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From "Aaron Kimball (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-5985) A single slow (but not dead) map TaskTracker impedes MapReduce progress
Date Sat, 06 Jun 2009 00:26:07 GMT
A single slow (but not dead) map TaskTracker impedes MapReduce progress
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                 Key: HADOOP-5985
                 URL: https://issues.apache.org/jira/browse/HADOOP-5985
             Project: Hadoop Core
          Issue Type: Bug
            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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