[Gllug] Opteron vs Xeon

[Richard Jones]

On Sun, Jul 29, 2007 at 12:07:07PM +0100, t.clarke wrote:
> A board that seems quite attractive (though not so readily available
> it seems) is the Asus KFN4-D16 - which is a dual Opteron Socket F
> board.  At least with the Opterons the choice of processor type is
> less bewildering than the host of different Intel variants!
>
> However, I note that the Opteron board has dedicated memory for each
> processor, as opposed to, as far as I can see, Intel processor
> boards having main memory serving both processors.  So, I am
> wondering how this affects performance.  As I understand it a lot of
> linux/unix processes will be using common 'library' code which will
> presumably therefore reside in only one memory location.  If this is
> the case, how does CPU B access memory locations in CPU A's memory
> 'bank' and does this materially affect performance ?
>
> Does anyone have any general views on Opterons vs Xeons ?

Yes, interesting question.  As you rightly said, Opterons contain
onboard memory controllers -- AIUI one memory controller per socket
(physical chip).  So each socket "owns" its own bank of memory, and
there is a small penalty if a socket needs to read or write memory
which is owned by the other socket.

In other words, this is a classic NUMA memory architecture.

http://en.wikipedia.org/wiki/Opteron#Multi-processor_features
http://en.wikipedia.org/wiki/Non-Uniform_Memory_Access

The Opteron architecture is a good thing once you start to have a lot
of nodes in your system.  It scales better than the Intel Xeon single
front-side bus approach.

All of this only really applies if (a) you are going to have a lot of
nodes in your machine (eg. >= 4), and (b) you are going to use Linux
utilities like numactl carefully to pin processes to sockets / memory
regions.

OTOH, Xeons scale fine up to a small number of cores, and in
particular seem to have much better single-threaded performance.  So
unless you are building a large multi-way server to handle many
clients at once, don't count them out.

Given all that, you should cost out both platforms and find out which
one gives you the best performance for what you are prepared to pay.

Rich.

-- 
Richard Jones
Red Hat
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