[Beowulf] Re: dual core (latency)

Stuart Midgley sdm900 at gmail.com
Mon Jul 18 20:05:44 PDT 2005

The first thing to note is that as you add cpu's the cost of the  
cache snooping goes up dramatically.  The latency of a 4 cpu (single  
core) opteron system is (if my memory serves me correctly) around  
120ns.  Which is significantly higher than the latency of a dual  
processor system (I think it scales roughly as O(n^2) where n is the  
number of cpu's).

Now, with a dual core system, you are effectively halving the  
bandwidth/cpu over the hyper transport AND increasing the cpu count,  
thus increasing the amount of cache snooping required.  The end  
result is drastically blown-out latencies.


On 19/07/2005, at 10:50, Vincent Diepeveen wrote:

> Hello Stuart,
> Thanks for your answer regarding numactl tools.
> Your answer doesn't necessarily explain why the dual core latency  
> (with or
> without numactl) is far worse, yes 30%+ worse, than that of single cpu
> opterons of the same speed, when benchmarking just 1 core (so the  
> others
> sitting idle).
> Any thoughts on that?
> Thanks,
> Vincent

Dr Stuart Midgley
Industry Uptake Program Leader
iVEC, 'The hub of advanced computing in Western Australia'
26 Dick Perry Avenue, Technology Park
Kensington WA 6151

Phone: +61 8 6436 8545
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