> Mach is not a very good microkernel at all, because the overhead is much higher than necessary. The L4 family’s IPC design is substantially more efficient, and that’s why they’re used in actual systems.

As opposed to Mach, which is not used in any actual systems

I mentioned XNU below. It doesn’t really count as a microkernel if you, you know, don’t actually use the microkernel part. At least for the 30 years between the FreeBSD collision and the introduction of DriverKit, which does most of its IPC through shared memory (because the mach ports are not efficient enough, I would assume).