Two single-cores versus a dual core:
http://www.geek.com/news/geeknews/2005Nov/bch20051108033166.htm
Benchmarks start on page 10:
http://www.tomshardware.com/2005/11/07/single/page10.html
The advantage of two single core processors, is the number
of chipset chips on the motherboard. The AMD processor is
a "hub" with Hypertransport busses sprouting out of it.
With two single cores, you can have two true PCI Express X16
slots, plus lots of tunnels for peripheral busses. If
there is only one CPU socket on a motherboard, generally
the I/O features are a bit less (they don't have to be,
but companies don't generally use large numbers of daisy
chained tunnel chips). This is mostly related to motherboard
marketing, and the proposed uses for single socket versus
dual socket systems (workstation versus server perhaps).
Example block diagram on page 2, of a dual socket board:
ftp://ftp.tyan.com/datasheets/d_s2895_100.pdf
Note that if the "H0" socket is not occupied by a processor,
you lose the use of a PCI Express x16 slot, an ethernet
interface, and the use of four more DIMM slots. The board
will still work with just one processor (single or dual core)
in socket "H1". If you buy a motherboard with a single socket,
well, you never have to worry about this, but you also have
fewer facilities of all sorts on the motherboard.
If your box is a pure compute engine, and the only peripheral
you have is a USB socket for your Ipod, the single socket
dual core processor wins. If the box is intended to hold
a wealth of hardware, then using two sockets might be a
better answer, even if it is computationally slightly slower.
With two sockets, you have two memory controllers, and
you would think, more memory bandwidth. A single socket
setup has access to fewer total sticks of RAM, which
might be important consideration if the problems you
handle need a lot of RAM.
"Cache-Coherent Nonuniform Memory Access (ccNUMA)" - the theory
http://cdrom.amd.com/21860/updates/...ext=OptguideHelp&file=OptguideHelp-09-04.html
"Non-Uniform Memory Architecture (NUMA): Dual Processor
AMD Opteron Platform Analysis" - benchmarks
(Conclusion - set node interleave off ; use "numa aware" OS)
http://www.digit-life.com/articles2/cpu/rmma-numa.html
That last article shows that you can get 12.8GB/sec memory
bandwidth, as long as each single core processor only accesses
its own local memory at 6.4GB/sec. While the dual socket
system has the ability to use any memory by either processor,
there is a penalty associated with going to the "other" memory.
If your system has ECC memory, I think features like scrubbing
and the like, may also affect whether things like node
interleave can be turned on.
Dual sockets have their complexities, as do setting up dual core
processors. For the chunk of cash you'll be paying for either
system, I hope the person setting it up does a good job.
Paul
