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https://issues.apache.org/jira/browse/MAPREDUCE-5168?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13656604#comment-13656604
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Vinod Kumar Vavilapalli commented on MAPREDUCE-5168:
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Sorry, missed this one.
bq. This is a case where 0.23 and branch-2/trunk have diverged. Before
MAPREDUCE-4808 the merge threads took a list of MapOutput objects to merge, and
these objects could linger around for quite some time waiting to be merged.
Ah, that explains it. I was looking at trunk and like you said, this is hardly
an issue in trunk.
bq. So this is an important fix for 0.23 and should be harmless for
trunk/branch-2. It's mostly preventative for the latter in case someone
refactors the code and MapOutput objects could linger again.
Yeah, please commit this to 0.23 only. Don't see a need for this in
trunk/branch-2. Thanks.
> Reducer can OOM during shuffle because on-disk output stream not released
> -------------------------------------------------------------------------
>
> Key: MAPREDUCE-5168
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-5168
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mrv2
> Affects Versions: 0.23.7, 2.0.5-beta
> Reporter: Jason Lowe
> Assignee: Jason Lowe
> Priority: Critical
> Attachments: MAPREDUCE-5168-branch-0.23.patch, MAPREDUCE-5168.patch
>
>
> If a reducer needs to shuffle a map output to disk, it opens an output stream
> and writes the data to disk. However it does not release the reference to
> the output stream within the MapOutput, and the output stream can have a 128K
> buffer attached to it. If enough of these on-disk outputs are queued up
> waiting to be merged, it can cause the reducer to OOM during the shuffle
> phase. In one case I saw there were 1200 on-disk outputs queued up to be
> merged, leading to an extra 150MB of pressure on the heap due to the output
> stream buffers that were no longer necessary.
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