[jira] Commented: (HADOOP-5589) TupleWritable: Lift implicit limit on the number of values that can be stored

["jv ning (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-5589?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12717744#action_12717744 ] 

jv ning commented on HADOOP-5589:
---------------------------------

How does the CompositeRecordReader change with this patch. I have been backporting to 18.2 and it looks like CompositeRecordReader uses a long to hold information also.

> 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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