opteron VS Itanium 2
hahn at physics.mcmaster.ca
Thu Oct 30 10:51:41 EST 2003
> > 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 particularly bad on "high-end" machines. for instance, several
machines have 4 it2's on a single FSB. there's a reason that specfprate
scales so much better on 1/2/4-way opterons than on 1/2/4-way it2's.
don't even get me started about those old profusion-chipset 8-way
PIII machines that Intel pushed for a while...
> 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.
huh? sharing data behaves pretty much the same on opteron systems
(broadcast-based coherency) as on shared-FSB (snoopy) systems. it's not
at all clear yet whether opterons are higher latency in the case where
you have *often*written* shared data.
it is perfectly clear that shared/snoopy buses don't scale, and neither does
pure broadcast coherency. I figure that both Intel and AMD will be adding
some sort of directory support in future machines. if they bother, that is -
the market for many-way SMP is definitely not huge, at least not in the
regards, mark hahn.
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