Dual-Core Servers: AMD or Intel?

AMD and Intel use different design methods for the motherboard chipset, and this further separates the two products in terms of performance. The Tyan/AMD motherboard shown in the table above uses a combination of Broadcom's HT-2000 and HT-1000 chips. Meanwhile, the Tyan/Intel motherboard uses Intel's 5000V Memory Controller Hub (MCH) and 6321ESB I/O Controller Hub (ICH).

Why are these chips important? Let's take a closer look at how they affect the performance of our server.

First, the AMD platform. The HT-2000 chip is a highly integrated device, bringing onto one chip all of our system interfaces: Gigabit Ethernet, PCI-Express, PCI-X, and HyperTransport.

This integrated HyperTransport interface is an essential part of AMD's Direct Connect Architecture, which permits communications between the processor and the HT-2000 at full CPU speed. In other words, as CPU clock speed increases, so does available bandwidth. This means faster performance without bottlenecks, up to 24 GB per second, or 3.0 GHz CPU clock speed. The HT-2000 chip can be thought of as the system's Northbridge chip, since it performs the same type of duties.

AMD's HT-1000 chip functions as the Southbridge chip, handling I/O such as SATA, USB, floppy-drive controller, parallel ATA, etc. The HT-1000 is connected to the HT-2000 seamlessly via HyperTransport, for high performance without bottlenecks in bandwidth.

The bottom line is higher efficiency in inter-chip communications, since there are no bottlenecks with the HyperTransport's point-to-point link. Data transactions flow more efficiently from storage I/O to memory to processor and back again, creating high throughput overall.

Next, the Intel chipset. Intel uses a different technique for inter-chip communications on the motherboard. As mentioned above, the Intel system uses a split controller for the memory, with the 5000V MCH handling the clock source and serial interconnect to the AMB chips on each of the FB-DIMMs.

Most I/O transactions occur across the FSB, which is a shared parallel bus. This limits available bandwidth as compared to the direct chip-to-chip HyperTransport links found in the AMD system design. It also creates the potential for resource contention when multiple resources are competing for bandwidth.

Each processor core inside the CPU chip has access to its own bus--what Intel refers to as Dual Independent Bus (DIB) architecture. But overall system-level performance is impacted once I/O leaves the CPU and tries to move data across the motherboard to other chips.

The Intel 6321ESB chip serves as the Southbridge for this arrangement. It is connected to the 5000V MCH chip via a PCI-Express interface and a proprietary ESI interface, which provides adequate bandwidth. The 6321ESB is analogous to the HT-1000 in the AMD system, since it also provides SATA, USB, PCI, PCI-Express, etc.

With Intel's design, the chipset depends on keeping the processors' large shared L2 caches full. It scales performance by raising the clock speed of the CPU chip itself. This is one explanation for Intel's release of Xeon chips with much higher clock speeds than AMD's competing Opterons. Performance falls off once data and instructions leave the CPU core and enter the realm of inter-chip communications across the FSB.

CONCLUSIONS

So, in an apples-to-apples comparison, with all the technical complexities aside, here's the bottom line: Intel dual-core servers are more expensive to build than those based on AMD. What's more, the costs rise as more memory and faster CPUs are added.

The fact that Intel Xeon servers have an advantage with higher CPU clock speeds actually increases the size of this hurdle. With the benefits of FB-DIMM memory technology not yet fully realized, this drawback is further compounded.

Also, the increased power consumption, cost and heat dissipation of the FB-DIMMs do not bode well for Intel dual-core server system efficiency when compared with AMD Opteron-based machines.

The final nail in the Xeon's coffin, in my opinion, is the fact that inter-chip communication performance encounters bottlenecks instead of continuing to scale upward, as it does on the AMD architecture.

In the absence of software that is highly optimized specifically for the new architecture of Intel Xeon multi-core processors, AMD holds the advantage in both performance per dollar and performance per watt of power expended.

Evidence of this is visible in the increased market share that AMD Opteron servers have been achieving since AMD's entry into the server market some time.

But this is a game of high-tech leap-frog. Both companies have already introduced quad-core server chips. Only time will tell who will field the ultimate server when the next round of product releases is announced.

DAVID GILBERT is the owner of Appalachian Computer Systems, a West Virginia system builder that specializes in multiprocessor SCSI RAID servers.