When Intel first unveiled its "Silverthorne" microarchitecture in 2008, it was clear that the low-power, in-order processors based on it (i.e., Atom) would be at the very bottom of the performance heap. It's fair to say that none of us in the tech press ever expected to see a server based on the design, much less one that would be aimed directly at Intel's high-end Xeon line. But we weren't the only ones watching the Silverthorne announcement.

SeaMicro is a Silicon Valley hardware startup that began work in July 2007 on a server that could gang together cheap, low-end processors like Atom in ways that would make sense for Web-centric server workloads. The result of that effort was unveiled today in the form of the SeaMicro SM10000, a datacenter server that squeezes 512 Atom processors into 10U of space and draws 2KW of power.

SeaMicro claims that the server provides the same SPECINT performance as a Dell Xeon server in one-fourth the space and at one-fourth the power. Its biggest advantage is that it runs existing x86 software, from the disk images on each node up to load-balancing and automation packages.

How it works

We've been over and over the economic case for the kind of server architecture that the SM10000 represents, so there's no need to recap that here (check "Further Reading" below for more on "physicalization"). Instead, let's take a practical look at how the SM10K works.

Each server node consists of one Atom Z530 chipset (Atom CPU plus I/O hub), a small pool of DRAM, and a special ASIC chip that attaches to the I/O hub's PCIe bus. This ASIC is what makes the SeaMicro idea work, and it does so by pretending to be a standard complement of PCIe-based storage and networking controllers.

So while the front side of the ASIC attaches to the Atom chipset via PCIe, the backside of it attaches to a high-bandwidth, proprietary bus that connects the invidual server nodes to each other and to a shared pool of storage and network interfaces. The ASIC does a bit of sleight-of-hand for the Atom node that it's attached to, making it appear as if the OS instance that's running on that node is directly looking at and talking to the storage and networking controllers; in reality, those controllers are being shared by all of the Atom nodes in the system.

In other words, the ASIC implements a kind of I/O virtualization in hardware.

By removing all of the storage and I/O controller hardware from each server node in order to centralize it in one place in the system, and by leaving a single ASIC at the node as the kind of ghost or avatar for that (now centrally located) controller hardware, the SM10K dramatically reduces the size of each individual server node. The picture below shows a single board that packs eight server nodes in a 5" x 11" space.

SeaMicro's approach makes for an incredibly low cost per node, both in terms of bill of materials and power consumption. On some kinds of workloads, like a Web application that features transient bursts of lightweight threads, the Atom-based server should indeed deliver better performance per watt and per dollar than a Xeon server running a bunch of virtual machines.

Right now, SeaMicro only offers a product based on Atom, which the company claims is head-and-shoulders above the dual-core ARM A9 parts it tested in performance per watt on server workloads. But there's nothing stopping the company from offering ARM-based servers, since the basic design should theoretically work with any chipset or SoC that talks to peripherals over PCIe.

What will really rock this design is Intel's upcoming Tunnel Creek SoC, which is an Atom-based SoC that has a PCIe controller on the main die, instead of in an I/O hub. A successor to the SM10K based on Tunnel Creek could do away with the I/O hub entirely, cutting down the number of components in each node to just an SoC, some DRAM, and the ASIC.

If SeaMicro moved to two SoCs per node in this case, they could instantly double the number of Atom cores in their design without increasing its size. Absolute power consumption would go up, of course, but so would power efficiency (the lack of an I/O hub chip would mean fewer wasted watts).

It's likely that SeaMicro is only the first of a barrage of similar startups that we'll see in the coming years. Plenty of system architects across Silicon Valley think in exactly these terms, and SeaMicro's exit from stealth mode shows that the approach is finally making the transition from a popular "what if..." notion and a few one-off products to an actual movement.