Intel Woodcrest, AMD's Opteron and Sun's UltraSparc T1: Server CPU Shoot-out

The New Intel Platform

The biggest advantage of Intel's newest Bensley platform is longevity: the Dempsey, Woodcrest and quad-core Clovertown Xeon all use the same socket and platform.



Bensley also eliminates the shared Xeon bus by giving each CPU an independent bus running at 1333 MHz. This is somewhat similar to the old Athlon MP platform, and it should be noted that this makes the Blackford Northbridge or MCH a pretty complex chip. Blackford also offers up to 4 memory channels and 24 PCI Express lanes.



The Dual Independent Bus (DIB) will not make much difference for Woodcrest and Dempsey as only some HPC applications are really limited by the FSB bandwidth. Three years of benchmarking tell us that most server and workstation application are not bottlenecked by the modern FSB speeds. The Opteron platform does not scale so much better thanks to NUMA in dual and quad core configurations. No, in most applications, the low latency integrated memory controller makes the difference, not FSB/NUMA bandwidth. Of course, with Clovertown, or two Woodcrests on one chip, a shared FSB might become a bottleneck, and in that case a DIB is a good idea.

The biggest innovation of Blackford is the introduction of fully buffered DIMMs (FB-DIMMs). On the FB-DIMM PCB we still find parallel DDR-2, but the Advanced Memory Buffer (AMB) converts this parallel data stream into a serial one to the Blackford chip. The serial links between the memory subsystem and the chipset not only eliminate skew problems but they also greatly simplify the routing on the motherboard. Routing quad-channel DDR-2 would be a nightmare.



The AMB, which you see under the heatsink in the middle of the DIMM, solves the skew and routing problems, and it comes with a relatively small price premium. The AMB also allows full duplex operation from the chipset to the AMB, where other memory bus designs are half duplex and introduce extra latency when alternating between send and receive modes. However, the AMB dissipates about 5 Watt and increases latency. This means that with 8 DIMMs or more, the advantage of using 65 Watt Woodcrest CPUs over 89-92 W Opterons will be gone.

The Blackford chipset uses X8 PCI Express links to talk to other various chips such as the ESB-2 I/O bridge, or "Southbridge" to keep it simple. The other PCI Express links can be used for 10 Gbit Ethernet or a SATA or SAS controller. A workstation version of Blackford, Greencreek will offer dual X16 PCI Express for running multiple workstation graphic cards.