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 <firstname.lastname@example.org> 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 <email@example.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
On Fri, Mar 29, 2013 at 9:42 AM, Edward Capriolo <firstname.lastname@example.org> wrote:
> Every map reduce task typically has a minimum Xmx of 256MB memory. See
> 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
> Wouldnt it be better if the input format spawned 10 map tasks instead of
> On Fri, Mar 29, 2013 at 10:28 AM, Jonathan Ellis <email@example.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 <firstname.lastname@example.org> 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 <email@example.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
Project Chair, Apache Cassandra