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https://issues.apache.org/jira/browse/MAPREDUCE-4827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13505979#comment-13505979
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Doug Cutting commented on MAPREDUCE-4827:
-----------------------------------------
This is an incompatible change; it will change the output of jobs. In most
cases this shouldn't matter, but there might be applications which expect,
e.g., the key '1' to go to the output file numbered '1'. This could be avoided
by, instead of modifying HashPartitioner, adding a new partitioner.
> 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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