RTAI vs PREEMT_RT

I am looking to upgrade to a newer version of Linux and then give Master 2.8 a try.

I'm a bit confused with RTAI and PREEMPT. From when I first started with LinuxCNC I thought that RTAI and PREEMPT were two different version of the kernel. You would use RTAI with a Mesa 5i25+7i76 and PREEMPT with a 7i76E. Am I all wet on this?
I look on a list of kernels Here and did not see preempt mentioned anywhere. All of the recent posts about upgrading seem to only talk about PREEMPT. What am I missing?

Currently I am running
Linux X3CNC 3.4-9-rtai-686-pae #1 SMP PREEMPT Debian 3.4.55-4linuxcnc i686 on a
Intel(R) Celeron(R) CPU J1800 @ 2.41GHz using a Mesa 5i25+7i76+7i83.

I just installed Linux X3CNC 4.9.0-9-rt-amd64 #1 SMP PREEMPT_RT Debian 4.9.168-1+deb9u2 (2019-05-13) x86_64 GNU/Linux
on a AMD Athlon(tm) 64 X2 Dual Core Processor 5000+ to play around with. If I actually wanted to use this to run my machine would I have to, or would it be desirable to, install the RTAI kernel?

Thanks
John

The are in fact two different _modifications_ of a kernels.
In general:
- RTAI for parallel port stepping machines.
- preempt_rt for Mesa/Pico based machines.

RTAI usually gives better latency numbers but you can't use it with as new a kernel.
RT gives worse latency but when using FPGA cards, it doesn't matter as much.

Chris M

Thanks Chris.

So I couldn't use RTAI with Debian9 anyway and RT is what I should be using.

BigJohnT wrote:

Just because they don't get updates don't mean they are broken.
JT

Bravo, that is some of the best advice I have seen given.

Very well said JT.


Johnny if the Latency looks a little off with the AMD CPU throw in a second hand Radeon Card, it works wonders.
Here are some of the combos I've tried (last MB still go for a bit of money)

Best results I've had (below 10K) are
Asus M4A785D-M PRO with an Athlon II x2 250, 4GB ram, Kingston 120GB SSD and on board video.

Asus M2N68-AM-PLUS with an Athlon 64 X2 5200+, 4GB ram, Kingston 120GB SSD and a Radeon HD5450 card.

Asus A8N-SLI/A8N-SLI Deluxe with a Athlon 64 3800+, 4GB ram, Kingston 120GB SSD and a Radeon HD5450 card

So true. I was doing some latency testing last night and I am getting WAY better results using Wheezy than I am with Stretch. I used a much better machine to install everything (AMD Athlon 64 5000+) on the drive and then put the drive on the J1800 machine. That may be part of the problem. On the Athlon machine the latency is good and the machine is very responsive but that motherboard won't fit into my cabinet. When I put that drive on my J1800 board the servo thread goes from 16000 to around 200000. Don't have the exact numbers with me. Using Debian 7 Wheezy the servo thread is 10655. My goal was to move to something that wasn't EOL and would allow me to play around with 2.8. On the J1800 it takes over 24 hours to compile a kernel. On the Athlon it took about half that.

Don't worry about wheezy being EOL....it's not going to stop working 3 months down the track. Most of the updates don't really affect linuxcnc....I just did a dist-upgrade on my stretch machine...aprt from the kernel upgrade (which wasn't as issue anyways) most of the updates were for libreoffice.
As for 2.8 it should compile under wheezy, the buildbot has 2.8 packages for wheezy.

hi,
I want to know what is the last version that i can choose the hva the parrallel port.

with linux mint19.1 + kernel preemt i belive that is not for the parrallel port ; can you confirm.

The parallel port is supported by both RTAI and Preempt-RT (uspace) versions on LinuxCNC
though RTAI has better latency on a wider range of CPUs than Preempt-RT.

The OS version is not really important though kernel support of plug-in parallel
port cards _may_ be required if the card needs non-standard initialization code.

For this reason, its easier in general to work with built-in parallel ports than plug-in cards.
(since the card initialization is done by the BIOs)

Ok if I understand i can install linux mint19.1 following the guide
gnipsel.com/linuxcnc/uspace/index.html and use the parrallel port
Do you confirm ?