A simple hierarchy: when there is deadline work to do that work takes precedence, fair work gets the rest of the time. This is effective because deadline work has bounded execution time, whereas fair work is elastic and can adjust to use the available bandwidth.
It's my perception that on current workstations, for example, that and "ionice -c3" in front of any build I do is far more useful then simply nicing it.
IO scheduling is a separate problem, though it shares the same fundamental properties. The same goes for network packet scheduling. All of these are ongoing efforts. Stay tuned! :)
Two of my primary areas of interest, audio engineering and VR both REALLY show how poorly modern operating systems do with user interactivity when it counts.
