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From "Zheng Shao (JIRA)" <>
Subject [jira] Commented: (HIVE-105) estimate number of required reducers and other map-reduce parameters automatically
Date Wed, 21 Jan 2009 00:43:59 GMT


Zheng Shao commented on HIVE-105:

> most hadoop installations have a non-zero value specified for this. hadoop-default.xml
comes with a value of 1. Did u mean if it is > 1?

User would need to override this value in the session or hive-default/site.xml.  The reason
for this is just for backward compatibility (it adds potential confusions because both hive
and hadoop are reading this "mapred.reduce.tasks" and hive might potentially change it in
case of reducer number estimation).

Otherwise it's much cleaner to use a separate variable (like "hive.exec.reduce.tasks"), with
the cost of backward compatibility.

> estimate number of required reducers and other map-reduce parameters automatically
> ----------------------------------------------------------------------------------
>                 Key: HIVE-105
>                 URL:
>             Project: Hadoop Hive
>          Issue Type: Improvement
>          Components: Query Processor
>            Reporter: Joydeep Sen Sarma
>            Assignee: Zheng Shao
> currently users have to specify number of reducers. In a multi-user environment - we
generally ask users to be prudent in selecting number of reducers (since they are long running
and block other users). Also - large number of reducers produce large number of output files
- which puts pressure on namenode resources.
> there are other map-reduce parameters - for example the min split size and the proposed
use of combinefileinputformat that are also fairly tricky for the user to determine (since
they depend on map side selectivity and cluster size). This will become totally critical when
there is integration with BI tools since there will be no opportunity to optimize job settings
and there will be a wide variety of jobs.
> This jira calls for automating the selection of such parameters - possibly by a best
effort at estimating map side selectivity/output size using sampling and determining such
parameters from there.

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