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From Arun C Murthy <...@yahoo-inc.com>
Subject Re: Choosing number of map/reduce slots (with hyperthreading)
Date Sun, 09 Jan 2011 08:39:25 GMT
Hyperthreading is interesting, but I'd put more emphasis on the amount  
of RAM you have on your boxes.

The JavaVM allocates all it's heap-size upfront, which means your node  
will starting thrashing on RAM if you put too many tasks per node.

Arun

On Jan 6, 2011, at 5:51 PM, Adam Phelps wrote:

> By scouring various web pages and lists via google I've found some
> general recommendations when it comes to setting the number of map and
> reduce slots for a cluster.  It seems to come down to setting them to
> roughly the number of cores on the machine, minus some if there will  
> be
> other processes active (such as HBase region servers), and to set the
> per-task memory usage so that the total will stay below that of the
> system.  Is this a reasonably general heuristic?
>
> One thing I haven't been able to find advice on is whether this
> heuristic should be adjusted for machines that have hyperthreading
> enabled.  My thought is that it wouldn't be beneficial to increase the
> number of slots (especially in a CPU-bound application) as slots equal
> to the # of cores would already be fully utilizing the CPU.  Are there
> alternative thoughts regarding that?
>
> - Adam


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