RTAI vs PREEMT_RT

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14 May 2019 23:44 - 15 May 2019 00:27 #133793 by JohnnyCNC
RTAI vs PREEMT_RT was created by JohnnyCNC
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
Last edit: 15 May 2019 00:27 by JohnnyCNC.

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15 May 2019 00:05 #133794 by cmorley
Replied by cmorley on topic RTAI vs PREEMT_RT
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

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15 May 2019 00:26 #133796 by JohnnyCNC
Replied by JohnnyCNC on topic RTAI vs PREEMT_RT
Thanks Chris.

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

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17 May 2019 12:22 #134045 by BigJohnT
Replied by BigJohnT on topic RTAI vs PREEMT_RT
While older OS's are past EOL I still have 2 machines running Ubuntu 10.04 with RTAI and you can have either 2.7 or 2.8 on any install. Just because they don't get updates don't mean they are broken.

I have a guide for installing LinuxCNC Uspace on recent OS's and I'm currently adding the light weight Lubuntu to that guide.

gnipsel.com/linuxcnc/uspace/index.html

JT
The following user(s) said Thank You: InMyDarkestHour

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17 May 2019 13:06 - 17 May 2019 13:14 #134046 by InMyDarkestHour
Replied by InMyDarkestHour on topic RTAI vs PREEMT_RT

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
Last edit: 17 May 2019 13:14 by InMyDarkestHour.

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17 May 2019 13:20 #134047 by JohnnyCNC
Replied by JohnnyCNC on topic RTAI vs PREEMT_RT
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.

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17 May 2019 16:42 #134064 by InMyDarkestHour
Replied by InMyDarkestHour on topic RTAI vs PREEMT_RT
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.

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05 Jun 2019 11:42 #135931 by Philippe
Replied by Philippe on topic RTAI vs PREEMT_RT
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.

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05 Jun 2019 13:23 - 05 Jun 2019 13:26 #135939 by PCW
Replied by PCW on topic RTAI vs PREEMT_RT
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)
Last edit: 05 Jun 2019 13:26 by PCW.

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05 Jun 2019 14:31 #135944 by Philippe
Replied by Philippe on topic RTAI vs PREEMT_RT
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 ?

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