[Beowulf] Re: dual core (latency)
sdm900 at gmail.com
Mon Jul 18 23:05:44 EDT 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
> sitting idle).
> Any thoughts on that?
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
Fax: +61 8 6436 8555
Email: industry at ivec.org
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