Reduce often attempts in memory merge with no work
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Key: HADOOP-3940
URL: https://issues.apache.org/jira/browse/HADOOP-3940
Project: Hadoop Core
Issue Type: Bug
Components: mapred
Reporter: Chris Douglas
Priority: Minor
ReduceTask.ReduceCopier.ShuffleRamManager initializes numRequiredMapOutputs to
0, so one of the predicates in ShuffleRamManager::waitForDataToMerge,
{{numPendingRequests < numRequiredMapOutputs}}, is false until the first map
output is fetched and false again after the last map output is fetched. The
InMemFSMergeThread thread will loop busily in this state.
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