dual Xeon issue

Martin Siegert siegert at sfu.ca
Thu Jun 5 15:17:03 EDT 2003

On Thu, Jun 05, 2003 at 08:20:47AM -0600, sfrolov at accufo.vwh.net wrote:
> Sorry for a noob-like question. We are in the process of buying a Xeon-based
> cluster for scientific computing (running MPI parallelized numerical models).
> We've benchmarked clasters offered by Microway and Aspen Systems and discovered
> that we are getting much better results using only one CPU per box (running the
> model on 6 boxes using one CPU in each box was about 50% faster than running it
> on 3 boxes using both CPUs). This was surprising to us since we've used to be
> limited by interprocessor communications, which should be a lot faster between
> CPUs in one box. Can anybody explain the reason for this and, more importantly,
> is there any way to improve this situation.

Since you mentioned that you are limited by interprocessor communications the
reason for the slowdown in the dual-processor configuration is probably that
the communication between processors on different nodes becomes the bottleneck.
Since the link between nodes is now shared between two processors on each node
you effectively halve the per processor bandwidth.

Remedy: either buy single processor boxes or double the bandwidth (cf channel

Martin Siegert
Manager, Research Services
Academic Computing Services                        phone: (604) 291-4691
Simon Fraser University                            fax:   (604) 291-4242
Burnaby, British Columbia                          email: siegert at sfu.ca
Canada  V5A 1S6
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