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https://issues.apache.org/jira/browse/MAPREDUCE-2162?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12965624#action_12965624
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Joydeep Sen Sarma commented on MAPREDUCE-2162:
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spent a lot of time coding and thinking about this. i am more to make a simple
change to cap the standardDeviation at some maximum value (say Mean/3).
i did a detailed analysis that seems to suggest that doing so would be roughly
equivalent to the scheme discussed above. we already have the notion of a
'speculative cap' - putting a speculative cap of 10% of the currently running
tasks would be roughly equivalent of speculating the bottom 10%. (The
LateComparator currently sorts speculatable tasks by remaining time (instead of
progress rate). if it were to sort based on progress rate - it would be very
similar to speculating the bottom 10%)
the conditions discussed here (runningTime >= mean/2 and remainingTime<mean)
are roughly equal to the current code (modulo the LateComparator) if stddev is
capped at mean/3 (it's a somewhat long deduction).
> 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
> Assignee: 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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