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https://issues.apache.org/jira/browse/MAPREDUCE-4827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13506064#comment-13506064
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Doug Cutting commented on MAPREDUCE-4827:
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> Most writables do not implement hashCode()
All WritableComparable (i.e., key) implementations included with Hadoop
implement hashCode(). Moreover a WritableComparable would be a poor key
implementation if it did not implement hashCode() and was used with
HashPartitioner since it wouldn't send equivalent values at the same reducer.
The WritableComparable documentation specifically advises implementing
hashCode().
http://hadoop.apache.org/docs/current/api/org/apache/hadoop/io/WritableComparable.html
> Increase hash quality of HashPartitioner
> ----------------------------------------
>
> Key: MAPREDUCE-4827
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-4827
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Reporter: Radim Kolar
> Attachments: betterhash1.txt
>
>
> hash partitioner is using object.hashCode() for splitting keys into
> partitions. This results in bad distributions because hashCode() quality is
> poor.
> These hashCode() functions are sometimes written by hand (very poor quality)
> and sometimes generated from by commons lang code (poor quality). Applying
> some transformation on top of hashCode() provides better distribution.
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