Bad latency performance ASRock Q1900B-ITX

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18 May 2021 18:40 #209378 by DoWerna
now I have to ask:

which text file do I have to edit,
google for a while now. but somehow not get any further

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18 May 2021 19:06 #209381 by ississ
To change kernel command line ?
/etc/default/grub

This is the default:
GRUB_CMDLINE_LINUX_DEFAULT="quiet"

Modified:
GRUB_CMDLINE_LINUX_DEFAULT="quiet isolcpus=2,3"

My system has now run ~6 hours, latencies: servo 31k, base ~31k with isolcpus=2,3.
I think this is enough, with isolcpus=1,2,3 ui feels slower. Tomorrow I'm going to test with some other window manager and different memories.
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18 May 2021 19:33 #209384 by DoWerna
Done,

that seems to have really helped now.

I'm still running the test, currently I have latencies of less than 13K with 2 cores

Let's see if the values still low, but for now it looks very good

Problem probably solved
Thanks

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18 May 2021 20:19 #209393 by ississ
Mine started at ~5000 and jitter is slowly increasing, now servo 32510, base 31148 after 7 hours.
Interesting to see the values tomorrow morning...
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19 May 2021 06:31 #209446 by DoWerna
8 hours later...


I would say it's fine and I can go on

Thanks
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19 May 2021 10:54 #209478 by ississ
I'll continue here with the memory tests:

1x 4G Samsung M471B5173QH0-YK0


2x 8G Corsair CMSO8GX3M2A1333C9 1333 Mhz CL9


2x 8G Hynix HMT41GS6BFR8A-PB 1600 CL11
No image, huge spikes quite far from center on servo thread, I say these should not be used.

1x 8G Corsair gives similar image as the 4G Samsung but a little worse latency.

=> So a big difference by just changing the memory.
These Corsairs are 1.5V (others 1.35V) and lower CL value. I belieave that optimal setup would be a low CL memory with 2 identical modules.
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19 May 2021 11:10 #209483 by tommylight

I would say it's fine and I can go on

Yes.

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20 May 2021 19:09 #209675 by ississ
Just a short follow- up for this case.

Do you @DoWerna have tried to run linuxcnc on your Q1900B ?
I have tested my board now quite a lot with different memories and boot parameters. Latency values are very good but when I start linuxcnc (even with a simple demo setup) I always got an unexpected realtime delay.
I also tried if uefi/non uefi makes adifference => no. This happens with the uspace version (I tried also the new rtai kernel but it crashed so no-go).
Old debian 7 works as a charm with all 4 cores enabled, no tweaks & oddities.
One real difference is that the old system uses X vesa driver (old intel driver does not support the chip) and probably does not have modeset.
New kernel with modeset/intel disabled is not usable (too slow) and the unexpected delay is still there.

I'll continue with the old system because it just works.
But please let me and others know if you are able to use the new one without errors.

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30 May 2021 11:39 - 30 May 2021 11:39 #210670 by h3ifri
Hi,

I have the same board an switched from Debian 7 RTAI to Debian 10 PREEMPT-RT.

Jitter starts at 5000ns on goes up to 30000ns after several hours with isolcpus=2,3.

I also get the "unexpected realtime delay on task 0". Setting BASE_PERIOD to really large values makes no difference.

Thanks.

@DoWerna: Can you share your BIOS settings please?
Last edit: 30 May 2021 11:39 by h3ifri.

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30 May 2021 20:07 #210730 by andypugh
30,000nS isn't terrible. It's certainly usable.

Are you actually _using_ a base thread?

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