Sounding out OS noise
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SAGE advice
Because the impact of OS noise is “very small for systems with a few thousand processors, few studies of this sort were done in the past,” ORNL’s Geist says. “Only recently have computers been built with a hundred thousand or more processors.”
Using a trio of applications that model climate, simulate explosions and test weapons safety – POP, SAGE, and CTH, respectively – the UNM-Sandia team used the lab’s Cray-based Red Storm high-performance computing system to measure the effects of noise.
With a technique of their own invention that injects interference at the kernel level, team members discovered that an application’s noise sensitivity is strongly correlated with two factors: the software program’s communication/computation ratio and the amount of collective communications it uses as measured in bytes, an approach researchers had never before considered.
A program’s communication/computation ratio is time spent computing relative to time spent exchanging data with other programs – processes that create noise. Collective communications are network communications in which a group of nodes collaborate on an operation.
“Noise is less likely to affect applications that do a lot of computation, because computation is an independent activity,” Brightwell says. “Noise is more likely to affect applications that communicate frequently, because communication is not independent.”
In other words, the higher the communication/computation ratio, the more likely noise is to disrupt an application.
