[TUHS] Is there a good, even definitive, list of reimplementations of the Unix kernel? What would good cut-off criteria be?

[Paul Winalski via TUHS]

On Sun, Apr 19, 2026 at 4:02 PM John R Levine via TUHS <tuhs at tuhs.org>
wrote:

> > OSX uses a microkernel, Mach. Thing is more and more code got moved from
> user space into the kernel to speed it up. First OSF1 moved more code into
> kernel space, then NeXT, then Apple. But the kernel definitely started out
> as the Mach micro kernel.
>
> That's what almost always happens, start with a microkernel, find that all
> the context switches are killing performance so you move all the pieces
> into one place and you end up with yet another macrokernel.
>
> QNX is probably the most successful microkernel system.  It's even POSIX
> compatible.
>
> I'd say the most successful microkernel system is Windows NT, if your
measure of success is marketplace presence.

Dave Cutler was a fan of microkernels.  His real-time OS for the
VAX--VAXeln--was microkernel-based.  The 32-bit OS he designed when he went
to Intel--Windows NT--has a microkernel.  Not unsurprisingly, much of the
stuff layered on that micorkernel resembles VAX/VMS, which was designed in
a large part by Cutler.

Like macOS and many other microkernel-based OSes, a lot of stuff crept into
the kernel for performance reasons.

-Paul W.