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From "Arun C Murthy (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5985) A single slow (but not dead) map TaskTracker impedes MapReduce progress
Date Mon, 22 Jun 2009 02:50:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12722454#action_12722454
] 

Arun C Murthy commented on HADOOP-5985:
---------------------------------------

Sorry for jumping in late, I somehow missed this one.

I thought I'd point out that currently reduces _do_ indeed provide feedback regarding 'failed'
shuffles to the JobTracker, which on receipt of sufficient number of complaints from reduces
does re-execute the map elsewhere.


> A single slow (but not dead) map TaskTracker impedes MapReduce progress
> -----------------------------------------------------------------------
>
>                 Key: HADOOP-5985
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5985
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.18.3
>            Reporter: Aaron Kimball
>
> We see cases where there may be a large number of mapper nodes running many tasks (e.g.,
a thousand). The reducers will pull 980 of the map task intermediate files down, but will
be unable to retrieve the final intermediate shards from the last node. The TaskTracker on
that node returns data to reducers either slowly or not at all, but its heartbeat messages
make it back to the JobTracker -- so the JobTracker doesn't mark the tasks as failed. Manually
stopping the offending TaskTracker works to migrate the tasks to other nodes, where the shuffling
process finishes very quickly. Left on its own, it can take hours to unjam itself otherwise.
> We need a mechanism for reducers to provide feedback to the JobTracker that one of the
mapper nodes should be regarded as lost.

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