Verdict: Who wants quad-core when you can have octo-core?
Although the availability of low-cost quad-core CPUs, such as the Core 2 Quad Q6600 has forced dual-core CPUs into the mainstream, anything beyond four cores remains elusive. Even the clever elves in Intel's fabs haven't made a retail-ready eight-core, single-package CPU yet. For an octo-core PC, you need two quad-core CPUs on one motherboard. This means using grown-up Xeon processors, rather than kiddie-toy Core 2 Quads or Xeon-wannabe Core 2 Extremes.
Motherboards that can house two quad-core CPUs have been available for over a year, but the launch of the Intel Xeon 5400-series makes dual-processor setups an attractive proposition. Xeon 5400-series CPUs are based on the awesome 45nm Penryn architecture, making them fast and energy efficient, with a massive 400MHz (1,600MHz effective) dual FSB. However, older motherboards based on the 5000-series chipsets don't support them. The first board to support Xeon 5400-series CPUs is the X7DWA-N, from SuperMicro.
Confusingly, the core logic chipset at the heart of the X7DWA-N is also known as the Intel 5400. However, the 5400 chipset is more than just a speed bump of the 5000-series. For starters, it provides two 16x PCI-E 2.0 slots to support two graphics cards in CrossFire. Its memory controller also supports FB-DIMM modules at speeds of up to PC2-6400 (800MHz), rather than the PC2-5300 (667MHz) FB-DIMMs of the 5000-series chipsets. A motherboard based on the 5400 chipset will therefore offer its CPUs considerably more memory bandwidth than a 5000-series chipset motherboard.
However, the most interesting feature is the Snoop Filter, which aims to eliminate the snooping of one CPU's cache by the other. Without a Snoop Filter, if CPU 0 wants to access data held in CPU 1's cache, it sends a request via the Northbridge, which passes it on to CPU 1. This causes CPU 1's cache to stall as the data is retrieved and sent back via the Northbridge to CPU 0. Snooping uses FSB bandwidth and can stall both processors, making it very inefficient. The Snoop Filter aims to eliminate unnecessary snoops by using its 24MB of memory to track data stored in each CPU's Level 2 cache, so that if CPU 0 needs data stored in CPU 1, it can get it from the Snoop Filter, instead of snooping CPU 1.
As well as two 16x PCI-E 2.0 slots, the X7DWA-N has two 64-bit 133MHz PCI-X slots and two PCI slots. The E-ATX-sized board also has eight FB-DIMM sockets, so it supports quad-channel RAM if you fit at least four sticks. It can handle up to 64GB, which should induce drooling in 64-bit OS users. It also has two Gigabit LAN ports, although Vista doesn't currently support them, as well as the usual S-ATA II ports, USB 2 ports, FireWire ports and mini-jacks for its eight-channel Intel HD Audio.
With a pair of 3.2GHz Xeon X5482 CPUs fitted, the X7DWA-N screamed through every benchmark, scoring an incredible 2,225 in our video-encoding test. It demolished the fastest AMD Opteron system, a pair of 3GHz Opteron 2222s, which scored only 1,189. As such, the X7DWA-N was the fastest non-overclocked PC we've tested.
To find out how much difference the Snoop Filter makes, we also tested the X7DWA-N with a pair of slower 3GHz Xeon X5365 CPUs (with 1,333MHz effective FSBs). We compared these results with those of the same 3GHz CPUs installed in an older SuperMicro X7DAE motherboard based on the Intel 5000X chipset (and hence with no Snoop Filter). There wasn't much of a performance difference (1-3 per cent) but the 5400 chipset's new features gave the X7DWA-N a slight edge over the X7DAE. Sadly, like most LGA771 Xeon boards, the X7DWA-N has no overclocking options in the BIOS, although you should be able to socket-mod slower Xeons.
Conclusion
Unlike AMD's dire new Opteron CPUs, the Xeon 5400-series CPUs are awesomely fast and efficient. They're compelling for a new workstation, server or monster folding rig, despite the price premium. The X7DWA-N is no more overclockable than any other LGA771 motherboard, but it has its advantages, such as support for PCI-E 2.0 graphics cards, faster FB-DIMMs and a Snoop Filter. It also offers a tantalising glimpse at SkullTrail, which will use a modified 5400 chipset and Xeon 5400-series CPUs when it launches in early 2008.
