Re: RT patch acceptance

Nick Piggin
2005-05-31T01:22:33Z

James Bruce wrote:
> Nick Piggin wrote:
>
>> Sorry James, we were talking about hard realtime. Read the thread.
>
>
> hard realtime = mathematically provable maximum latency
>
> Yes, you'll want a nanokernel for that, you're right. That's because
> one has to analyze every line of code, and protect against introduced
> regressions, which is almost impossible given the pace that Linux-proper

Thank you, James. Now please tell that to Bill. It would seem
that I haven't written enough "RT media apps" for him to take
me seriously ;)

>
> If you look at your first two messages in this thread however, you seem
> to be offering a nanokernel approach (in particular RTAI as suggested by
> Cristoph) as an alternative to the RT-patch. This is sort of confused
> by the fact that Ingo called it "hard realtime" because he measured a
> maximum latency during a stress test. Unfortunately that's not really
> hard realtime if you are just measuring it; Rather its "really damn good
> soft realtime". An analysis of code paths could be done to determine if
> something really does satisfy hard-RT constraints, but to my knowledge
> that's not on the table at this point. So you're discussing soft
> realtime if you're dicussing the RT patch.
>

No, I clarified the point that the direction the RT people want
to go in is hard-realtime in the Linux kernel.

I'm very well aware of what the actual current PREEMPT_RT patch is,
and I was never talking about that particular patch.

> So its really just a misunderstanding; Nanokernels certainly still have
> a place for some applications even with the RT patches applied (Ingo has
> said as much). However expecting audio applications such as Jack to
> have to use RTAI is kind of silly, and would end up annoying the authors
> of both (I'm sure the RTAI people have better things to do than support
> ALSA drivers in RT mode).
>

Yes, Jack is more of a soft realtime application, and in that case
Linux supports it already today (although perhaps not very well -
something the RT patch aims to improve).

[snip rest]

>
> I really hope we understand each other now, but if not I guess it wasn't
> to be. Hopefully someone got something out of reading this discussion,
> but I won't be posting on this branch of the thread anymore either.
>

It seems that you do understand my position now, yes.
I'll try to refrain from posting further, too.

Nick

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