User-managed concurrency groups

That could be said about e.g. user-mode network stacks. I don't get the point myself but people are evidently getting enough mileage out of that to justify having it in Linux.

Sometimes you can't scale vertically with faster hardware or horizontally with more hardware (especially in present times), and then the only option left is to scale inwards by re-examining old software assumptions. Or scaling backwards, by asking for less - but that doesn't lead to cool hacks...