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
