Turbo and Hyperthreding gernate a ton of noise when benchmarking. Disabling both results in a one or two order of magnitude reduction in outlier results (maybe

Turbo and Hyperthreding gernate a ton of noise when benchmarking. Disabling both results in a one or two order of magnitude reduction in outlier results (maybe you want to call it noise - but it is not symmetric like classic noise). The primary way turbo introduces variance is due to the turbo transitions forced by the "max turbo ratio varies by active core count" behavior. Eg on my Skylake chip, the turbo speed is either 3.5, 3.3, 3.2 or 3.1 GHz depending on whether 1, 2, 3 or 4 cores are active. Let's say you are running a threaded benchmark with "nothing else" running on the box. Of course other cores will still occasionally fire up, to handle scheduler ticks, interrupts from you network card, background processes, whatever. Every time this happens, the chip has to immediately undergo a frequency transition which leaves it running at 3.3 GHz. The main problem isn't the lowered speed, it's the fact that the transition itself puts the chip into a halted state for 10 to 20 us, presumably to allow the multiplier transition to occur, for voltages to stabilize, etc. Especially for short, precise benchmarks, this shows up as a lot of noise. Turning off turbo fixes it, but so does just setting the max turbo ratio to the "all cores" value (3.1 GHz in my case) since then you don't have these forced transitions.