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https://issues.apache.org/jira/browse/MAPREDUCE-1838?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12874862#action_12874862
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dhruba borthakur commented on MAPREDUCE-1838:
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The random shuffle will work only if speculative-execution is switched on for
this job. Otherwise there is a non-trivial probability that one long mapper
will delay the job completion to a large extent.
> DistRaid map tasks have large variance in running times
> -------------------------------------------------------
>
> Key: MAPREDUCE-1838
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-1838
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: contrib/raid
> Affects Versions: 0.20.1
> Reporter: Ramkumar Vadali
> Priority: Minor
>
> HDFS RAID uses map-reduce jobs to generate parity files for a set of source
> files. Each map task gets a subset of files to operate on. The current code
> assigns files by walking through the list of files given in the constructor
> of DistRaid
> The problem is that the list of files given to the constructor has the order
> of (pretty much) the directory listing. When a large number of files is
> added, files in that order tend to have the same size. Thus a map task can
> end up with large files where as another can end up with small files,
> increasing the variance in run times.
> We could do smarter assignment by using the file sizes.
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