Faster PC longer latency. Is that normal?

Hi all,

I have been running my machine with LinuxCNC on an old PC for a while and it works fine but there have been a few times when it is obvious that the machine was struggling and actually stuck and then released the apps running a few times. When it did that of course it screwed up the job I was running but it was the only PC I had available.

I recently traded a buddy at work for another PC that was newer. Both systems are loaded from a disk installed with the latest ISO and each system only has the same minimal software. Pretty much just LinuxCNC and almost no other apps.

I ran sysbench on each and the newer system is nearly 4 times faster executing CPU tasks and fileIO.

So far so good.

Then I ran latency tests while configuing the new machine. My old clunker machine had jitter base that was typically around 6000. The newer faster PC has a jitter base of over 22000.

I was expecting the newer PC to have lower jitter under similar load and with the same basic software loaded and running.

Does it make sense to you guys that a 4 times faster PC similarly loaded would have a jitter base that was 4 times higher (23000 versus 6000)?

I'm about to take it out to the shop and hook it up but wanted to see if you guys with more experience thought this was normal.

Thanks a bunch.
Jeff

This is not unusual.
speed of the cpu and latency of the system is two different things.
latency includes how long it takes a system to stop what it's doing and respond to a request.
It's literally a measure of lateness of a periodic task.

New computers add more and more functions ( in the hardware side I mean) to aid it's job as a general utility machine.
These features tend to negatively affect latency.

Are you using software stepping ? 22000 will limit the top speed more then a 6000 system.
Whether that's a problem depends on your requirements. (6000 is actually excellent/rare)
If using a hardware stepping/servo system 22000 is fine.

You may be able to improve the latency.
There are some links on this page:
wiki.linuxcnc.org/cgi-bin/wiki.pl?Latency-Test

Chris M

Thanks Chris,

So I always thought that the latency on that old computer was too good based on all I had read.

I will look at the link you sent and see if I can improve things and I will probably try it out to see how it goes.

I am not currently using stepping but wanted to be able to at some point. So far even full stepping the cuts have been good.

So the million dollar question... based just on those numbers, would you think swapping to the newer PC would be worthwhile from a stability viewpoint? With the higher latency if I am full stepping, will it slow things down? I can get about 45 inches per second now but typically cut at 30 or less depending on the material. I wouldn't mind more speed but have been careful up till now.

Thanks for the help
Jeff

Here are another couple of links regards latency

www.linuxcnc.org/index.php/english/forum...-the-latency-problem
www.linuxcnc.org/index.php/english/forum...me-latency-solutions

Not knowing what the new computer is, cannot be specific what you might need to try.

So the million dollar question... based just on those numbers, would you think swapping to the newer PC would be worthwhile from a stability viewpoint? With the higher latency if I am full stepping, will it slow things down? I can get about 45 inches per second now but typically cut at 30 or less depending on the material. I wouldn't mind more speed but have been careful up till now.

If the old kept crashing then it is no use to you even if the latency was low.

You will only be able to output approx 30% of the pulses per time period that the old computer could, so yes it will have a proportionate effect on max velocity, acceleration etc.
I assume the 45"/sec reference is acceleration, quite frankly it is way too high for a small stepper machine anyway.

Having single stepping will save some speed, but at the expense of smoothness and resolution.

If you went for hardware step generation via a FPGA board, then the servo thread is probably fine, the base thread becomes irrelevant and it will serve quite well.

regards

The newer machine is a Dell Inspiron 560.

The old one only crashed a few times and it was when I tried to pull a file across the network while it was cutting. The machine is too far to practically run net cables so I am using a wireless card and that sometimes affects LinuxCNC during a run. Now I just turn off wireless before I start a cut and I haven't had it crash.

The 30"/Minute and 45" per minute is the actual cut speed or at least it is the cut speed I input into CAMBAM when I convert my patterns to GCode. I know that 30"/minute is slower than 45"/minute, 10"/minute is slower than 30"/minute, etc etc but no idea if it is actually 30"/minute for real. In LinuxCNC wizard, my X axis (large stepper) is running at .3"/second max (18"/minute) and the Y and Z axes are twice that (36"/min).

I will take a look at the links you provided.

Thanks again for the help
Jeff

The 30"/Minute and 45" per minute is the actual cut speed.......

45"/minute rather than /second makes much more sense

Sounds like the option of the original machine is open if it was just the wireless that caused problems.
I never use the computer running a program for anything else whilst it is running, too fraught with potential problems.

regards

The newer machine is a Dell Inspiron 560.

There are at least 4 different processor options from Celeron to core i2

Most, if not all will be multi-core, if it is look at the isolcpus kernel option for a start
wiki.linuxcnc.org/cgi-bin/wiki.pl?The_Is..._Parameter_And_GRUB2

regards

I have decided I will test the newer PC just to see but will likely stick with the older and just disable the network during runs. It hasn't had any issues other than that so maybe it's not sowrth the swap.

Thanks for all the info. Still researching the links.

Jeff