Latency and dedicated PC

I understand the latency problem (I think) but not the testing. OK - you need to obtain a correct latency figure. Well - you need to obtain a sufficiently high latency figure to ensure the correct procession of commands through the parallel port. But providing a latency figure that is very large is "dumbing down" the PC and allowing it to twiddle its thumbs. Correct me if I'm wrong.
But if my PC is a dedicated linuxcnc machine that has no other function - why in the testing phase do I need to play music, surf the web, race he mouse to the 4 corners of the screen etc. None of this will be going on during the running of linuxcnc - so why don't I just let the test run with background calm? - no interruptions and then input the resultant Jitter factor - its going to be a better approximation to operational reality than the test recommendation. Please enlighten me.

I believe the reasoning is to try to replicate worst case scenarios, that could lead to a problem during normal machining.

But if my PC is a dedicated linuxcnc machine that has no other function - why in the testing phase do I need to play music, surf the web, race he mouse to the 4 corners of the screen etc. None of this will be going on during the running of linuxcnc - so why don't I just let the test run with background calm? - no interruptions and then input the resultant Jitter factor - its going to be a better approximation to operational reality than the test recommendation. Please enlighten me.

No real argument here, I think people do get a bit carried away. So if you want your computer to play music and you are going to surf the web while machining you got that covered.

However the Axis interface and linuxcnc do use computer resources, so running glxgears, etc. for the test does make sense and as Todd pointed out it is better to error on the safe side.

Rick G

A bit off-topic but does playing with the "nice" priority process within Linux have any effect upon the efficiency of linuxcnc within a dedicated PC?

No, "nice" adjusts priorities of the normal linux processes.
It does not affect the real time processes that run under RTAI that LinuxCNC uses