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https://issues.apache.org/jira/browse/MAPREDUCE-5110?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13616779#comment-13616779
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Karthik Kambatla commented on MAPREDUCE-5110:
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If we want to strictly guarantee serial execution of task attempts (say, when
speculative execution is turned off), we want to kill the task first before
re-scheduling on another node. If we treat these as decoupled events, the JT
will schedule another attempt. Meanwhile, before the TT kills it, the task
might make some progress violating the above guarantee.
> Long task launch delays can lead to multiple parallel attempts of the task
> --------------------------------------------------------------------------
>
> Key: MAPREDUCE-5110
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-5110
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: tasktracker
> Affects Versions: 1.1.2
> Reporter: Karthik Kambatla
> Assignee: Karthik Kambatla
> Attachments: expose-mr-5110.patch, mr-5110.patch, mr-5110.patch
>
>
> If a task takes too long to launch, the JT expires the task and schedules
> another attempt. The earlier attempt can start after the later attempt
> leading to two parallel attempts running at the same time. This is
> particularly an issue if the user turns off speculation and expects a single
> attempt of a task to run at any point in time.
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