Comment by eru

> Isn't that the entire point?

The exokernel people would disagree.

You see, an operating system as commonly conceived has at least two major jobs:

- abstract away underlying hardware

- safely multiplex resources

And do the above with as little overhead as possible.

Now the thing is: whenever you have multiple goals, you need to make trade-offs, and you aren't as good at any one goal as you could be.

So the exokernel folks made a suggestion in the 90s: let the OS concentrate on safely multiplexing resources, and do all the abstracting in user level libraries.

See eg https://www.classes.cs.uchicago.edu/archive/2019/winter/3310... or https://people.eecs.berkeley.edu/~kubitron/cs262/handouts/pa...

Normal application programming would mostly look the same as before, your libraries just do more of the heavy lifting. But it's much easier to swap out different libraries than it is to swap out kernel-level functionality.

That vision never caught on with mainstream OSes. But: widespread virtualisation made it possible. You can see hypervisors like Xen as exokernel OSes that do the bare minimum required to safely multiplex, but don't provide (many) abstractions.