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MapReduce >> mail # user >> Distributing Keys across Reducers

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Re: Distributing Keys across Reducers
On 07/20/2012 09:20 AM, Dave Shine wrote:
> I have a job that is emitting over 3 billion rows from the map to the reduce.  The job is configured with 43 reduce tasks.  A perfectly even distribution would amount to about 70 million rows per reduce task.  However I actually got around 60 million for most of the tasks, one task got over 100 million, and one task got almost 350 million.  This uneven distribution caused the job to run exceedingly long.
> I believe this is referred to as a "key skew problem", which I know is heavily dependent on the actual data being processed.  Can anyone point me to any blog posts, white papers, etc. that might give me some options on how to deal with this issue?

Hadoop lets you override the default partitioner and replace it with
your own.  This lets you write a custom partitioning scheme which
distributes your data more evenly.