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https://issues.apache.org/jira/browse/MAPREDUCE-1797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12869007#action_12869007
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Arun C Murthy commented on MAPREDUCE-1797:
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A read-write lock in the JobTracker doesn't really make much of a difference,
because the vast majority of the operations on a JobInProgress (mostly during
heartbeat processing) will need the write lock.
OTOH, the scenario you describe with a client calling getCounter() is already
fixed with MAPREDUCE-1354.
> The JobTracker lock can be a reader/writer lock
> -----------------------------------------------
>
> Key: MAPREDUCE-1797
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-1797
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: jobtracker
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> The Jobtracker has a global lock and a per-job JobInProgress lock. The aim
> for the JobInprogress lock is to support the ability to lock a single job's
> metadata without blocking out the entire JobTracker. However, many code
> paths acquire the JobTracker lock and then acquire the JobInProgress lock
> while keeping the JobTracker lock. This somewhat defeats the benefit of
> having a per-job lock.
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