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MapReduce, mail # user - Re: How do map tasks get assigned efficiently?

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Re: How do map tasks get assigned efficiently?
Michael Segel 2012-10-24, 11:51

Data locality only works when you actually have data on the cluster itself. Otherwise how can the data be local.

Assuming 3X replication, and you're not doing a custom split and your input file is splittable...

You will split along the block delineation.  So if your input file has 5 blocks, you will have 5 mappers.

Since there are 3 copies of the block, its possible that for that map task to run on the DN which has a copy of that block.

So its pretty straight forward to a point.

When your cluster starts to get a lot of jobs and a slot opens up, your job may not be data local.

With HBase... YMMV
With S3 the data isn't local so it doesn't matter which Data Node gets the job.



On Oct 24, 2012, at 1:10 AM, David Parks <[EMAIL PROTECTED]> wrote:

> Even after reading O’reillys book on hadoop I don’t feel like I have a clear vision of how the map tasks get assigned.
> They depend on splits right?
> But I have 3 jobs running. And splits will come from various sources: HDFS, S3, and slow HTTP sources.
> So I’ve got some concern as to how the map tasks will be distributed to handle the data acquisition.
> Can I do anything to ensure that I don’t let the cluster go idle processing slow HTTP downloads when the boxes could simultaneously be doing HTTP downloads for one job and reading large files off HDFS for another job?
> I’m imagining a scenario where the only map tasks that are running are all blocking on splits requiring HTTP downloads and the splits coming from HDFS are all queuing up behind it, when they’d run more efficiently in parallel per node.