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https://issues.apache.org/jira/browse/MAPREDUCE-1342?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12796092#action_12796092
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Amar Kamat commented on MAPREDUCE-1342:
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Simply making _potentiallyFaultyTrackers_ a concurrent HashMap and removing the
*synchronized* keyword might introduce more issues. I think the reason for
synchronizing on _potentiallyFaultyTrackers_ was to perform some operations in
an atomic manner. Have you checked if the semantics remain same after removing
the synchronized keyword? I think making _potentiallyFaultyTrackers_ as
concurrent HashMap is better but might be dangerous.
One other way to avoid the deadlock would be by marking few non-private apis in
JobTracker.FaultyTrackerInfo as synchronized. Mainly
{code}
JobTracker.FaultyTrackerInfo.incrementFaults // called via Heartbeat and
testcases
JobTracker.FaultyTrackerInfo.markTrackerHealthy // called via Heartbeat
JobTracker.FaultyTrackerInfo.shouldAssignTasksToTracker // called via Heartbeat
and testcases
JobTracker.FaultyTrackerInfo.isBlacklisted // called in multiple cases .. need
to check
JobTracker.FaultyTrackerInfo.getFaultCount // called via Heartbeat and testcases
JobTracker.FaultyTrackerInfo.getReasonForBlackListing // never used!
JobTracker.FaultyTrackerInfo.setNodeHealthStatus // called via Heartbeat and
testcases
{code}
So except JobTracker.FaultyTrackerInfo.isBlacklisted(), all the calls are
centrally locked on JobTracker. Hence adding the synchronized keyword in the
method signature wouldnt introduce any overhead. Need to check on
JobTracker.FaultyTrackerInfo.isBlacklisted().
> Potential JT deadlock in faulty TT tracking
> -------------------------------------------
>
> Key: MAPREDUCE-1342
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-1342
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: jobtracker
> Affects Versions: 0.22.0
> Reporter: Todd Lipcon
> Attachments: cycle0.png, mapreduce-1342-1.patch
>
>
> JT$FaultyTrackersInfo.incrementFaults first locks potentiallyFaultyTrackers,
> and then calls blackListTracker, which calls removeHostCapacity, which locks
> JT.taskTrackers
> On the other hand, JT.blacklistedTaskTrackers() locks taskTrackers, then
> calls faultyTrackers.isBlacklisted() which goes on to lock
> potentiallyFaultyTrackers.
> I haven't produced such a deadlock, but the lock ordering here is inverted
> and therefore could deadlock.
> Not sure if this goes back to 0.21 or just in trunk.
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