RT System CPU Usage vs. loop speed

[David XIII]

Running LV8.2 RT on cRIO (VxWorks), host runs XP SP2.

I have a control vi running on CompactRIO in a timed loop with 10ms period (100 Hz).  Based on the difference between a usec clock in the first frame of the sequence (inside the loop) and the last frame, the code appears to only take around 2800 usec (2.8ms) out of the 10ms allotted.  Yet the RT System Manager shows CPU Usage at 92-98%.  Should I be concerned?

Is TCP/IP overhead included in this measure of CPU usage?  In case it matters, I am using four shared variables to communicate data (one) and  control program status (three) with the host.

Thanks for any insight you can share.

[Nadim R]

Hi David,

The Real Time System Manager may consume a significant amount of CPU usage since it constantly polls your RT system for usage information.  Since the RT target is being polled, there are TCP/IP threads that flying back and forth from the target and host for monitoring purposes.  Have a look at this KB that talkes about the RT system manager. A more major concern would be if your CPU usage or memory constantly treks up, indicating maybe some sort of memory leak or other problem.

I hope this helps,

Regards,

Nadim
Applications Engineering
National Instruments

[Ben]

"Should I be concerned?"

If this was prior to LV 8, yes.

In LV 8 and above, all availabe CPU cycles are used to maintain the various protocols (PSP etc). These functions should yeild CPU to your task when required.

So for LV 8 and up, this is "normal".

Ben