Priorities
In addition to the kernel’s per-process CPU and I/O priorities, it would be nice to have memory residency priorities. That way, we could hint the kernel into keeping proportionally more pages of latency-sensitive desktop processes in RAM - like the main menu, the taskbar, the file manager and maybe some applets. Disk cache could have its own priority, a la the swappiness setting in /proc/sys/vm/swappiness. It could be a practical* way to mitigate the “my main menu takes several seconds from click to paint” problem.
* Since I’m not a kernel dev, I’m just guessing here.