Latency questions

Hi all,
I am running Linux CNC compiled from the main git repo using the master branch (current as of today, 29 April 2019). I am running this on a Gentoo Linux kernel 4.19.10-rt8. The computer is a Dell Precision 390 with 6Gb RAM wuth the 82801GR/GH/GHM (ICH7 Family) chipset. I use isolcpus to reduce it to one CPU. I have been using Linux CNC on Gentoo for quite a while so far and it's working well. I have a couple of questions on latency:

1) my latency is about 60,000us which I understand is high. Regardless, I am not seeing this causing any issues when machining. My machine is a homemade 500x600mm 3-Axis made from the Solsylva plans. It runs NEMA23 steppers with lead screws. I run it at about 1800mm/min. The question is: what issues should the high latency be causing? How will it manifest itself as a problem? I suppose the real question is, what benefit will lowering the latency give me?

2) It is usual for the latency to spike after about 5 minutes of the computer running and then settle down? I get 3 60,000us spikes within 5 minutes and then no matter how hard I work the computer it never goes beyond 20,000. Is this common? Does this help me identify the cause of the spikes?

Thanks,
Allistar.

If I run LinuxCNC within a few minutes of the computer starting I always get the error. I ignore it and never have any issues with the machining.

What would you expect issues of latency being too high be? Is this about the machine losing steps and therefore not being positioned where the software thinks it is?

I always rehome the axes when I turn it on. I have run 14 hour programmes before and it X0Y0 is always spot on at the end.

Will setting it to 100000 force LinuxCNC to run slower? That is, can I still get 2000mm/min out of it with the same acceleration? Or will this high base period limit the speed?

no problem to set it that high on your numbers
you shoudt see 3600mm/min at latency 200000
we are talking on nanoseconds
so it is enoph time for you to get a clear step signal
you can also increase the numbers of LETS say Parport reset or the signal length
to a good always stable working mashine
if you see no steploss over hoourers of movement it is perfect
if you like you can go faster and test out the edges of the mashine

No matter what I set it to Axis will always complain that the max latency has been reached. When I run the latency tool is sits at about 20,000 for quite a while and has to odd spike of 60k. Yet Axis reports a latency check immediately, and every time. I have never measured a latency of > 70k and that's with a 1 hour check while hammering the computer (e.g. compiling the kernel), yet with latency set to 80k in the ini file Axis complains immediately. This is with LinuxCNC up to date (as of today) on the master branch.

Any ideas as to why it has this behaviour?

if you pst here your ini and hal we can take a lok at it
in ini file
[EMCMOT]
BASE_PERIOD = 100000

[TRAJ]
DEFAULT_VELOCITY = 10
MAX_LINEAR_VELOCITY = 40

[AXIS_0]
TYPE = LINEAR
HOME = 0.0
FERROR = 1
MIN_FERROR = 0.05
MAX_VELOCITY = 40
MAX_ACCELERATION = 120.0
STEPGEN_MAXACCEL = 200.0
# these are in nanoseconds
DIRSETUP = 10000
DIRHOLD = 10000
STEPLEN = 5000
STEPSPACE = 5000

try this then we get a better look at it

Here is the ini and hal files.

Can you try testing the machine using one of the live Linuxcnc ISOs?
(We don't really know if there is a problem with your Gentoo real time kernel or if it is something in your hardware or it's configuration.)