[Gllug] Opteron - dual core or two chips?

[Daniel P. Berrange]

On Thu, Mar 30, 2006 at 06:15:42PM +0100, Tet wrote:
> Anyone have any opinions on this? Dual core would intuitively seem a
> better option, but are there any hidden catches that I'm not aware of
> (shared cache, for example)? There's so much information out there,
> but little in an organised format that tells the relatively uninformed
> buyer what to go for.
> 
> The application is I/O bound (mostly to memory, but some disk too).

You'd need a bit more info on how the app works to say for certain
whether dual core or two chips would be better. With dual-core the
two CPUs share the bus to their memory, but also potentially benefit
from shared cache too (can't remember offhand whether the opterons
have a shared L1 or L2 cache or not).

With separate chips, and a hypertransport & NUMA memory layout, there 
is no cache sharing but each CPU has a dedicated bus to its local
memory node. 

So, say the memory bus has 1 GB/s bandwidth, with two chips you have a
theoretical peak throughput of 2 GB/s if all memory access is local. If
memory access is always remote then you get bottleneck on the hypertransport
so reduce peak throughput down to a worst case of 1 GB/s. Remote access
also has higher latency, so the worst case of 1GB/s for two chips is worse
than the 1 GB/s of dual-core.

So, if you have an application which is both parallelizable, and can
have its working data partitioned in memory, then you can CPU lock
individual processes and also use NUMACTL to apply a memory policy
ensuring allocation is on the local NUMA node. So allowing your app
to approach the peak 2 GB/s throughput expected with 100% local memory
access.

So it really all comes down to your application - if its easily 
parallelizable & partionable, then two chips is proabably worth
the extra money & admin complexity, otherwise save some cash and
get a dual-core.

Dan.
-- 
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