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https://issues.apache.org/jira/browse/HIVE-105?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12667051#action_12667051
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Joydeep Sen Sarma commented on HIVE-105:
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a few things:
- seems like we are missing the code that uses mapred.reduce.tasks if it's
greater than 0
- Driver.java has some informational stuff that needs to be updated:
boolean hasReduce = hasReduceTasks(sem.getRootTasks());
if (hasReduce) {
console.printInfo("Number of reducers = " +
conf.getIntVar(HiveConf.ConfVars.HADOOPNUMREDUCERS));
console.printInfo("In order to change numer of reducers use:");
console.printInfo(" set mapred.reduce.tasks = <number>");
}
- we can change conf variable names to have better consistency. for example:
hive.exec.reducers.max and hive.exec.reducers.bytesperreducer
> 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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