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 >
