It should be easy to control the number of map tasks. http://wiki.apache.org/hadoop/HowManyMapsAndReduces. It standard HDFS you might run into a directory with 10,000 small files and you do not want 10,000 map tasks. This is what the CombinedInputFormat's do, they help you control the number of map tasks a job will generate. For example, imagine i have a multi-tenant cluster. If a job kicks up 10,000 map tasks, all those tasks can starve out other jobs. Being able to say "I only want 4 map tasks per c* node regardless of the number of vnodes" would be a meaningful and useful feature.
On Fri, Mar 29, 2013 at 2:17 PM, Edward Capriolo <edlinuxg...@gmail.com>wrote: > Yes but my point, is with 50 map slots you can only be processing 50 at > once. So it will take 1000/50 "waves" of mappers to complete the job. > > > On Fri, Mar 29, 2013 at 11:46 AM, Jonathan Ellis <jbel...@gmail.com>wrote: > >> My point is that if you have over 16MB of data per node, you're going >> to get thousands of map tasks (that is: hundreds per node) with or >> without vnodes. >> >> On Fri, Mar 29, 2013 at 9:42 AM, Edward Capriolo <edlinuxg...@gmail.com> >> wrote: >> > Every map reduce task typically has a minimum Xmx of 256MB memory. See >> > mapred.child.java.opts... >> > So if you have a 10 node cluster with 256 vnodes... You will need to >> spawn >> > 2,560 map tasks to complete a job. >> > And a 10 node hadoop cluster with 5 map slotes a node... You have 50 map >> > slots. >> > >> > Wouldnt it be better if the input format spawned 10 map tasks instead of >> > 2,560? >> > >> > >> > On Fri, Mar 29, 2013 at 10:28 AM, Jonathan Ellis <jbel...@gmail.com> >> wrote: >> >> >> >> I still don't see the hole in the following reasoning: >> >> >> >> - Input splits are 64k by default. At this size, map processing time >> >> dominates job creation. >> >> - Therefore, if job creation time dominates, you have a toy data set >> >> (< 64K * 256 vnodes = 16 MB) >> >> >> >> Adding complexity to our inputformat to improve performance for this >> >> niche does not sound like a good idea to me. >> >> >> >> On Thu, Mar 28, 2013 at 8:40 AM, cem <cayiro...@gmail.com> wrote: >> >> > Hi Alicia , >> >> > >> >> > Cassandra input format creates mappers as many as vnodes. It is a >> known >> >> > issue. You need to lower the number of vnodes :( >> >> > >> >> > I have a simple solution for that and ready to write a patch. Should >> I >> >> > create a ticket about that? I don't know the procedure about that. >> >> > >> >> > Regards, >> >> > Cem >> >> > >> >> > >> >> > On Thu, Mar 28, 2013 at 2:30 PM, Alicia Leong <lccali...@gmail.com> >> >> > wrote: >> >> >> >> >> >> Hi All, >> >> >> >> >> >> I have 3 nodes of Cassandra 1.2.3 & edited the cassandra.yaml for >> >> >> vnodes. >> >> >> >> >> >> When I execute a M/R job .. the console showed HUNDRED of Map tasks. >> >> >> >> >> >> May I know, is the normal since is vnodes? If yes, this have slow >> the >> >> >> M/R >> >> >> job to finish/complete. >> >> >> >> >> >> >> >> >> Thanks >> >> > >> >> > >> >> >> >> >> >> >> >> -- >> >> Jonathan Ellis >> >> Project Chair, Apache Cassandra >> >> co-founder, http://www.datastax.com >> >> @spyced >> > >> > >> >> >> >> -- >> Jonathan Ellis >> Project Chair, Apache Cassandra >> co-founder, http://www.datastax.com >> @spyced >> > >
