Jason Lowe created MAPREDUCE-4228:
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Summary: mapreduce.job.reduce.slowstart.completedmaps is not
working properly to delay the scheduling of the reduce tasks
Key: MAPREDUCE-4228
URL: https://issues.apache.org/jira/browse/MAPREDUCE-4228
Project: Hadoop Map/Reduce
Issue Type: Bug
Components: applicationmaster, mrv2
Affects Versions: 0.23.1
Reporter: Jason Lowe
Assignee: Jason Lowe
If no more map tasks need to be scheduled but not all have completed, the
ApplicationMaster will start scheduling reducers even if the number of
completed maps has not met the mapreduce.job.reduce.slowstart.completedmaps
threshold. For example, if the property is set to 1.0 all maps should complete
before any reducers are scheduled. However the reducers are scheduled as soon
as the last map task is assigned to a container. For a job with very
long-running maps, a cluster with enough capacity to launch all map tasks could
cause reducers to launch prematurely and waste cluster resources.
Thanks to Phil Su for discovering this issue.
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