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https://issues.apache.org/jira/browse/MAPREDUCE-2162?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12927729#action_12927729
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Joydeep Sen Sarma commented on MAPREDUCE-2162:
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agree in general although i would need to collect some data to see how any
particular scheme would turn out. if we just shifted to percentiles, we could
do it in terms of progress rates as well.
> if we let each task run for some amount of time (say half the average task
> length)
seems like a good one to add to the list.
> 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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