InformationWeek

In a NUMA (Non-Uniform Memory Architecture) system, a server's multiple processors share the same memory, just as they do on more traditional systems, but that memory is spread among several component boards with high-speed interconnect building blocks and software. The result: Processors access memory both near and remote while retaining linear scalability. By contrast, when traditional SMP (symmetric multiprocessing) architectures add processors, they sacrifice the amount of memory that can be placed on the bus. That, in turn, slows the server's overall performance.

NUMA vendors such as Data General, Sequent Computer Systems, and Silicon Graphics are playing up the distinction between NUMA and traditional SMP, and are winning over some IS managers. "I see NUMA replacing SMP," says John Hnanicek, CIO of Hollywood Entertainment Inc. in Portland, Ore. "What I see is the simplicity of SMP combined with the scalability of MPP [massively parallel processing]."

Now that NUMA systems are in production at customer sites, competitors Hewlett-Packard and Sun Microsystems-which don't offer NUMA products-are countering with detailed explanations of why they think NUMA is unsuitable for commercial applications.

Fortunately, when the sniping over ba se architectures and underlying technology is over, customers will still be able to choose scalable servers based on price and performance. "There's a trade-off with every architecture," says Michael Goulde, a senior analyst with Patricia Seybold Group in Boston.

NUMA isn't the first architecture that aims to improve on SMP systems. Both MPP systems and clusters aim to overcome shared memory bus limitations by moving to a shared-nothing architecture. But clusters are complex, and software residing on an SMP system has to be rewritten before it can run on a cluster. Despite the interest in clusters driven by Windows NT, Unix customers prefer SMP's simplicity. For some Unix customers already operating at an enterprise level, NUMA is a viable way to extend the scalability of SMP.

Hollywood Entertainment is an early customer of NUMA Q systems from Sequent Computer Systems, in Beaverton, Ore. Hnanicek invested in two NUMA Q systems because he expects them to expand with his rapidly growing business. Hollyw ood is opening a video rental store a day, and Hnanicek oversees nightly updates from 1,000 stores. So far, Hollywood caps out at 12 processors on one of the systems, but Hnanicek expects that to grow. It's early, but NUMA systems are in production, and customers are enjoying the benefits. Hnanicek also knows the limitations of other Unix systems. "I was completely Unix-based in 1991, and over the years I became frustrated with the diminishing returns of SMP," he says. "With NUMA, the attraction is linear scalability."

Is There A Need For NUMA?
Still, HP and Sun claim their work on SMP scalability is obviating any need for NUMA systems. Customers can listen in on the debate, but Goulde of Seybold reminds them that price/performance is still the most important factor in choosing a server platform.