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From "patrick white (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MAPREDUCE-3403) Speculative Execution: enabling multiple reduce tasks inhibit spec exec launch of mappers
Date Tue, 15 Nov 2011 19:11:52 GMT
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
10 node test cluster
            Reporter: patrick white

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():


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