Question

I have a (soft) realtime system which queries some sensor data, does some processing and then waits for the next set of sensor data. The sensor data are read in a receiver thread and put into a queue, so the main thread is “sleeping” (by means of a mutex) until the new data has arrived.

There are other tasks like logging or some long-term calculations in the background to do. These are implemented to run in other threads.

However, it is important that while the main thread processes the sensor data, it should have highest priority which means that the others threads should not consume any CPU resources at all if possible (currently the background threads cause the main thread to slow down in an unacceptable way.)

According to Setting thread priority in Linux with Boost there is doubt that setting thread priorities will do the job. I am wondering how I can measure which effect setting thread priorities really has? (Platform: Angstrom Linux, ARM PC)

Is there a way to “pause” and “continue” threads completely?

Is there a pattern in C++ to maybe realize the pause/continue on my own? (I might be able to split the background work into small chunks and I could check after every chunk of work if I am allowed to continue, but the question is how big these chunks should be etc.)

Thanks for your thoughts!

Answer 1

Your problem is with OS scheduler, not the C++. You need to have a real real-time scheduler that will block lower priority threads while the higher priority thread is running.

Most “standard” PC schedulers are not real-time. There’s an RT scheduler for Linux – use it. Start with reading about SCHED_RR and SCHED_FIFO, and the nice command.

In many systems, you’ll have to spawn a task (using fork) to ensure the nice levels and the RT scheduler are actually effective, you have to read through the manuals of your system and figure out which scheduling modules you have and how are they implemented.