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Pricing and availability
Here's a quick overview of the new dual-socket Xeons models, along with key features and pricing.

Model Clock
speed
Cores L3 cache QPI
link
speed
Max
DDR3
speed
TDP Turbo? Hyper-
Threading?
Price
Xeon W5580 3.2GHz 4 8MB 6.4 GT/s 1333MHz 130 W Y Y $1600
Xeon X5570 2.93GHz 4 8MB 6.4 GT/s 1333MHz 95 W Y Y $1386
Xeon X5560 2.8GHz 4 8MB 6.4 GT/s 1333MHz 95 W Y Y $1172
Xeon X5550 2.66GHz 4 8MB 6.4 GT/s 1333MHz 95 W Y Y $958
Xeon E5540 2.53GHz 4 8MB 5.86 GT/s 1066MHz 80 W Y Y $744
Xeon E5530 2.4GHz 4 8MB 5.86 GT/s 1066MHz 80 W Y Y $530
Xeon E5520 2.26GHz 4 8MB 5.86 GT/s 1066MHz 80 W Y Y $373
Xeon L5520 2.26GHz 4 8MB 5.86 GT/s 1066MHz 60 W Y Y $530
Xeon E5506 2.13GHz 4 4MB 4.8 GT/s 800MHz 80 W N N $266
Xeon L5506 2.13GHz 4 4MB 4.8 GT/s 800MHz 60 W N N $422
Xeon E5504 2.00GHz 4 4MB 4.8 GT/s 800MHz 80 W N N $224
Xeon E5502 1.86GHz 2 4MB 4.8 GT/s 800MHz 80 W N N $188

Nehalem has a plethora of knobs and dials available for product differentiation, and Intel has apparently decided to twiddle with them all. Each of them impacts performance in its own way, so choosing the right processor for your needs may prove to be something less than straightforward.

On top of all of the possibilities you see in the table above, there's the issue of L3 cache speed, a notable attribute that impacts performance, but one Intel hasn't opted to document too clearly (as we learned with the Core i7.) As I understand it, the uncore elements in Nehalem chips can be clocked independently of one another, so the speed of the memory controller or the QPI link doesn't necessarily correspond to the frequency of the L3 cache. The pair of processors we have for this first review, of the decidedly ultra-high-end, workstation-oriented Xeon W5580 variety, have a 2.66GHz L3 cache. So does the Xeon X5570, the top server model.

The Opteron also edges forward
Unfortunately, we don't have a direct Opteron competitor to test against the Xeon W5580, primarily because AMD doesn't make a dual-socket CPU that expensive. We do, however, have a pair of new "Shanghai" Opterons, model 2389, with a 2.9GHz core clock frequency (and a 2.2GHz L3/north bridge clock.) These are not "SE" parts, so they offer higher performance within the same power/thermal envelope as most mainstream Opterons, with a 75W ACP rating.

The bigger news here may be the addition, at last, of HyperTransport 3.0 support to these Opterons. HT3 essentially doubles the bandwidth of a HyperTransport link, and at 2.2GHz, the link between our Opteron 2389s should operate at 4.4 GT/s and provide a total of 17.6 GB/s of bandwidth—quite close to the 19.2 GB/s supplied by the 4.8 GT/s QPI link on mainstream Nehalem variants. Upgrading to HT3 was as simple as dropping these new Opterons into our existing test system. This system's Nvidia core logic chipset doesn't support HT3, but the socket-to-socket interconnect automatically came up with HT3 enabled.

The Opteron 2389 currently lists for $989, which makes it a direct competitor for the Xeon X5550. I'd certainly like to show you a performance comparison between these two chips, but unfortunately, time constraints and a minor flood in my office have prevented me from pursuing the matter. Perhaps soon.