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Date: Mon, 28 Nov 2011 18:05:40 +0000 (UTC)
From: "Eric Payne (Assigned) (JIRA)" <jira@apache.org>
To: mapreduce-issues@hadoop.apache.org
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Subject: [jira] [Assigned] (MAPREDUCE-3403) Speculative Execution: enabling
 multiple reduce tasks inhibit spec exec launch of mappers
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     [ https://issues.apache.org/jira/browse/MAPREDUCE-3403?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Eric Payne reassigned MAPREDUCE-3403:
-------------------------------------

    Assignee: Eric Payne
    
> Speculative Execution: enabling multiple reduce tasks inhibit spec exec launch of mappers 
> ------------------------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-3403
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3403
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: job submission
>    Affects Versions: 0.23.0
>         Environment: Hadoop version is: Hadoop 0.23.0.1110031628
> 10 node test cluster
>            Reporter: patrick white
>            Assignee: Eric Payne
>
> When forcing multiple reduce tasks to be launched by applying the setNumReduceTasks() method on a Job object, and
> running on input data which has one significantly longer map (and consequently reduce) task;
> - a speculative reduce task was not launched, even with a longer running reducer only 4 reduce tasks were launched
> - the spec launch of map tasks was inhibited by the setNumReduceTasks() method applied, so even with
> -Dmapreduce.job.maps.speculative.execution=true we only had 4 map tasks launched. The exact same code with the
> setNumReduceTasks() method taken out, and on the same input data set, consistently launched 5 mappers as expected.
> Testing info:
> 3. modified WordCount to force 4 reducers being launched, by adding:
>     job.setNumReduceTasks(4); // hardwire 4 reducers for now
>     System.out.println("\nTESTDEBUG: using 4 reduce tasks for now\n\n");
> to the Job object. This causes 4 reduce tasks to be launched, oddly though it inhibits the map task from speculative
> launch. So the same job code, without the setNumReduceTasks() method, will launch 5 mappers as described in case #2.
> When this method is added, that same job will only launch 4 mappers, as well as 4 reducers, otherwise the job
> successfully completes.
> output snippet with setNumReduceTasks():
>         org.apache.hadoop.mapreduce.JobCounter
>                 TOTAL_LAUNCHED_MAPS=4
>                 TOTAL_LAUNCHED_REDUCES=4
>                 RACK_LOCAL_MAPS=4
>                 SLOTS_MILLIS_MAPS=190787
>                 SLOTS_MILLIS_REDUCES=572554

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