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https://issues.apache.org/jira/browse/HADOOP-5589?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12717752#action_12717752
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Jingkei Ly commented on HADOOP-5589:
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It looks like you would probably need to backport HADOOP-3721 as well, as this
incorporated a change to CompositeRecordReader.JoinCollector that removed the
limitation from using longs.
> TupleWritable: Lift implicit limit on the number of values that can be stored
> -----------------------------------------------------------------------------
>
> Key: HADOOP-5589
> URL: https://issues.apache.org/jira/browse/HADOOP-5589
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Affects Versions: 0.19.1
> Reporter: Jingkei Ly
> Assignee: Jingkei Ly
> Fix For: 0.21.0
>
> Attachments: HADOOP-5589-1.patch, HADOOP-5589-2.patch,
> HADOOP-5589-3.patch, HADOOP-5589-4.patch, HADOOP-5589-4.patch
>
>
> TupleWritable uses an instance field of the primitive type, long, which I
> presume is so that it can quickly determine if a position has been written to
> in its array of Writables (by using bit-shifting operations on the long
> field). The problem with this is that it implies that there is a maximum
> limit of 64 values you can store in a TupleWritable.
> An example of a use-case where I think this would be a problem is if you had
> two MR jobs with over 64 reduces tasks and you wanted to join the outputs
> with CompositeInputFormat - this will probably cause unexpected results in
> the current scheme.
> At the very least, the 64-value limit should be documented in TupleWritable.
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