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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-1838) DistRaid map tasks have large variance in running times
Date Fri, 04 Jun 2010 18:46:57 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1838?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12875705#action_12875705
] 

dhruba borthakur commented on MAPREDUCE-1838:
---------------------------------------------

I agree, speculation actually does not make this go any faster!

> 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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