[cluster-l] Single- vs. Dual- vs. Quad-core CPUs

[Nils Oberg]

At 14:30 3/28/2007, Jim Phillips wrote:
>Everything you're seeing makes sense for a memory-limited code.

Will a memory-limited code benefit significantly by adding a 
higher-speed interconnect?  Do vendors lend demo equipment to test 
these sorts of things?

>Each Opteron core has its own cache, and each chip has its own 
>memory interface.  Each pair of Xeon cores shares a single cache, 
>and all chips share a single memory interface.  Thus the Opteron 
>system scales more linearly to higher numbers of cores.  Of course, 
>when you use fewer cores it also slows down linearly, as IBM pointed 
>out a few years ago.  It's a trade-off, and all that really matters 
>is the maximum performance you can get when using all of the cores on a node.

Is there any way to determine how jobs are scheduled?  I'm thinking 
of the case where a user runs an mpich job with 10 processors on a 
5-node cluster with 8 cores in each node.  Will mpich put 2 processes 
on each node, or will it bunch them all on the first two nodes in its 
machines file?

Thanks,

Nils

>On Wed, 28 Mar 2007, Nils Oberg wrote:
>
>>Thanks for your help Jim.
>>
>>I performed some benchmarks on demo equipment from AMD and Intel 
>>and there are some interesting differences between the two 
>>platforms for our code.  All times are in seconds.
>>
>>Here are some results:
>>
>>2x2 Opteron 2218 2.6 GHz with 4GB RAM:
>>1 core:  697
>>2 cores: 323
>>4 cores: 211
>>
>>4x2 Opteron 875 2.2 Ghz with 8 GB RAM:
>>1 core: 531
>>2 cores: 333
>>4 cores: 181
>>6 cores: 143
>>8 cores: 139
>>
>>1x4 Xeon 5355 2.6 GHz with 4 GB RAM:
>>1 core:  510
>>2 cores: 343
>>4 cores: 251
>>
>>2x4 Xeon 5355 2.6 GHz with 8 GB RAM:
>>1 core:  516
>>2 cores: 314
>>4 cores: 228
>>6 cores: 195
>>8 cores: 167
>>
>>
>>I don't understand why the Xeon performs better than the Opteron on 
>>one core, but worse than the Opteron on 4 cores.  I tried a 
>>different CFD code and the same pattern emerged.  Why might this be happening?
>>
>>
>>I was getting better than linear speedup results for one of our 
>>programs.  Is this possible?  Here are some results:
>>
>>2x4 Xeon 5355 2.6 GHz with 8 GB RAM:
>>cores: 1  7590
>>cores: 2  4523   speedup: 1.68
>>cores: 4  2060   speedup: 3.68
>>cores: 8   916   speedup: 8.29
>>
>>2x2 Opteron 2218 2.6 GHz with 4GB RAM:
>>cores: 1  8497
>>cores: 2  4360   speedup: 1.95
>>cores: 4  1883   speedup: 4.51
>>
>>
>>Does this make sense?
>>
>>Thanks for any help.
>>
>>Nils
>
>--
>Nils Oberg, Research Programmer
>Civil & Environmental Engineering, University of Illinois at U-C
>phone: 217-333-8365, web: http://vtchl.uiuc.edu