I'd like to see a clone of the QNX kernel, which is very tiny, implemented in Rust. It has no drivers or file systems in kernel space; those are all user processes. But it offers a POSIX-like API. Unlike L4, you don't have to run another OS on top of it to do anything.
This would be good for embedded applications on Raspberry Pi sized machines, where the Arduino environment is too weak and Linux is too much.
Aren't there patents protecting it? I remember reading something about the fast interprocess call mechanism... Microkernels are prone to high overhead because everything requires context switching. Maybe this isn't true anymore.
US 8,434,086, "Process scheduler employing adaptive partitioning of critical process threads", is still in force until 2030. That's a more recent feature for hard real time, where you want to guarantee that some thread gets, say, 2ms of CPU time every 10ms of elapsed time. Few schedulers do that sort of thing at such fine granularity, but that's what you may need if you're controlling some actuator. Good for things like audio players, too. It's not a commonly used feature, and went into QNX some time after 2005. I remember it being listed as a future feature back then.
The fast interprocess call mechanism is what makes it all go. It's mostly about it being simple. Mach tried to get too cute, moving pages from one address space to another. Turns out that copying is usually cheaper than stalling out the CPU and flushing caches while you mess with the page tables of recently active pages. L4 just offers shared memory, dumping the problem on the next level up. QNX offers useful send/receive message functionality that goes fast. Yes, there's a penalty, maybe 20%. But you usually get that back because you're not forced to do interprocess communication with something far slower, like HTTP.
QNX has been around for a long time. Back in the mid-90s I did some consulting work porting some Unix utilities to QNX, and if there were patents back then, they have now expired.
This would be good for embedded applications on Raspberry Pi sized machines, where the Arduino environment is too weak and Linux is too much.