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From "Carl Steinbach (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-535) Memory-efficient hash-based Aggregation
Date Wed, 24 Feb 2010 23:47:28 GMT

    [ https://issues.apache.org/jira/browse/HIVE-535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12838095#action_12838095
] 

Carl Steinbach commented on HIVE-535:
-------------------------------------

The folks working on Mahout seem to think the CERN license is compatible with Apache. They
have
already imported cern.colt*, cern.jet* and cern.clhep into their source tree. See MAHOUT-222.

Check out the update to their LICENSE.txt file: http://svn.apache.org/repos/asf/lucene/mahout/trunk/LICENSE.txt

> Memory-efficient hash-based Aggregation
> ---------------------------------------
>
>                 Key: HIVE-535
>                 URL: https://issues.apache.org/jira/browse/HIVE-535
>             Project: Hadoop Hive
>          Issue Type: Improvement
>    Affects Versions: 0.4.0
>            Reporter: Zheng Shao
>
> Currently there are a lot of memory overhead in the hash-based aggregation in GroupByOperator.
> The net result is that GroupByOperator won't be able to store many entries in its HashTable,
and flushes frequently, and won't be able to achieve very good partial aggregation result.
> Here are some initial thoughts (some of them are from Joydeep long time ago):
> A1. Serialize the key of the HashTable. This will eliminate the 16-byte per-object overhead
of Java in keys (depending on how many objects there are in the key, the saving can be substantial).
> A2. Use more memory-efficient hash tables - java.util.HashMap has about 64 bytes of overhead
per entry.
> A3. Use primitive array to store aggregation results. Basically, the UDAF should manage
the array of aggregation results, so UDAFCount should manage a long[], UDAFAvg should manage
a double[] and a long[]. The external code should pass an index to iterate/merge/terminal
an aggregation result. This will eliminate the 16-byte per-object overhead of Java.
> More ideas are welcome.

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