[jira] Commented: (HIVE-900) Map-side join failed if there are large number of mappers

["Ning Zhang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12796105#action_12796105 ] 

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

Closing this issue since HDFS-767 is committed.

> 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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