Speculative Execution: enabling multiple reduce tasks inhibit spec exec launch
of mappers
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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
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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