Jitter 40.000 and "unexpected realtime delay on task 0 with 57000" good enought?

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02 Jun 2021 18:10 - 02 Jun 2021 18:17 #211010 by Roman Simon
Hello,

I am new to linux cnc and I bought before installation a minicomputer (and sherline mill) and installed
Debian 10 Buster PREEMPT-RT ISO. I didn't know anything about jitter .... my computer is

ASRock J4005B-ITX,
Intel Celeron Dual-Core J4005, 2.0GHz (Turbo: 2.7GHz),
Intel HD600 Graphics
with parallel port

I have disabled every processor energy saving option in the bios. This droped the Max Jitter only a little bit (-4.000)
I have read "If the Max Jitter is more like 30-50 microseconds, you can still get good results..."
but the displayed error unsettles me.

Thanks for any advice. If someone sees something that I could safely turn of in the bios would be great also.



Jitter starts around 25.000 and then gets higher after a few minutes.

Thank you, Best Regards
Roman
Last edit: 02 Jun 2021 18:17 by Roman Simon. Reason: linux version

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02 Jun 2021 18:43 #211015 by Joco
Are you intending to use parallel BOB to control the drivers/motors or a mesa card?
Those measurements might not be great for a parallel port/BOB based system.

As for getting more consistency what settings do you have on the linux boot options?
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02 Jun 2021 20:35 #211025 by Roman Simon
I will need the parallel port. Boot option ... I found this
BOOT_IMAGE=/boot/vmlinuz-4.19.0-11-rt-amd64 root=UUID=56a0a235-95ea-4827-8f16-5d4340212beb ro quiet
under /proc/cmdline. I have installed the official linuxcnc image and attached the dmesg output.
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02 Jun 2021 21:22 #211029 by tommylight
Share memory from auto to whatever, 16 or 32 or 64MB...
If left in auto it will mess up latency s it will resize the memory reserved for video when it needs it.
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03 Jun 2021 00:05 - 03 Jun 2021 00:06 #211039 by Joco
On the share memory for video also try different sizes. I have found better prerformance with a smaller value on some systems.

Given you are dual core you might need to look into cpu isolation. I would also do an explicit check on the MHz of the cpu when running your latency tests. Just to make sure that even though you have disabled speed stepping in the bios that you are in fact running at max MHz.

On the max MHz front I have read comments about the fact that the Turbo mode can actually hurt latency not help it so look for you natural top speed (2GHz).

I used this command (might be easier to wrap in a shell script) to check on MHz of cpus I have tested.

watch "grep -E '^model name|^cpu MHz' /proc/cpuinfo"


These are the kernerl boot flags I have used when tuning a dual core system which might be of use:

intel_idle.max_cstate=0 processor.max_cstate=0 isolcpus=1 noirqbalance

you may also need to consider idle=poll if you are not seeing max cpu speed OR use a CPU hog script to drive the CPU to max speed.

Cheers - J.
Last edit: 03 Jun 2021 00:06 by Joco.
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03 Jun 2021 05:19 #211058 by Roman Simon
I set the shared memory to 64MB (it is the lowest possible setting).
Both two cores run at 1996.8 Mhz. But max jitter is only bit lower ~ 39.000

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03 Jun 2021 05:22 #211059 by Joco
Did you try the boot up kernel parameters?

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03 Jun 2021 06:09 #211062 by Roman Simon
oh, now i have added the flags, so my call is

BOOT_IMAGE=/boot/vmlinuz-4.19.0-11-rt-amd64 root=UUID=56a0a235-95ea-4827-8f16-5d4340212beb ro quiet intel_idle.max_cstate=0 processor.max_cstate=0 isolcpus=1 noirqbalance

max. Jitter starts then right away with ~34.000 and goes up 38.000-40.000
before this I had a minute ~25.000

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03 Jun 2021 06:44 #211065 by Joco
have you black listed all the sound modules and disabled sound in bios? This is also another possible latency source.

check to see what is loaded with:
lsmod | grep snd

You should probably blacklist them in a file in /etc/modprobe.d

For example I have a file in that dir called "snd-blacklist.conf". It contains these entries:
blacklist snd_seq_midi
blacklist snd_hda_codec_realtek
blacklist snd_hda_codec_generic
blacklist snd_hda_codec_hdmi
blacklist snd_seq_midi_event
blacklist snd_hda_intel
blacklist snd_rawmidi
blacklist snd_hda_codec
blacklist snd_hda_core
blacklist snd_hwdep
blacklist snd_seq
blacklist snd_pcm
blacklist snd_seq_device
blacklist snd_timer
blacklist snd
blacklist soundcore
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03 Jun 2021 07:20 #211071 by Roman Simon
I have disabled "Onboard HD Audio" in the bios. With lsmod I do not finy "snd" or "sound"
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