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From "Ning Zhang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-900) Map-side join failed if there are large number of mappers
Date Wed, 02 Dec 2009 20:54:20 GMT

    [ https://issues.apache.org/jira/browse/HIVE-900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12784995#action_12784995
] 

Ning Zhang commented on HIVE-900:
---------------------------------

Tried different approaches in Hive and it turns out none of them is perfect. Finally the solution
should be in the HDFS side. The JIRA HDFS-767 is trying to solve that. 

> Map-side join failed if there are large number of mappers
> ---------------------------------------------------------
>
>                 Key: HIVE-900
>                 URL: https://issues.apache.org/jira/browse/HIVE-900
>             Project: Hadoop Hive
>          Issue Type: Improvement
>            Reporter: Ning Zhang
>            Assignee: Ning Zhang
>
> Map-side join is efficient when joining a huge table with a small table so that the mapper
can read the small table into main memory and do join on each mapper. However, if there are
too many mappers generated for the map join, a large number of mappers will simultaneously
send request to read the same block of the small table. Currently Hadoop has a upper limit
of the # of request of a the same block (250?). If that is reached a BlockMissingException
will be thrown. That cause a lot of mappers been killed. Retry won't solve but worsen the
problem.

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