powerd++: Even Better After EuroBSDcon

To create my slides for EuroBSDcon 2016, I produced a number of graphs to illustrate the positive effects of using a low pass filter to mitigate the noise problem powerd is so suceptible to. Load recording and replay were the critical tools I developed to show this. A side effect of being able to perform load replays was the ability to create and test arbitrary loads.


One such test I devised, was a simple linear growth in load over time. This test revealed an effect that I was aware of, but underestimated, because the utility of filtering the load overshadowed its negative impact. The following plots are based on completely noise free data, so the effect is clearly visible:

powerd++ 0.1.x load/freq plot

The plots show load and clock frequency over time. The load (abs) plot shows the absolute load, the load (rel) plot the load in relation to the current clock frequency. The latter is what powerd++ measures. And you can see that the output signal, the clock freq plot, is ringing.

powerd++ overshoots its load target and actually takes the clock frequency down for two notches, while the real (absolute) load is still rising.

What is happening here is that the low pass filtering creates a feedback with the sampled load. E.g. a sampled load of 0.8 at 800 MHz and 0.4 at 1600 MHz would be combined into a load of 0.6, ignoring that 0.8 at 800 MHz are just 0.4 at 1600 MHz. So a stable load gets rated as a load increment.

An Absolute Measure of Load

There is a simple solution to this issue — changing the way load is measured from a relative to an absolute scale. If you look at the above plot you might notice, that a unit for such a scale is readily available to us, the MHz scale on the right.

Take the relative load, multiply it with the current clock frequency and it results in the absolute load — just in the unit MHz instead of a fraction.

That means the 0.2.0 release of powerd++ will measure load as million CPU cycles consumed per second. The following plot shows, it has the desired effect:

powerd++ 0.2.0 load/freq plot

The ringing has completely been eliminated.

Another change that slipped in with the same commit is that the load sample buffer is initialised with the target load, so the algorithm has some bias to stay at the original frequency, when powerd++ starts.