Am Samstag, 5. Januar 2019 schrieb Arnt Karlsen:
> On Sat, 5 Jan 2019 04:13:22 -0500, Steve wrote in message
> <20190105041322.13e04671@???>:
>
> > 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.

Nik


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