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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: Vnodes - HUNDRED of MapReduce jobs
Date Fri, 29 Mar 2013 14:42:22 GMT
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 <jbellis@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 <cayiroglu@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 <lccalicia@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
>

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