Thread scheduler for PID period control ?

[Greg McKaskle]

The timer doesn't control the rate of the loop. It merely measures the time from one execution to the next. If you have code within teleop that takes more than 20ms to run, you will skip DS updates. I see this quite often on team DS logs, even teams from Einstein. It would be odd that the periodic is not a multiple of 20ms. And unless the network and/or DS are stressed out, I'd expect jitter numbers like Joe showed above.

I'm not sure how Java PID does its scheduling, and when teams spawn another thread and set its rate, I'm not sure what they are using to schedule.

Team 358's article is about timing using LabVIEW. LabVIEW has two different schedulers. One has ms level resolution and is used for general VI execution. It uses a pool of threads at various priorities, and the LV wait and wait ms functions and a plain while loop will operate in this scheduler. The TimedLoop uses its own scheduler that runs at an elevated priority and has advanced scheduling options -- quite advanced. It can schedule at sub millisecond periods and implements a variety of policies. Only Timed structure code runs in this scheduler.

The timed loop is also instrumented so that it is easy to determine start and stop times, missed deadlines, adjust priorities and other things. I'd highly recommend that you instrument your PID not only to measure its actual period, but also its execution time. You could also do this with a profiler if you have one. This will let you know the cost of the operation you are scheduling.

Greg McKaskle