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https://issues.apache.org/jira/browse/MAPREDUCE-2162?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12925646#action_12925646
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Scott Chen commented on MAPREDUCE-2162:
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For example, there are three finished task with progress rate (1, 2, 30)
mean = 11
std = 13.4
std > mean
The fourth task will never get speculation even if it stuck forever.
> speculative execution does not handle cases where stddev > mean well
> --------------------------------------------------------------------
>
> Key: MAPREDUCE-2162
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-2162
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Reporter: Joydeep Sen Sarma
>
> the new speculation code only speculates tasks whose progress rate deviates
> from the mean progress rate of a job by more than some multiple (typically
> 1.0) of stddev. stddev can be larger than mean. which means that if we ever
> get into a situation where this condition holds true - then a task with even
> 0 progress rate will not be speculated.
> it's not clear that this condition is self-correcting. if a job has thousands
> of tasks - then one laggard task, inspite of not being speculated for a long
> time, may not be able to fix the condition of stddev > mean.
> we have seen jobs where tasks have not been speculated for hours and this
> seems one explanation why this may have happened. here's an example job with
> stddev > mean:
> DataStatistics: count is 6, sum is 1.7141054797775723E-8, sumSquares is
> 2.9381575958035014E-16 mean is 2.8568424662959537E-9 std() is
> 6.388093955645905E-9
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