Nicholas Mc Guire wrote: >> Nicholas Mc Guire wrote: >>>>>> well thats true for ADEOS/RTAI/RTLinux as well - we are also only >>>>>> black-box testing the RT-kernel - there currently is absolutley NO >>>>>> prof for worst-case timing in any of the flavours of RT-Linux. >>>>> >>>>> Nope, it isn't. There are neither sleeping not spinning lock nesting >>>>> depths of that kind in Xenomai or Adeos/I-pipe (or older RT >>>>> extensions, >>>>> AFAIK) - ok, except for one spot in a driver we have scheduled for >>>>> re-design already. >>> >>> that might be so - never the less there is no formal-proof that the >>> worst >>> case of ADEOS/I-pipe is X-microseconds, the latency/jitter numbers are >>> based on black-box testing. In fact one problem is that there are not >>> even >>> code-coverage tools (or I just did not find them) that can provide >>> coverage data for ADEOS - thus how can one guarantee worst-case ? > >> The fact that tool support is "improvable" doesn't mean that such an >> analysis is impossible. You may over-estimate, but you can derive >> numbers for a given system (consisting of real-time core + RT >> applications) based on a combined offline system analysis and runtime >> measurements. But hardly anyone is doing this "for fun". > > > with the current status I don't think a off-line analysis is resonable > I don't think a model of ADEOS is resonably duable, alteast not a > modleing that would lead to any usable results - I might be wrong - do > you know of any such successfull approaches ? All testing is really > inherently limited, from black-box testing you simply don't get any > guarantees.
We are no longer black-box testing - thanks to our "KFT". I'm trying to advertise this model heavily to users, but it still requires a bit too much system knowledge. Still, modelling a system of I-pipe + Xenomai remains an open challenge AFAIK. > > <snip> >>> its not a corener case demonstration, Ive been doing benchmarks on rt >>> preempt now for quite some time, there is still an advantage if you run >>> simple comparisons (jitter measurements) - but it is clearly going down, >>> The problem I have with RT-preempt being 50us and ADEOS is 15us is >>> simply that the sector that does need those numbers that RT-preempt will >>> most likely >>> never reach is generally interested in guaranteed times, and thats where >>> it becomes tough to argue any of the hard-realtime extensions at this >>> point - that is not saying RT-preempt can replace ADEOS/RTAI/RTLinux-gpl >>> Im just saying that the numbers are no longer 2/3 orders of >>> magnitude,which they were in 2.2.X/2.4.X and where arguing the use was >>> simple. > >> Granted, arguing becomes more hairy when you have to pull out low-level >> system details like I posted (and not discussing individual issues of >> certain patches). There are scenarios where I would recommend -rt as >> well, but so far only few where RT extensions are fitting too. > >>> >>> Don't get me wrong Im not trying to argue away ADEOS/RTAI or I would >>> have given up RTLinux/GPL quite some time ago - but I belive if these >>> low-jitter/latency systems want to keep there acceptance in industry a >>> key issue will be to improve the tools for verification/validation - > >> Ack, and I'm sure they will emerge over the time. I don't expect this to >> happen just because someone enjoys it (adding features is always >> funnier), but because users will at some point really need them. It's a >> process that will derive from the steadily growing professional user >> base in both industry and academia. > > let see - I hope you are right - I'm just starting into a FMEA/HAZOP for > XtratuM "for fun" ;) Will be interesting to hear/read about practical experiences. > >>> >>> <snip> >>> >>> THAT is a problem in arguing for ADEOS/I-pipe - WHAT is the worst case >>> now ? what is the cause of the worst case ? and can I really demonstrate >>> by strong evidence that the worst case on this system is actually XXXX >>> microseconds under arbitrary load and will not be higher in some strange >>> corner cases ? > >> Leaving the completely formal proof aside (that's something even >> microkernels still cannot provide), you may go to the drawing board, >> develop a model of your _specific_ system, derive worst-case >> constellations, and trace the real system for those events (probably >> also stimulating them) while measuring latencies. Then add some safety >> margin ;), and you have worst-case numbers of a far higher quality then >> by just experimenting with benchmarks. This process can become complex >> (ie. costly), but it is doable. > >> The point about co-scheduling approaches is here, that they already come >> with a simpler base model (for the RT part), and they allow to "tune" >> your system to simplify this model even further - without giving up an >> integrated non-RT execution environment and its optimisations. We will >> see the effect better on upcoming multi-core systems (not claiming that >> Xenomai is already in /the/ perfect shape for them). > > >> However, if you have suggestions on how to improve the current tool >> situation, /me and likely others are all ears. And such improvements do >> not have to be I-pipe/Xenomai-specific... > > well one thing Im looking into for RTLinux is to extend things like > kernel GCOV into RTLinux and KFI/KFT to RTLinux as this allows much > better assessment. I guess that those extensions would equally be worth > while > for ADESO/I-pipe/Xenomai. > > refs: > > KFT www.celinuxforum.org/CelfPubWiki/PatchArchive last one for 2.6.12 > GCOV-Kernel part of LTP now (last one is for linux-2.6.16-gcov.patch.gz > [Quick glance at GCOV patch] Hmm, the thrilling thing is typically locking, but I don't see a single spinlock, just some semaphores that cannot be called from arbitrary contexts anyway. Hmm. Did you already played with it for some kernel? Regarding KFT: we have such thing already. Partly derived from Ingo Molnar's work, but with less impact during freeze, the function tracer is in I-pipe since more than a year. It's heavily used (at least by the core team) for application and kernel debugging, and for latency spotting of course. Available for most I-pipe archs, even for the latest x86_64-WiP. The funny thing is that even RTAI could make use of it - if they only realised that it's in their patches. Next to come (yeah, long announced) is LTTng support, i.e. patch and front-end extensions for Xenomai. There is a working version lying around somewhere in Canada, I just need to kick the guy again who did that work for his thesis so that he roles out a release and we can start discussing the patch integration. Good to be reminded... So there is definitely not nothing - but surely still enough to do :). If you see some potential in cooperating on front-ends (given that you still seem to head for your own kernel-patch path), let us know. I guess there should be common ground. Jan
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