opteron VS Itanium 2

Richard Walsh rbw at ahpcrc.org
Thu Oct 30 11:07:25 EST 2003


On Thu, 30 Oct 2003 07:57, Andreas Boklund wrote:

>Just a note,
>
>> For bandwidth/memory intensive codes, I think the Opteron is a clear
>> winner in a dual processor configuration because of its dual channel
>> to memory design.  Stream triad bandwidth during SMP operation is
>> ~50% more than a one processor test.  Both the dual Pentium 4 and Itanium 
>> 2 share their memory bus and split (with some loss) the bandwidth in 
>> dual mode. 
>
>This is true as long as you are using an applicaiton where one process has its own
>memory area. If you would have 2 processes and shared memory the Opt, would
>behave like a small NUMA machine and a process will get a penalty for accessing
>another process (processors) memory segment.
>
>To quote D. Barron, "If it seems to be to good to be true, it probably is!", i have never
>yet seen true linear scalability, and with Ahmdahl out there i doubt that i ever will.

 Agreed.  Of course, in the case of dual Pentium and Itaniums, even non-
 overlapping memory locations buy you nothing bandwidth-wise.  Small or large 
 scale perfect cross-bars to memory are tough and expensive. The Cray X1, with
 all its customer design effort and great total bandwidth on the node board,
 targeted only 1/4 of peak-data-required iin its design and delivers less under 
 the full load of its 16-way SMP vector engines.

 And it's node board is probably the best bandwidth engine in the world at the 
 moment.

 Regards,

 rbw

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