[geofft]

Right, the claim is that no one has actually demonstrated that, though. It sounds like it should be possible in theory but no microkernel has actually done so.

One big reason, I suspect, is that very little of the work in the VFS layer touches hardware. Reads of directory structure, metadata, and data are all handled from cache whenever possible, and writes are buffered. When reading, blocks are prefetched so nearby data is in memory, and when flushing writes, the kernel optimizes and orders them to be most efficient. A library that always handles reads and writes from hardware will be slower because it actually goes to hardware, no matter how many context switches it saves. And if you can't share a read cache and write buffers across processes, you're effectively going to hardware for all initial reads and when the program exits.

(Also, context switches aren't that expensive. They're not free, sure, but it's easily possible for software costs to outweigh it.)