Wolfgang Grandegger wrote:
On 10/12/2005 04:39 PM Philippe Gerum wrote:

Wolfgang Grandegger wrote:

We have linux-2.4.14-rc3 running on all AMCC eval boards (see
http://www.denx.de). But the kernel supported by RTAI/Fusion,
linuxppc-2.6.10rc3, does not boot on Ebony. The main problem is the
missing support for U-Boot but there might be others. And it's simply
not worth the effort to port it, I think.

Open question: to your opinion, is 2.6 on low-end embedded hw doomed "by design" and why, or do you think that part of the reluctance to move to 2.6 is mostly explained because 2.4 is just fine and up to the task, IOW it's kind of a "don't fix if it ain't broken" perception?


As Wolfgang (Denk) already pointed out, 2.6 is less attractive on low
end systems, because it's bigger, slower, ... This is also true for
Xenomai (RTAI/fusion). It's difficult to beat the latency value of the
old RTAI/RTHAL under 2.4. You need more CPU power and resources, that's
how thing are going. Nevertheless, compared to the realtime preemption
patch, Xenomai is _lightweight_ :-).

I think so too; that's the problem with strictly native real-time support in the kernel: you must end up with some kind of SMPish structure which virtually exhibits an infinite number of processors (one per task basically), so it's not going to help reducing the cpu footprints and the various noisy artefacts implied by the generalized mutex approach (which is otherwise sound, that's no the issue). This is also why there is still space for real-time extensions, provided - I think - they run as symbiotically as possible with Linux, so that we don't end up telling people to ignore they have Linux while running their apps over it.

As far as Xeno is concerned, we should be able to continue to reduce those footprints. From my window, I see two aspects we need to work on:
- impact of the Adeos pipelining on cache especially for hw with sluggish
memory bandwidth
- a better placement of the hot data that are accessed inside the fast interrupt path (mainly those of the scheduler).

Looking at the ppc figures since early 2005 or so, the raw latency has continuously been reduced, i.e. we went from ~120 us on a Freescale's Icecube running the user-space test, to 53 us as measured recently with 0.9.1+r8c4. I did not manage to check again on the Sandpoint (connection problem to the Vlab) which is very representative of the low-end hw issues we could face [and basically made me cry when I first looked at the latency reports], but I suspect that thing might have progressed there too. I've recently ported 0.9.1 over a Mvista kernel (experimental PREEMPT_RT-like stuff + other patches) on a mpc8541, and the figures for user-space are ~22 us worst-cast lat. Of course, this is not what one would call a sluggish low-end hw and I agree that a more structured design like Xeno can't beat a flat ISR-based design, but still, in any case, I'm optimistic enough to think that we likely have a margin of improvement there.


Furthermore I think, that part of the reluctance is also due to
"development in progress" including features like the realtime
preemption patch, especially on embedded PowerPC systems. People are
waiting that things get available and stable.


Well, we might all have the same problem here...

--

Philippe.

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