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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Updated: (MAPREDUCE-969) NullPointerException during reduce freezes job
Date Thu, 10 Sep 2009 23:57:59 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-969?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Todd Lipcon updated MAPREDUCE-969:

    Attachment: reduce_task_logs

Attaching sanitized logs from this incident.

The event that seems to be a red flag is the lost task tracker xx05.

The null pointer exception is caused by u.getHost() being null - this URI is the taskTrackerHttpAddress
in a TaskTrackerStatus. The job event output doesn't show any with a malformed URL, so I suspect
some kind of race.

Aside from this issue, I find it odd that GetMapEventsThread ignores exceptions. In cases
like this it will cause the ReduceTask to spin forever while still reporting progress until
the user intervenes.

> NullPointerException during reduce freezes job
> ----------------------------------------------
>                 Key: MAPREDUCE-969
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-969
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: jobtracker, task, tasktracker
>    Affects Versions: 0.20.2
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: bad_job_events, bad_job_jt_logs, reduce_task_logs
> We experienced several jobs stuck in Reduce on a cluster. All of the stuck reduce tasks
had a similar were stuck at "Need another 2 map output(s) where 0 is already in progress"
despite all of the mappers having completed, and 0 scheduled. The stuck reducers had experienced
the following exception early in the shuffle:
> java.lang.NullPointerException
> 	at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:768)
> 	at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$GetMapEventsThread.getMapCompletionEvents(ReduceTask.java:2747)
> 	at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$GetMapEventsThread.run(ReduceTask.java:2670)
> Will attach more information and logs momentarily.

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