Friday May 08, 2009

Observing Intel's "Turbo Boost" feature with PowerTOP

One of the most interesting features of Intel's Core i7 and the Xeon 5500 processors (Nehalem) is the ability of the processor to go into a mode called "Turbo Boost". While most modern processors are capable of being power managed to run at various clock frequencies, "Turbo Boost" is different, as it allows the processor to autonomously run at a higher clock speed than would otherwise be available in the processor's maximum performance power management state (P0).

Here's how it works. When one or more of the cores on the processor is in the P0 (max perf) state, those cores may enter Turbo Boost mode, allowing them to run at the faster clock frequency. "How much faster" depends on how much power and thermal headroom is available, but in general, the more cores there are on the socket that are idle and power managed (via. C-states), the faster the remaining running core(s) will go. With the introduction of OpenSolaris support for Deep C-states which integrated in build 110, we're certainly seeing the effects...since the system is now readily taking advantage of the deeper C-states, turbo boost happens all the time. :)

But how does one observe this? It's certainly useful to know when Turbo Boost is happening, and how much of an "overclock" the processor is achieving. Fortunately, in build 110 Rafael Vanoni pushed some changes to PowerTOP that provides this observability:

This is a screenshot of PowerTOP running on a Xeon 5500 (Nehalem) based system. Notice in the P-states (Frequencies) column that the highest clock speed has (turbo) next to it. As turbo mode is entered, PowerTOP will track the average frequency of the system's processors over the sampling interval. You can actually watch that top end frequency fluctuate as utilization across the system changes.

Clearly, this observability is important for understanding system performance (but more importantly, for performance determinism). Very cool stuff...and yes, this will be present in the upcoming OpenSolaris release. Here's a video we did in which Rafael talks more about this...


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