Sounding out OS noise

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Red Storm rising

Differentiating OS noise based on frequency and duration meant designing an out-of-the-box experiment with so-called kernel level noise injection. Few researchers have tried this technique, and Ferreira says he believes it is because they “lack the unique capabilities we have at Sandia.”

Those capabilities include Red Storm, a Cray XT3/4 series machine with more than 13,000 nodes, each containing a 2.4 gigahertz dual-core AMD Opteron processor and either 2 or 4 gigabytes of main memory. They also include the Catamount lightweight kernel, an operating system specifically designed for high-performance scientific parallel computing, Brightwell says.

“ ‘Lightweight’ means it has fewer lines of code, a smaller memory footprint and supports fewer functions,” he explains.

The Catamount kernel is like a laboratory control. It has “a very low native noise signature,” Ferreira says, and it can be controlled, interrupting an application only when requested to do so.

Says ORNL’s Geist: “Catamount is widely regarded as the lowest-noise OS in the world, so it makes sense to use it in a study like this.”

Interactive Supercomputing’s Reinhardt agrees: “This is a thorough study measured with real applications across a range of system sizes and noise patterns. It gives an excellent baseline understanding of raw OS noise.”

But he adds an important caveat. “The Catamount results are very useful in telling us what problems to avoid, but we still have to avoid them in the commodity OS’s” such as Windows and Linux –“a very different and much more difficult problem.”

Byte-sized morsels

Diet gurus say to eat lots of small meals throughout the day rather than two or three large meals to keep off the weight.

Brightwell offers the same advice to software and operating-system designers: “Our work showed that parallel applications tend to be less impacted when larger tasks that run for a long time less often are broken down into smaller tasks that run for a short time more often. It’s better for the OS to take lots of small bites than one big bite.”

Georgia Tech’s Schwan also sees implications for what he calls the “common noisy OS’s,” or the commodity operating systems Steve Reinhardt finds troubling.

“These results suggest that full-scale versions of Linux or Windows are unlikely to scale to petascale – or beyond – unless they are changed substantially,” Schwan says. “The results also suggest the continuing importance of micro-kernels or lightweight kernels for high-performance computers.”

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