You could do worse than Mach ports

[ibotty]

> :I know you said you didn't want to go all the way to Mach ports for
> :the messaging, but... you could do a lot worse.
> [snip]
>     Well, I used mach messaging long ago on the NeXT machine.  The basic
>     problem with mach messages is that they are 'heavy weight'.  The
>     messaging system has far too much knowledge about the information
>     being sent, and it presumes fairly expensive memory mapping operations
>     which I believe can be avoided.

have you considered using (or maybe only borrowing ideas from) l4?

l4 is a microkernel api, that tries to do 'super-fast local IPC' (quoted
from [1]) and (to my understanding) maps to the light weight messaging, you
propose.

there is a sample implementation of the current (X.2) api codenamed
pistachio [2]

afaik, it does implement threads, so you may not be happy with it, but i
think it is worth at least a second (and a first, if you were not aware of
it) look.
btw: i recommend the whitepaper, it makes things short ;) [3]

there exists a linux port (linux running in one l4 process only though)
guys over at hurd seem to also like this project.
i've also seen someone mentioning a netbsd port, but only once, so it may
not be planed currently.
l4 is performant too, according to some benchmarks.

references:
[1] http://www.l4ka.org/
[2] http://www.l4ka.org/projects/pistachio/
[3] http://www.l4ka.org/projects/pistachio/pistachio-whitepaper.pdf

yours,
~ibotty
(looking for an os, that is message based and runs on his pc)