lag time for reduce speculation should start from completion of first mapper
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Key: MAPREDUCE-2133
URL: https://issues.apache.org/jira/browse/MAPREDUCE-2133
Project: Hadoop Map/Reduce
Issue Type: Bug
Reporter: Joydeep Sen Sarma
Priority: Minor
reducers don't make any progress until first mapper finishes. if the first
mapper finishes after the reduce speculation lag time - a bunch of (waiting)
reducers may make some (non-zero) progress while others stay at 0 progress.
this causes a few reducers to be speculated (unnecessarily) at this stage in
the job.
it seems that we should not start counting towards reduce stage lag time until
at least one mapper is done.
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