On 11/4/2013 10:01 AM, EBo wrote: > On Nov 4 2013 8:44 AM, Michael Haberler wrote: >> Am 04.11.2013 um 16:30 schrieb EBo <e...@sandien.com>: >> >>> >>> What hard real-time ARM kernels have been found suitable for >>> LinuxCNC? >> >> The only one I found both to have some build support for raspberry >> and beaglebone, and which works fine is Xenomai; the mailinglist >> should be helpful, and they have rather good docs >> >> I am not aware of any progress on the RTAI/ARM front >> >> I did hear someone was working on a RT-PREEMPT build for the >> beaglebone, you might want to check the #beagle group on freenode >> >> all this has been discussed here a while back, you might want to look >> through the archives - the point really was: if there's a Xenomai >> platform patch available, there isnt much point redoing the exercise >> for RT-PREEMPT > > Good to know. I will follow up on these are soon as I get a chance > (which is likely a couple of weeks). > > If RT-PREEMPT works for my needs I would prefer to go with it as it is > supperted directly in the kernel and does not require patching. The > real question is when is good enough -- which will always be open to > debate. > > Anyway, I'll try to report back with what I find.
RT-PREEMPT kernels rely on good quality driver code written for high-performance SMP. Results vary based on the hardware you're running, and are generally pretty good on x86 machines and not so good on ARM (where the driver code hasn't had as much time to mature in SMP environments). Xenomai preempts the whole kernel, so doesn't really depend on the hardware and provides the best latency numbers on the ARM. At the start of the LinuxCNC on BeagleBone project, the RT-PREEMPT kernel had _really_ lousy latency numbers (measured in mS!), but that has improved dramatically since then. There's an OSADL test system running a 3.2 kernel (the BeagleBone needs 3.8) on an AM335x board that shows what's possible, but Xenomai still provides better numbers: https://www.osadl.org/Profile-of-system-in-rack-7-slot-5.qa-profile-r7s5.0.html For comparison, typical latency on the Xenomai kernel is around 25 uS with worst case around 75-80 uS. -- Charles Steinkuehler char...@steinkuehler.net
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