On 5/6/2013 8:13 AM, Charles Steinkuehler wrote: > Yes, but if you've got a Mesa card, you don't gain that much on an x86 > platform. Unless you're really pushing your servo rate, rtai and > xenomai are IMHO really only necessary on x86 if you're trying to do > software stepgen. But make sure you test your board, latency numbers > for rt_preempt can vary a*LOT* based on BIOS settings and your > particular hardware (since for rt_preempt to work well, the driver > code needs to be written to work well under SMP).
and, just prior, Ebo wrote: > There are people poking at RT-PREEMPT, and depending on your exact > setup getting very good results. Like others have said, RTAI and > Xenomai get better latencies and will probably always do so because of > technical reasons of their implementation. That being said, if the > rt-preempt latencies is good enough for your application I personally go > for that -- rt-preempt is now part of the stock kernel and there is a > variant which is now supposed to provide hard real-time. Anyway, them's > my thoughts on the matter. > and, just prior to that, Michael wrote: > Yes, RTAI and Xenomai have significantly better latency behaviour than > RT-PREEMPT. I think this is relevant mostly for software stepgen setups The question of what is good enough has intrigued me since I first started reading LinuxCNC nee EMC2 documentation. For many years we have had essentially only one quantitative criterion. IIRC correctly it shows up several places but this particular quote comes from the Latency-Test page on the Wiki: " If the numbers are 100 uS or more (100,000 nanoseconds), then the PC is not a good candidate for software stepping. Numbers over 1 millisecond (1,000,000 nanoseconds) mean the PC is not a good candidate for LinuxCNC, regardless of whether you use software stepping or not." Is this still the best we can do for guidance to prospective users? I realize that details matter so it's difficult to get very specific about what is "good enough for your application" but I keep hoping we could say more. For that matter, we could say more about the impact of high jitter. Currently, we say only : "...your maximum step rate might be a bit disappointing...." If I thought I had the knowledge I would write more about the impact myself, but I don't and I haven't. MIchael and I briefly discussed the subject a year ago in the context of stepper drives and I realized after considerable Internet searching that little useful information is available (one limited calculation by Proctor et al and some postings by Mariss) so maybe it's just too hard a subject? As a practical matter, we haven't collected much latency/jitter data for the Xenomai and RT-PREEMPT kernels on different CPU-motherboard-BIOS combinations. It feels like it is time to start gathering such data in new tables on the Latency-test page on the Wiki. If I get time later today (and if the Wiki is more responsive than it was yesterday), I'll copy in my Xenomai results for several systems and Charles' RT-PREEMPT results for several other systems which have been posted elsewhere. Maybe the presence of new tables will embolden others to contribute their results. Regards, Kent PS - if new material on this subject already has been added to the LCNC documentation please point it out. My blind spots are widening with age. ------------------------------------------------------------------------------ Introducing AppDynamics Lite, a free troubleshooting tool for Java/.NET Get 100% visibility into your production application - at no cost. Code-level diagnostics for performance bottlenecks with <2% overhead Download for free and get started troubleshooting in minutes. http://p.sf.net/sfu/appdyn_d2d_ap1 _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
