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From "Kihwal Lee (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-5958) Wrong reduce task progress if map output is compressed
Date Thu, 06 Nov 2014 21:54:35 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-5958?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14201012#comment-14201012
] 

Kihwal Lee commented on MAPREDUCE-5958:
---------------------------------------

+1 the patch looks good. Thanks for adding the test case, Jason.

> Wrong reduce task progress if map output is compressed
> ------------------------------------------------------
>
>                 Key: MAPREDUCE-5958
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5958
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>    Affects Versions: 2.2.0, 2.3.0, 2.2.1, 2.4.0, 2.4.1
>            Reporter: Emilio Coppa
>            Assignee: Emilio Coppa
>            Priority: Minor
>              Labels: progress, reduce
>         Attachments: HADOOP-5958-v2.patch, MAPREDUCE-5958v3.patch
>
>
> If the map output is compressed (_mapreduce.map.output.compress_ set to _true_) then
the reduce task progress may be highly underestimated.
> In the reduce phase (but also in the merge phase), the progress of a reduce task is computed
as the ratio between the number of processed bytes and the number of total bytes. But:
> - the number of total bytes is computed by summing up the uncompressed segment sizes
(_Merger.Segment.getRawDataLength()_)
> - the number of processed bytes is computed by exploiting the position of the current
_IFile.Reader_ (using _IFile.Reader.getPosition()_) but this may refer to the position in
the underlying on disk file (which may be compressed)
> Thus, if the map outputs are compressed then the progress may be underestimated (e.g.,
only 1 map output ondisk file, the compressed file is 25% of its original size, then the reduce
task progress during the reduce phase will range between 0 and 0.25 and then artificially
jump to 1.0).
> Attached there is a patch: the number of processed bytes is now computed by exploiting
_IFile.Reader.bytesRead_ (if the the reader is in memory, then _getPosition()_ already returns
exactly this field).



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