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From "Joydeep Sen Sarma (JIRA)" <j...@apache.org>
Subject [jira] Created: (MAPREDUCE-2062) speculative execution is too aggressive under certain conditions
Date Sat, 11 Sep 2010 01:21:33 GMT
speculative execution is too aggressive under certain conditions

                 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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