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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 20:54:19 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-4827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13534292#comment-13534292
] 

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

I'm fine with a separate partitioner (RehashPartitioner?) for historically "broken" hashCode
impls. OTOH, I think the goal should strive for better hashCode impls moving forward, otherwise
we'll be paying for the cost of rehash even if we have good hashCode impls.
                
> 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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