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From "Luke Lu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-4827) Increase hash quality of HashPartitioner
Date Mon, 17 Dec 2012 18:12:13 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-4827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13534135#comment-13534135
] 

Luke Lu commented on MAPREDUCE-4827:
------------------------------------

I don't see this as generally useful, it only works with hashCode already returns reasonable
numbers (enough cardinality). It'll do absolutely nothing to improve quality but wasting CPU
cycles for bad hashCode implementations. I suggest that we close this as won't fix, as it's
not a good place to do this.
                
> 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, betterhash2.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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