Home | About | Sematext search-lucene.com search-hadoop.com
 Search Hadoop and all its subprojects:

Switch to Threaded View
Hadoop >> mail # dev >> Number of reducers

Copy link to this message
Re: Number of reducers
Ah, well, my bad. See instead the description for mapred.reduce.tasks
in mapred-default.xml, which states this: "Typically set to 99% of the
cluster's reduce capacity, so that if a node fails the reduces can
still be executed in a single wave."

FWIW, I set it manually to the level of parallelism I require (given
my partitioned data, etc.).

On Tue, Aug 28, 2012 at 8:43 PM, abhiTowson cal
> hi harsh,
> Thanks for the reply.I get your first and second points and coming to
> third point how is it specific to a job?
> My question was specific to job.
> Regards
> Abhishek
> On Mon, Aug 27, 2012 at 11:29 PM, Harsh J <[EMAIL PROTECTED]> wrote:
>> Hi,
>> On Tue, Aug 28, 2012 at 8:32 AM, Abhishek <[EMAIL PROTECTED]> wrote:
>>> Hi all,
>>> I just want to know that, based on what factor map reduce framework decides number of reducers to launch for a job
>> The framework does not auto-determine the number of reducers for a
>> job. That is purely user-or-client-program-supplied presently.
>>> By default only one reducer will be launched for a given job is this right? If we explicitly does not mention number to launch via command line or driver class.
>> Yes, by default the number of reduce tasks is configured to be one.
>>> If i choose to decide number of reducers to mention explicitly, what should I consider.Because choosing in appropriate number of reducer hampers the performance.
>> See http://wiki.apache.org/hadoop/HowManyMapsAndReduces
>> --
>> Harsh J

Harsh J