[
https://issues.apache.org/jira/browse/MAPREDUCE-2162?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12925692#action_12925692
]
Matei Zaharia commented on MAPREDUCE-2162:
------------------------------------------
This is interesting. The original LATE algorithm actually used a percentile
(for example, the 25th percentile) instead of a standard deviation to determine
tasks that are "slow enough". That guarantees that there are always tasks that
are considered "slow enough". Maybe we should go back to that. I believe that
Arun and Andy changed it to the current approach in HADOOP-2141 because they
thought that the stddev would be more robust and they were afraid that
computing the 25th percentile takes time. However, it doesn't take that long
and it only needs to be computed periodically.
> 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
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
