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Date: Fri, 10 Sep 2010 21:21:33 -0400 (EDT)
From: "Joydeep Sen Sarma (JIRA)" <jira@apache.org>
To: mapreduce-dev@hadoop.apache.org
Subject: [jira] Created: (MAPREDUCE-2062) speculative execution is too
 aggressive under certain conditions
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speculative execution is too aggressive under certain conditions
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                 Key: MAPREDUCE-2062
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2062
             Project: Hadoop Map/Reduce
          Issue Type: Bug
          Components: jobtracker
            Reporter: Joydeep Sen Sarma


The function canBeSpeculated has subtle bugs that cause too much speculation in certain cases.

- it compares the current progress of the task with the last observed mean of all the tasks. if only one task is in question - then the progress rate decays as time progresses (in the absence of updates) and std-dev is zero. So a job with a single reducer or mapper is almost always speculated.
- is only a single task has reported progress - then the stddev is zero. so other tasks may be speculated aggressively.
- several tasks take a while to report progress initially. they seem to get speculated as soon as speculative-lag is over. the lag should be configurable at the minimum.


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