Improving Base Thread Jitter

I would like some advice please on how to proceed with the computer I am using to run LinuxCNC.  I have just installed the latest version of LinuxCNC (2.9.3 Debian12 Bookworm Preempt -RT) and have run the Stress Test for an hour - the figures are not encouraging: 85953ns max jitter.  I know this is way too high, and if I were to use this computer I would have to make some changes, or it might be that no matter what I do it will not be suitable for software stepping.  If so I might have to save-up for a Mesa Board and do the stepping there.  The problem is that  I have no real feel for what scale of improvement various changes might have. 

The computer is a Dell Optiplex 745 with an Intel(R) core (TM)2 CPU 4300@1.8GHz.  It has 6GB of DDR2 SDRAM and is using the on-board graphics card.  I know the graphics card is the first place I should look to make an improvement, and I have the opportunity to buy an AMD Radeon 450 with 4GB GDDR5 - but it is not that cheap. I have a simple 3 axis cnc mill with just 3 Nema 23 stepper motors, but I would like to upgrade it at some future point.

My first question is: in very general figures, is it realistic to think that a new graphics card might reduce the jitter figure by, say 60000ns, or is this too optimistic - is it even worth a try?  If any new graphics card could not make this scale of improvement it might be better not to buy the Radeon and put the money towards a Mesa board.

My second question is: if I were to buy the Mesa board would I still need to upgrade the graphics card anyway, or could I stick with the on-board card?

Finally,(preparing my self for the worst !), I have been looking at the range of Mesa boards available and it is a little confusing for a novice.  The advice on this site seems to be a year or 2 old, and so I would be most grateful if you could recommend the most suitable board for me.  I cannot imagine I will ever require more than 5 axis - I probably will stick with 3, but I might double up the motors on Y.  I need the ability to control the spindle (1500Kw 24000rpm currently using a Chinese VFD), and the mist (just a compressed air feed).  
Grateful for any pointers
Mitch

Have you done all the suggested BIOS changes to reduce latency?

(disabling all power management, speed-step, hyperthreading, C state > C1 etc etc)

Thank you for replying so quickly. I am afraid the answer is no. I just loaded Linux and did the stress test because I had rather suspected this old computer would not be very good. I had not realised that BIOS changes were necessary. I will try and find some instructions and give it a go. Is there a list anywhere of the required changes please? If those changes reduce the latency then may be the graphics card might be worth a try.
thanks again

Mitch66 wrote:

Is there a list anywhere of the required changes please? If those changes reduce the latency then may be the graphics card might be worth a try.
thanks again

Try my list
docs.google.com/document/d/1jeV_4VKzVmOI...diY/edit?usp=sharing

thank you for the link to your document. I have a 2 core computer which I was rather hoping I could interface through a parallel port, and so I am not sure how much applies to me. From the Optional Steps section I find 'Disable energy efficient ethernet' and 'disable active state power management', I don't think the 'Isolate 2 Cores' is applicable. I was rather looking for things I should do in the BIOS, but the only reference is the Review Latency section which says the BIOS setting are important in reducing latency.

My lack of knowledge is almost certainly causing me to read the document incorrectly and I apologise for that. I will go into the BIOS and try to disable the power management and I will not have the computer on-line, but otherwise still not sure how to proceed.

Again! Many thanks. I have deselected as many items as possible in the BIOS and selected 'disable multicore technology' so presumably it is now running on just 1 core. I also presume this will have the same effect as disabling core2 with the Isolcpus Parameter and that it will no longer be necessary to use that parameter?

The new stress test reading is 65738 a reduction of around 20000ns. The next stage is to install the Wheezy iso you advised. Just need to find out how I can have both Wheezy and Bookworm available in the GRUB start menu so I can select either OS.

Since Rodw's post I have done a lot of reading - I had not expected the learning curve to be so steep! But very interesting nevertheless, and if I am to get to grips with Linux and stop wasting your time, I will need to overcome my 'Windows induced fear' of changing system files and do much more reading.

If I may, I will report back once I have tried it with Wheezy. If this also makes a difference I will buy the new graphics card and maybe I will get the figure down to something useable. Finger Crossed!

I suspect that the future is hardware based stepping and so I will probably go down the Mesa card route in due course. Bu first, now I have started with this I will do my best to make this computer suitable for software pulse generation - it will be a useful learning tool
thank you again
Mitch

After installing Wheezy the jitter reading is 24899 - much improved. interestingly after all the BIOS changes the installation process took much longer - but it worked .
Do you think this reading is good enough to proceed with this computer in its current state?