[jira] [Commented] (MAPREDUCE-5817) mappers get rescheduled on node transition even after all reducers are completed

["Sangjin Lee (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MAPREDUCE-5817?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13955750#comment-13955750 ] 

Sangjin Lee commented on MAPREDUCE-5817:
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We're talking about two options for this: (1) modify JobImpl.actOnUnusableNode() so that if all reducers are completed do not reschedule mappers, and (2) modify checkReadyForCommit() so that it transitions to COMMITTING if all reducers are completed (if reducers exist) instead of checking all tasks are completed.

Either approach seems to have some downsides.

For (1), the change is pretty narrow (only affects the rescheduling scenario). However, it still lets the mapper tasks that were rescheduled prior to reducer completion run. So the job may linger until those mapper tasks run to completion. And if those mapper tasks fail for any reason, it may render the job as failed (even though all reducers may have succeeded in reality).

For (2), it would be effective and would make the job finish much more quickly. On the other hand, we'd need to do something about the mapper tasks that are running at that point. They may need to be killed. Also, if the original mapper tasks were successful, we may need to "resurrect" their status from KILLED to SUCCESSFUL to avoid confusion.



> mappers get rescheduled on node transition even after all reducers are completed
> --------------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-5817
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5817
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: applicationmaster
>    Affects Versions: 2.3.0
>            Reporter: Sangjin Lee
>
> We're seeing a behavior where a job runs long after all reducers were already finished. We found that the job was rescheduling and running a number of mappers beyond the point of reducer completion. In one situation, the job ran for some 9 more hours after all reducers completed!
> This happens because whenever a node transition (to an unusable state) comes into the app master, it just reschedules all mappers that already ran on the node in all cases.
> Therefore, if any node transition has a potential to extend the job period. Once this window opens, another node transition can prolong it, and this can happen indefinitely in theory.
> If there is some instability in the pool (unhealthy, etc.) for a duration, then any big job is severely vulnerable to this problem.
> If all reducers have been completed, JobImpl.actOnUnusableNode() should not reschedule mapper tasks. If all reducers are completed, the mapper outputs are no longer needed, and there is no need to reschedule mapper tasks as they would not be consumed anyway.



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