Rahul Jain created MAPREDUCE-4560:
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Summary: Job can get stuck in a deadlock between mappers and
reducers for low values of mapreduce.job.reduce.slowstart.completedmaps (<<1)
Key: MAPREDUCE-4560
URL: https://issues.apache.org/jira/browse/MAPREDUCE-4560
Project: Hadoop Map/Reduce
Issue Type: Bug
Reporter: Rahul Jain
Fix For: 2.0.0-alpha
This issue has been seen with MapReduceV2, never with MapReduceV1 in our lab
systems.
The parameter mapreduce.job.reduce.slowstart.completedmaps=0.05 (the default
value).
We found Application master stuck in a deadlock between mappers and reducers
with no progress in the job; the sequence appears to be:
1. Initial available map/reduce slots were allocated to mappers
2. Once mappers made progress and few of them completed, reducers started
occupying few of the slots due to low values of above config param.
3. The scheduler appears to not give priority to mappers over reducers; after a
while in our system we saw all slots occupied by reducers.
4. Since there were still mapper tasks not yet assigned any slot, the map phase
never completed.
5. The system entered a deadlock state where reducers occupy all available
slots, but are waiting for mappers to be complete; mappers cannot move forward
because of no slot available.
The workaround in our system was to set
mapreduce.job.reduce.slowstart.completedmaps=1 and the issue was no longer seen.
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