Re: RT patch acceptance

James Bruce
2005-05-31T10:59:09Z

Nick Piggin wrote:
> I have never been in any doubt as to the specific claims I have
> made. I continually have been talking about hard realtime from
> start to finish, and it appears that everyone now agrees with me
> that for hard-RT, a nanokernel solution is better or at least
> not obviously worse at this stage.

It is only better in that if you need provable hard-RT *right now*, then
you have to use a nanokernel. The RT patch doesn't provide guaranteed
hard-RT yet[1], but it may in the future. Any RT application programmer
would rather write for a single image system than a split kernel. So if
it does eventually provide hard-RT, just about every new RT application
will target it (due to it being easier to program for). In addition it
radically improves soft-RT performance *now*, which a nanokernel doesn't
help with at all. "Best" would be getting preempt-RT to become
guaranteed hard-RT, or if that proves impossible, to have a nanokernel
in addition to preempt-RT's good statistical soft-RT guarantees.

I think where we violently disagree is that in your earlier posts you
seemed to imply that a nanokernel hard-RT solution obviates the need for
something like preempt-RT. That is not the case at all, and at the
moment they are quite orthogonal. In the future they may not be
orthogonal, because *if* preempt-RT patch becomes guaranteed hard-RT, it
would pretty much relegate nanokernels to only those applications
requiring formal verification.

- Jim Bruce

P.S. Preempt-RT is a sight to behold while updatedb is running. The
difference between it and ordinary preempt is quite impressive. Nothing
currently running has so much as a hiccup, even though / is using the
non-latency-friendly ReiserFS. The only way I even notice updatedb is
running at all is through my CPU monitor and the fact that disk IO is
slower.

[1] By this I mean on a system loaded with low priority tasks doing the
relatively arbitrary things one might do on a live system.