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https://issues.apache.org/jira/browse/MAPREDUCE-2062?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Joydeep Sen Sarma updated MAPREDUCE-2062:
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Environment: hadoop-20 with HADOOP-2141
> 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
> Environment: hadoop-20 with HADOOP-2141
> 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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