[newhouseb]

maxcpus needs to be set at boot time. You can turn CPUs on/off logically at runtime but that's not equivalent to "take this CPU out of the default scheduler rotation", it's equivalent to "power it off, I may unplug the entire CPU and swap it out". The latter would seem fancier than the former, so not sure why there isn't the means to set maxcpus at runtime, but there doesn't appear to be.

The author does gloss over the fact that if you have a hyperthreading enabled processor, you want to make sure you either turn it of or don't schedule a pair of processes in different virtual cores on the same logical core (hyperthread pair), because you will see a slowdown because the two virtual cores are actually sharing boatloads of state.