Abysmal latency test on reasonably powerful PC

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12 Jun 2021 01:52 #211841 by thanks4opensource
Thanks, @BeagleBrainz. Yes, I saw the warning regarding Buster+RTAI.

I'm not going to continue with Buster on the Core2 Duo x86_64 machine for right now because I found a very good alternate solution. I installed linuxcnc-2.7.14-wheezy.iso on a Pentium4 box and got the best latency (jitter) numbers I've seen so far: 23000/36000 for the Servo/Base threads with lots of processes running but more like 7000/10000 as the machine will normally be used (dedicated to running the mini-mill, not connected to the internet, no other user software). This is ideal for me because I can use the Core2 for other things whereas the P4 is too old to be a normal daily desktop. As linuxcnc.org/docs/2.8/html/install/latency-test.html says, "Note: LinuxCNC does not require bleeding edge hardware."

I still think understanding and predicting LinuxCNC performance from a machine's specs is black magic. Before I got the above numbers, I installed the same ISO on a different Pentium4 box. The BIOSes between the two are very different, but I did my best to turn off all hyperthreading, etc., and neither has built-in graphics on the motherboard. The "good" one is 3.3GHz compared to 3.0 on the other, but that 10% difference doesn't explain the latency on the "bad": 3million/16000.

One more question: Can I update 2.7.14 Wheezy RTAI on the 32-bit box using the repository listed at linuxcnc.org/docs/2.8/html/getting-start..._setting_apt_sources ("deb linuxcnc.org wheezy base 2.8-rt")? Does that contain 32-bit binaries? I'd like to start with LinuxCNC using the current 2.8 version, but installing a full development environment on the P4 and building from source is more than I want to take on right now.

Update: Looks like I can use the manual method at linuxcnc.org/docs/2.8/html/getting-start...ed:update-no-network and go to linuxcnc.org/dists/wheezy/2.8-rt/binary-i386/ to make sure I'm getting the right package.

Thanks again for all the help, and for reading about my failures and successes.

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12 Jun 2021 03:17 - 12 Jun 2021 03:35 #211848 by BeagleBrainz
Bitness is done automagically

But just for completeness:
www.linuxcnc.org/dists/wheezy/2.8-rt/binary-i386/Packages

Look for the heading on the left titled Architecture: for each package, but you will get the idea.


Yeah clock speed is not the best thing to predict jitter by. There is a lot of things that contribute, best idea is to try and take in what others recommend.

Some of the newish itx boards with Celerons get good results, but then you run into problems some only support UEFI boot (no legacy option), which makes installations fail. I spent a lot of time working thru this when I rolled a Mint 19.3 ISO that ran linuxcnc out of the box when installed. There are ways around it but it can get a bit complicated & confusing.

And we wont discuss the issues on newer M\B that don't have an on board parallel port trying to get an add in card to work. That's why I jumped over to a MESA ethernet card. Least I knew if one machine went bang finding a suitable machine would be easier.....and jitter isn't so critical once the step & direction signals timings are generated off board from the machine.
The boards that work with Mach3 and the like actually do the motion control, which doesn't work with the ecosystem of Linuxcnc.

There I go waffling again.

Anyways good luck, keeps us all informed on how you're going.
Last edit: 12 Jun 2021 03:35 by BeagleBrainz.

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