[jira] Resolved: (HADOOP-4360) Reducers hang in SHUFFLING phase due to duplicate completed tasks in TaskTracker.FetchStatus.allMapEvents

["Zheng Shao (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HADOOP-4360?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Zheng Shao resolved HADOOP-4360.
--------------------------------

    Resolution: Duplicate

Duplicate of http://issues.apache.org/jira/browse/HADOOP-3155

> Reducers hang in SHUFFLING phase due to duplicate completed tasks in TaskTracker.FetchStatus.allMapEvents
> ---------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-4360
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4360
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.17.2
>            Reporter: Zheng Shao
>
> On our cluster we have seen JobTracker went to a weird state that a lot of TaskTrackers are getting duplicate entries in TaskTracker.FetchStatus.allMapEvents.
> Since TaskTracker fetches new completed map tasks using the size of the allMapEvents as starting index, this prohibits the tasktracker from getting all completed map tasks. And as a result, reducer just hangs in the shuffling status.
> The problem does not get fixed by killing and restarting TaskTracker, and when it happens a lot of TaskTrackers will show the same problem.
> It seems some problems happen to the communication between JobTracker and TaskTracker.
> An easy preventive fix will be to include the starting index of the list of completed map tasks from JobTracker to TaskTracker, so that TaskTracker can just throw away the data if the starting index does not match the current size of the array.

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