todd Lipcon

Looking at profiling results, it became clear that the ReduceTask has the following busy-loop
which was causing it to suck up 100% of CPU in the fetch phase in some configurations:
- the number of reduce fetcher threads is configured to more than the number of hosts
- therefore "busyEnough()" never returns true
- the "scheduling" portion of the code can't schedule any new fetches, since all of the pending
fetches in the mapLocations buffer correspond to hosts that are already being fetched (the
hosts are in the {{uniqueHosts}} map)
- {{getCopyResult()}} immediately returns null, since there are no completed maps.
Hence ReduceTask spins back and forth between trying to schedule things (and failing), and
trying to grab completed results (of which there are none), with no waits.

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