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". >>>> The essence for me is that with >>>> the work in 2.6.X I don't see the big performance jump provided by teh >>>> hard-RT variants around - especially with respect to guaranteed worst >>>> case (and not only "black-box" results). >>> >>> Could it be a bit too enthusiastic to base such an assessment on a >>> corner-case demonstration? > > 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. > just take this discussion - it started out with: > > <snip> >> RTD| -1.585| 7.556| 16.275| 0| >> -1.585| 16.275 > > Latencies are mainly due to cache refills on the P4. Have you already > put load onto your system? If not, worst case latencies will be even > longer. As pointed out earlier in this thread, those numbers doesn't tell much without appropriate load and a significant runtime. We are maintaining documentation on this in Xenomai, but it may be too tricky to find. And as always, such a test only represents one simple snapshot. At least you have to redo this on the target hardware with all peripheral devices in use. > > <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... Jan
signature.asc
Description: OpenPGP digital signature
_______________________________________________ Adeos-main mailing list [email protected] https://mail.gna.org/listinfo/adeos-main
