On Sat, 5 Jan 2019 14:07:21 +0100, Dr. wrote in message <201901051407.21071.dr.kl...@gmx.at>:
> Am Samstag, 5. Januar 2019 schrieb Arnt Karlsen: > > On Sat, 5 Jan 2019 04:13:22 -0500, Steve wrote in message > > <20190105041322.13e04...@mydesk.domain.cxm>: > > > > > Hi all, > > > > > > Someone asked whether you can run a 3.4.9 rtai kernel on Devuan. > > > So I'm asking here. > > > > ..why an old a 3.4.9 rtai kernel and not e.g. > > linux-image-4.18.0-0.bpo.3-rt-amd64, now at version 4.18.20-2~bpo9+1 > > with PREEMPT, RT etc? > > > > > > PREEMPT-RT gives absolutely horrible latency figures on older > hardware and amd64 (this is from a T60): RTAI ~ 2000ns, RT ~ 150000ns > on the same hardware, with latency spikes now and then of up to 5 > seconds (again, depending on individual hardware, even seamingly > identical systems do not give consistent results). The latest > linuxcnc-patched PREEMPT still gives ~ 20000ns on the same hardware, > that's ~ 10 times worse than RTAI. Additionall, when running 2 > realtime threads, RTAI gives a reliable latency over different > baseperiods. On Linuxcnc that means that latency of the servothread > at 1000000ns and the basthread of ZZZ ns (default 25000, but usually > you adjust it to be just 20% longer than the measured latency) are > independent. With PREEMPT that's not the case: Latency spikes > erratic, depending on the ratio servoperiode to baseperiode and the > airpressure in the vally of the kings. when baseperiode 25000 gives a > latency of 10000 then on RTAI you could savely run a baseperiode of > 12000. On PREEMPT you most likely will lock your system doing the > same. Even worse, enlarging the baseperiode to 30000 does not > guarnatee that the latency stays the same or gets better, just on the > contrary: it can spike up to 100000, and a baseperiode of 50000 might > give again 10000 latency. This behavour is most irritating and points > to a well hidden bug just waiting to suck your blood and time or ruin > your parts. If you get a RTAI kernel working for your system, you > will definitly want to stick with it. With PREEMPT you are on the > "interesting" side of live. ..there is _some_ improvement on the PREEMPT-RT side: http://linuxgizmos.com/real-time-linux-explained/ -- ..med vennlig hilsen = with Kind Regards from Arnt Karlsen ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. _______________________________________________ Dng mailing list Dng@lists.dyne.org https://mailinglists.dyne.org/cgi-bin/mailman/listinfo/dng
