Home | About | Sematext search-lucene.com search-hadoop.com
 Search Hadoop and all its subprojects:

Switch to Plain View
MapReduce, mail # user - Questions with regard to scheduling of map and reduce tasks


+
Vasco Visser 2012-08-30, 17:41
Copy link to this message
-
Re: Questions with regard to scheduling of map and reduce tasks
Vinod Kumar Vavilapalli 2012-08-30, 18:19

Since you mentioned containers, I assume you are using hadoop 2.0.*. Replies inline.

> When running a job with more reducers than containers available in the
> cluster all reducers get scheduled, leaving no containers available
> for the mappers to be scheduled. The result is starvation and the job
> never finishes. Is this to be considered a bug or is it expected
> behavior? The workaround is to limit the number of reducers to less
> than the number of containers available.

No, you don't need to limit reducers yourselves, MR ApplicationMaster is smart enough to figure out available cluster/queue capacity and schedule maps/reduces accordingly. If ever it runs into a situation where it has outstanding maps but reduces happen to occupy all available resources, it will preempt reduces and start running maps.

> Also, it seems that from the combined pool of pending map and reduce
> tasks, randomly tasks are picked and scheduled. This causes less than
> optimal behavior. For example, I run a task with 500 mappers and 30
> reducers (my cluster has only 16 machines, two containters per machine
> (duo core machines)). What I observe is that half way through the job
> all reduce tasks are scheduled, leaving only one container for 200+
> map tasks. Again, is this expected behavior? If so, what is the idea
> behind it? And, are the map and reduce task indeed randomly scheduled
> or does it only look like they are?

No, again MR ApplicationMaster is smart and the scheduling isn't random. It runs maps first, and slowly ramps up reduces as maps finish.

HTH

+Vinod Kumar Vavilapalli
Hortonworks Inc.
http://hortonworks.com/

+
Vasco Visser 2012-08-30, 23:38
+
祝美祺 2012-08-31, 02:07
+
Vinod Kumar Vavilapalli 2012-08-31, 03:51
+
Vasco Visser 2012-08-31, 11:17
+
Vinod Kumar Vavilapalli 2012-08-31, 22:59
+
Vasco Visser 2012-09-02, 16:41