isolcpus Setting in Buster

I'm following the guide for GRUB2 isolcpus setup.

wiki.linuxcnc.org/cgi-bin/wiki.pl?The_Is...rameter_And_GRUB2i'm using

For quad core it instructs RTAI_OPTS="isolcpus=1,2,3"
For dual core it instructs RTAI_OPTS="isolcpus=1"

In some of the latency testing posts I see quad cores set to RTAI_OPTS="isolcpus=2,3" This is opposite the wiki that the last core should be used. Just wondering why?

Lastly, I'm running a Mesa 7i96e card. What would be the recommended isolcpus setting?

Unless you have an actual latency issue I would not use any isolcpus
options at all

PCW wrote: Unless you have an actual latency issue I would not use any isolcpus
options at all

Under Mint I had issues, more of a intermittent spike. Being a hobby machine I just doubled the Servo Period and that seemed to take care of it.

Now that I'm seeing better latency numbers under Buster I was hoping to run the default servo period.

With Buster, all 4 cores, latency test jitter Servo=15K Base=14K. Using isolcpus I'm seeing lower numbers yet again. I understood these number to be good.

However with the new scrip test...
forum.linuxcnc.org/18-computer/39371-res...ith-linuxcnc?start=0
the results don't seem as good comparative to other peoples' results. Am I making to much of it? Whats the potential down side to running isolcpus?

Do not worry to much about latency, LinuxCNC will inform when it is not OK.
Get the machine working, if you get latency warnings, raise the base thread/period a bit and test again.
That is if using the Parallel port.

tommylight wrote: Do not worry to much about latency, LinuxCNC will inform when it is not OK.
Get the machine working, if you get latency warnings, raise the base thread/period a bit and test again.
That is if using the Parallel port.

No parallel port, I'm running a Mesa 7i96e ethernet card.

Project_Hopeless wrote: That is if using the Parallel port.

No parallel port, I'm running a Mesa 7i96e ethernet card.[/quote]
That makes latency even less relevant, up to the point of loosing connection to the board.

tommylight wrote: Do not worry to much about latency, LinuxCNC will inform when it is not OK.
Get the machine working, if you get latency warnings, raise the base thread/period a bit and test again.

The last time I had a latency issue I simply doubled the Servo Period. If I raise the default Servo Period are there specific increments or is any value valid. For example will some value or multiplier lead to a rounding error in the math or a harmonic in the response?

Not sure really, but i do test and use some at 500K and sometimes 2mill on two of the laptops.
Otherwise i would think that any divisible number would not cause issues.

Project_Hopeless wrote:

tommylight wrote: Do not worry to much about latency, LinuxCNC will inform when it is not OK.
Get the machine working, if you get latency warnings, raise the base thread/period a bit and test again.

The last time I had a latency issue I simply doubled the Servo Period. If I raise the default Servo Period are there specific increments or is any value valid. For example will some value or multiplier lead to a rounding error in the math or a harmonic in the response?

No there is no need to worry. In a hal component there is a variable called period and also fperiod which report the servo period in nanoseconds and seconds. So any component requiring timing info "SHOULD" use these, not blindly count periods...