Best build for a P4 w/Ethernet?

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15 Jan 2020 17:39 #154937 by blockbuffer
Hi!

I'd like some clarification on the best OS/linuxcnc combination for my situation, please.

The desired end state is:
Optiplex Gx270 (P4) ->Ethernet-> Mesa 7i92-> Gecko G540-> Taig CNC Mill

I don't have a preference regarding the linux distro used, beyond 'it just works', with the above hardware. I am the original owner of the Gx270, and my unit has been a solid machine. I already have the 7i92, the G540, and the Taig.

I tried the 2.7 Debian Stretch 9 download (linuxcnc-stretch-uspace-i386.iso), which seemed to be the appropriate choice. I had no difficulty installing it on the Gx270, but the jitter test results were not that great. Started low, but stepped up gradually, and grew over time. Not good.

The original video card was an nvidia, which I understand can be problematic (on-board was worse..). I have a Radeon 9250 on order, that will replace it.

While searching for jitter remedies, I came across posts opining that older, uniprocessor linux kernels did better on uniprocessors (like my P4), than newer builds intended for multi-core processors. Seems reasonable, and I think I saw a post from someone who rebuilt their kernel just for that and was happy with the results - although unfortunately I didn't think to bookmark it for details at the time.

Indeed, the jitter benchmark page (which led me to think that the Gx270 would be a very good choice), was said to have used those older builds...

My intention is to replace the video card and re-test, but then the questions are:
1. Is linuxcnc-stretch-uspace-i386.iso the best build for my situation?
2. What about a uniprocessor kernel? Would one even work with this build?
3. If it would work and potentially be helpful, is a prebuilt kernel available? If not, how would someone who's never rebuilt a kernel do that?

Thanks for your help with this!

- Bob

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15 Jan 2020 18:03 #154941 by PCW
Replied by PCW on topic Best build for a P4 w/Ethernet?
What was the actual jitter?

As long as you do not get real time errors on the 1 ms servo thread,
the actual jitter figures is really not significant.

And even if you _do_ get real time errors at a 1 ms servo thread
if you do not have extremely high acceleration, a 2 ms thread is OK
for step/dir systems,. This will run on practically anything

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15 Jan 2020 22:27 - 15 Jan 2020 22:51 #154962 by blockbuffer
Thanks for the response!

The Base thread jitter with the nvidia reinstalled, was between 36k and 39k on two separate 30-minute tests just now, with gears, browser, youtube, etc. running.

What bothered me is that every once in a while on prior tests (I've been at this a while...), I'd get a spike up in the 100k range - no idea why, and this just happened a few times. This is after the Optiplex had hardware unnecessary for its new role, stripped out.

The Radeon will arrive in a week or so. At that time, I'll swap video cards, do a fresh software install, and see how the tests go then. My 100k+ problem *might* be as simple as fans coming on shortly after a restart, but I just don't have it bracketed down that close yet.

You didn't mention anything about linuxcnc-stretch-uspace-i386.iso's suitability, so I take it that build is ok for use with the 7i92? And don't worry about uniprocessor vs. SMP kernels?

When you say real time errors, you're speaking of run time, during machining operations? So linuxcnc can detect a missed step? This is open loop (steppers), so it never occurred to me that that would be available - I was expecting a latency/jitter issue to turn up as a machining flaw on the workpiece.

What is the on-error behavior, then? If it's a halt, that's probably fine.

Please confirm on the questions above, but I think you're basically advising me to keep calm and carry on. :-)

Thanks!

- Bob
Last edit: 15 Jan 2020 22:51 by blockbuffer. Reason: Clarification: Was uniprocessor/SMP, now uniprocessor vs. SMP

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15 Jan 2020 23:12 #154968 by PCW
Replied by PCW on topic Best build for a P4 w/Ethernet?
1. If you are using step generator hardware (like a 7I92) you dont need a base thread, only a 1 ms servo thread, so base thread latency is dont-care

2. As long as the host computer can reliably update the 7I92 at a 1 KHz rate
that all that is required, the latency does not matter unless its so bad that it interferes with this 1 KHz communication

3. Latency errors will not normally cause step loss unless they are so bad that LinuxCNC times out on multiple cycles

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16 Jan 2020 00:05 #154978 by blockbuffer
Ah! Thank you!!

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