[Beowulf] Re: scalability

[Greg Keller]

On Dec 12, 2009, at 10:41 AM, beowulf-request at beowulf.org wrote:

>
> From: Gus Correa <gus at ldeo.columbia.edu>
>
> Hi Amjad
>
> amjad ali wrote:
>> Hi Gus,
>>
>>    I was told that some people used to run two processes only on
>>    dual-socket dual-core Xeon nodes , leaving the other two cores  
>> idle.
>>    Although it is an apparent waste, the argument was that it paid
>>    off in terms of overall efficiency.
>>
>>
>> I guess I fully agree with this.
>>
The other reason I see folks choose this is licensing.  In some cases  
the "cost" of the license tokens to use the extra cores is too  
expensive given the minimal benefit since they still compete for  
Memory Bandwidth, CPU Cache, or less commonly Network Bandwidth/Latency.
...
>>
>> But still if it is a shared cluster (as in my case) then the cores  
>> you
>> left unbusy may be allocated to another process of another user by  
>> the
>> Batch scheduler. Right??
>
> Unless you request full nodes, as Chris Samuel suggested:
> #PBS -l nodes=10:ppn=8
>
> However, beware that this greedy and wasteful  behavior
> may drive your system administrator and
> the other cluster users mad at you!        :)
> Well, you can always justify it in the name of science, of course. ;)
>
They may thank you for not sharing a "maxed out" node.  If someone is  
doing benchmarking or running a similarly sensitive code you're saving  
everyone a lot of head scratching.  In my world users sharing nodes is  
almost always trouble looking for a way to ruin a weekend.

Cheers!
Greg
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