Mesa 7i76e - more computer/jitter/latency questions

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29 Apr 2020 17:07 #166106 by warreng
I have a 7i76e and was planning to use it with:

HP Compaq 8000 Elite.
Specs:
E7500 Core 2 Duo 2.93Ghz
Intel 82567LM-3 Ethernet
Kernel 4.14.28-rt23-kona-rt
Linux Mint 19.3
Using usb wifi dongle to connect to internet while ethernet port is directly wired to Mesa board.

note: I am using the wifi internet connection to play a youtube video to stress the machine. My intention long term is to use the wifi for file transfer purposes. I suppose it could be unplugged while a job is actually running...

I got this all installed and ran the basic latency test. It typically runs below 100k on the servo thread but very occasionally will spike far above. I even got a 445k at one point! Since this didn't seem so good, I decided to try linuxCNC with a Mac Mini. I simply cloned the drive and put the original drive in the Mac. I was able to boot from it without any fuss.

Late 2009 Mac Mini Server.
Specs:
P8700 Core 2 Duo 2.53Ghz
Nvidia MCP79 ethernet ??
Kernel 4.14.28-rt23-kona-rt
Linux Mint 19.3

With this kernel I found that the built-in wifi would not work correctly with the suggested driver so I left it alone and used the same usb dongle as with the HP. I found that my jitter with this arrangement was typically under 50k with one odd ball spike at one point jumping to 110k. Overall the latency test servo reading seems to be significantly better with the Mac than the HP.

After further reading about the ethernet Mesa cards I decided to look at the servo.thread.tmax from the hal meter while my "machine" is running. If I understand correctly, this will show the latency through the I/O in addition as well as the basic processor latency. With this arrangement I find that the HP machine gives me numbers in the range of 950,000 which divided out with the processor speed gives me about 0.33ms. The Mac on the other hand gives me numbers in the range of 1,500,000 and divided out it's closer to 0.6ms. And, with the Mac, the hal meter starts at about 1,260,000 as soon as I can get it loaded whereas the HP works its way up to the 950,000 range.

Neither of these numbers feel very good given the goal of keeping below 1ms. But I'm new to the Mesa/linuxCNC system so I'm looking for comments/suggestions.

btw, I'm suspect of that Nvidia ethernet on the Mac. I was surprised to find that was the vendor.

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29 Apr 2020 17:59 #166117 by tommylight

warreng wrote: HP Compaq 8000 Elite.
Specs:
E7500 Core 2 Duo 2.93Ghz
Intel 82567LM-3 Ethernet
Kernel 4.14.28-rt23-kona-rt
Linux Mint 19.3
Using usb wifi dongle to connect to internet while ethernet port is directly wired to Mesa board.

I have the same system here working on it for the last month or so, i have a Mesa 6i25 in it, and occasionally i connect a Mesa 7i92 to it through ethernet, so far i have seen only one error with the 7i92 and none with 6i25. Mine has E8400 processor @3GHz

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29 Apr 2020 18:26 - 29 Apr 2020 18:29 #166123 by PCW
For Ethernet, as long as you can maintain a 1 ms servo thread (or even 2 ms on low acceleration machines) the jitter is not significant. Note that on the 8000 you need to disable IRQ coalescing since its has Intel Ethernet hardware.

I would also suggest a kernel command line of: "quiet splash processor.max_cstate=1 idle=poll"
Last edit: 29 Apr 2020 18:29 by PCW.

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30 Apr 2020 20:02 #166246 by warreng

PCW wrote: For Ethernet, as long as you can maintain a 1 ms servo thread (or even 2 ms on low acceleration machines) the jitter is not significant. Note that on the 8000 you need to disable IRQ coalescing since its has Intel Ethernet hardware.

This i have done.

I would also suggest a kernel command line of: "quiet splash processor.max_cstate=1 idle=poll"


This I have not done. I'll have to look into it.

But, it seems that neither of you are concerned about the numbers I'm seeing from this computer. I'll move forward then...

btw, tommylight, can you share some more info about the error you've seen with 7i92? Is it a timing problem?

Thanks,

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30 Apr 2020 20:12 - 30 Apr 2020 20:13 #166248 by tommylight

warreng wrote: btw, tommylight, can you share some more info about the error you've seen with 7i92? Is it a timing problem?

It lost connection once as do not have coalescing set to 0 yet and i was using a 15M long cable wound in a bundle, pretty low quality.
Most probably that happened as i was stepping on that cable a lot as it was dragged across the workshop.
BTW, did you disable everything not needed in the BIOS?
Last edit: 30 Apr 2020 20:13 by tommylight. Reason: info

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