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?

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.

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?

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...