Re: [CAD_CAM_EDM_DRO] LinuxCNC laptops

Michael Fagan wrote:

>I don't know about this particular issue, but one of the main
>differences between the EMC2 packaged Ubuntu version and the normal
>Ubuntu distro is that it runs a real time kernel which is designed to
>help prevent this sort of interruption, at least at the OS level. It
>makes the computer slower for GUI and other tasks, but a lot better
>for CNC controls.
>
>

This issue still crops up on the realtime kernel. There are workarounds
that work on some computers (but not others).

The System Management Mode is a higher priority interrupt than any of
the timers available to the RT system. (it's actually higher priority
than NMI, the "non-maskable interrupt" - that's very very high priority
:) ) A system management interrupt will halt execution of an RT task
just as easily as a userspace task. Even if it doesn't interrupt an RT
task, as soon as the RT system yields the CPU to the non-realtime
processes, the SMI processing will be done, and that will delay the next
execution of the RT tasks.

Note: I haven't stuck a modern CPU on a logic analyzer to test this
stuff - I can't afford the equipment :) If anyone has more info on the
subject, I'm all ears.
- Steve