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From "Eric Payne (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (MAPREDUCE-3403) Speculative Execution: Kill a reducer when queue is full and a mapper
Date Tue, 17 Jan 2012 16:21:40 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-3403?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Eric Payne updated MAPREDUCE-3403:

    Issue Type: New Feature  (was: Bug)
       Summary: Speculative Execution: Kill a reducer when queue is full and a mapper   (was:
Speculative Execution: enabling multiple reduce tasks inhibit spec exec launch of mappers

The reason we see this behavior is because the queue's containers are all in use at the time
of the speculation. So, the speculated mapper gets requested, but as long as all of the containers
are occupied, the speculated mapper task will never be scheduled.

For e.g., 
|Requested Tasks|Used Containers|Completed Tasks|
|Map0 prime|Map0 (long running||
||Reduce 0|Map1, Map2|
||Reduce 1|Map3|

If all of the containers of a queue are consumed with running mapred task attempts (both maps
and reduces) and
speculation of a mapper needs to run, would it be reasonable to have the MRAM kill one of
the reduce tasks so the
speculative map task can run?

> Speculative Execution: Kill a reducer when queue is full and a mapper 
> ----------------------------------------------------------------------
>                 Key: MAPREDUCE-3403
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3403
>             Project: Hadoop Map/Reduce
>          Issue Type: New Feature
>          Components: job submission
>    Affects Versions: 0.23.0
>         Environment: Hadoop version is: Hadoop
> 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)
> - 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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