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From "Jason Lowe (JIRA)" <j...@apache.org>
Subject [jira] [Assigned] (MAPREDUCE-5958) Wrong reduce task progress if map output is compressed
Date Fri, 10 Oct 2014 19:12:36 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-5958?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jason Lowe reassigned MAPREDUCE-5958:
-------------------------------------

    Assignee: Emilio Coppa

Thanks for the report and patch, Emilio.  We just ran across this as well, and my apologies
for missing this earlier.  Looks like this was caused by MAPREDUCE-2264 when the getRawDataLength
stuff was added.

Patch looks good overall, and I manually verified it fixes the issue with compressed map outputs.
 However it would be nice to have a unit test to verify this doesn't break again in the future.
 I see there's already unit tests in TestMerger that were intended to test this, but the tests
are ineffective because they create no segments.  The {{for (int i = 1; i < 1; i++)}} code
in the test ends up creating no segments so nothing is merged.

[~sandyr] [~devaraj.k], could you also take a look since you were familiar with the changes
in MAPREDUCE-2264?

> 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
>             Fix For: 2.4.1
>
>         Attachments: HADOOP-5958-v2.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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