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AMD's Quad FX Processor: Is Something Wrong?

by Drew Dunn

AMD released the quad core "QuadFX" system yesterday. There has been a fair amount of publicity around the web about this new four-processor system, particularly for applications that are processor bound, such as video editing and complex computer simulations.

Intel, too, has a quad processor offering, but the two platforms couldn't be more different. Rather than a single quad-core processor, AMD's system uses a pair of matched, dual-core processors that are linked together through a special bus on the Nvidia-designed chipset. For the eagle-eyed out there, that may seem to be sort of a head scratcher, since AMD bought out erstwhile chipset-maker ATI earlier this year, but development of this project preceded that transaction by a long time. But more on that in a bit.

Physically and logically, this is a really complicated affair. There is a single motherboard that supports the QuadFX, the Asus L1N64-SLI WS. It uses the Nvidia 680a SLI chipset, which is itself a pair of 570 chipsets that ties two Socket F sockets together. Out of this, you get two PCIe x16, two PCIe x8 and eight PCIe x1 lanes and a grunch of other I/O ports. Asus didn't put everything on the board - there's only so much space and, at $300.00, it's fairly spendy already.

One of the processor sockets connects to the chipset through two cHT x16 buses and the other socket connects to the first socket.

The system uses DDR2 memory and, indirectly, this is, perhaps, what causes problems for the QuadFX. Each CPU has a built in memory controller, something that had really helped AMD's processors perform extremely well - after all, if the CPU controls the memory, that's one less step and less distance in the chain of memory access. However, the Nvidia chipset uses the Northbridge memory controller, which takes that advantage away from the AMD CPUs. In fact, it appears that the AMD HyperTransport protocol supports the dual physical processors, so it would seem that rather than the complicated mess of two chipsets (and their associated hardware), a simple Southbridge is really all that should be necessary to provide a high performance, low cost motherboard.

And that's where the question about ATI comes into play. Since ATI makes chipsets (in addition to video hardware), that may be the next step in the QuadFX evolution. A less hardware-bloated motherboard would be a step in the right direction for this platform. And that's important in light of the QuadFX's performance.

Performance reviews are all over the web, so we elected to peruse the information that's out there, rather than reinventing the wheel. The direct competitor to the QuadFX is Intel's QX6700, a quad core "Extreme Edition" code-named "Kentsfield". And, from the reviews, Intel should be very happy. In fact, Intel should be jumping for joy, because their Core 2 Duo processors outperformed the QuadFX on many of the benchmarks. Of course, AMD's dual core Socket AM2 CPUs also outperformed the QuadFX, too, so all is not lost.

What's the problem? Besides the chipset bloat and offboard memory controller, the QuadFX uses memory in a way that isn't particularly well supported by Windows XP. The QuadFX's implementation of non-uniform memory architecture (NUMA) causes some fairly substantial latency under Windows XP. Intel's processors and the dual core AMD processors use a unified pool of memory, which XP was designed to support. Windows Vista doesn't have this problem, and it appears that, under this OS, the difference between the QuadFX and its Intel counterpart is much smaller. Linux users should also see a much smaller performance difference.

What else? Well, choice, perhaps. You have exactly one choice of motherboard at this point in time for the QuadFX. Intel has several, and you ought to know that motherboard design makes a difference. No knock on Asus for this - their board could very well be the paragon of motherboard design, but with nothing else to compare it with, we just can't know at this time.

So what's the conclusion? A QuadFX processor set costs between $600 and $1000 (depending on clock speed). Add another $300 for the motherboard. A Kentsfield processor is about $1000, with a less expensive motherboard. A Core 2 Duo or dual core AM2 processor is even less expensive. In both the bang-for-buck and outright performance categories, it just looks like the QuadFX isn't a great deal. At this time, we have to recommend against it.


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