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https://issues.apache.org/jira/browse/HIVE-105?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12667533#action_12667533
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Zheng Shao commented on HIVE-105:
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> seems like we are missing the code that uses mapred.reduce.tasks if it's
> greater than 0
We don't need any code for that - hadoop directly picks up
"mapred.reduce.tasks" I think.
> Driver.java has some informational stuff that needs to be updated:
This will be moved into ExecDriver since the number of reducers will be
different from each map-reduce job.
> we can change conf variable names to have better consistency. for example:
> hive.exec.reducers.max and hive.exec.reducers.bytesperreducer
Changed to:
hive.exec.reducers.max
hive.exec.reducers.bytes.per.reducer
Will upload a new patch after testing.
> estimate number of required reducers and other map-reduce parameters
> automatically
> ----------------------------------------------------------------------------------
>
> Key: HIVE-105
> URL: https://issues.apache.org/jira/browse/HIVE-105
> Project: Hadoop Hive
> Issue Type: Improvement
> Components: Query Processor
> Reporter: Joydeep Sen Sarma
> Assignee: Zheng Shao
> Attachments: HIVE-105.1.patch
>
>
> 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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