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From "David Parks" <davidpark...@yahoo.com>
Subject RE: How can I limit reducers to one-per-node?
Date Mon, 11 Feb 2013 02:29:39 GMT
I guess the FairScheduler is doing multiple assignments per heartbeat, hence
the behavior of multiple reduce tasks per node even when they should
otherwise be full distributed. 


Adding a combiner will change this behavior? Could you explain more?






From: Michael Segel [mailto:michael_segel@hotmail.com] 
Sent: Monday, February 11, 2013 8:30 AM
To: user@hadoop.apache.org
Subject: Re: How can I limit reducers to one-per-node?


Adding a combiner step first then reduce? 



On Feb 8, 2013, at 11:18 PM, Harsh J <harsh@cloudera.com> wrote:

Hey David,

There's no readily available way to do this today (you may be
interested in MAPREDUCE-199 though) but if your Job scheduler's not
doing multiple-assignments on reduce tasks, then only one is assigned
per TT heartbeat, which gives you almost what you're looking for: 1
reduce task per node, round-robin'd (roughly).

On Sat, Feb 9, 2013 at 9:24 AM, David Parks <davidparks21@yahoo.com> wrote:

I have a cluster of boxes with 3 reducers per node. I want to limit a
particular job to only run 1 reducer per node.

This job is network IO bound, gathering images from a set of webservers.

My job has certain parameters set to meet "web politeness" standards (e.g.
limit connects and connection frequency).

If this job runs from multiple reducers on the same node, those per-host
limits will be violated.  Also, this is a shared environment and I don't
want long running network bound jobs uselessly taking up all reduce slots.

Harsh J


Michael Segel <mailto:msegel@segel.com>   | (m) 312.755.9623

Segel and Associates


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