[jira] [Commented] (MAPREDUCE-5958) Wrong reduce task progress if map output is compressed

["Kihwal Lee (JIRA)" <ji...@apache.org>]

    [ 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).



--
This message was sent by Atlassian JIRA
(v6.3.4#6332)