Background: I'm working on porting an Ada project from Vxworks to a Linux platform. The project heavily relies on usage of Ada's dynamic task priorities.
I've done a couple experiments which have left me confused. Ada task priorities seem to have no effect on the underlying Linux priority/niceness. Two identical tasks with different priorities take equally long to complete, even when pinned to one CPU core.
Question: What are the semantics of Ada tasks on Linux? Why am I seeing no effect from setting task priorities?
Question: What are the semantics of Ada tasks on Linux? Why am I seeing no effect from setting task priorities?
The first question has bascially been answered by Shark8. I figured I'd post the answer to my second question.
Answer: Using Ada task priorities on Linux requires using a Linux "real time" scheduling algorithm. The default is typically SCHED_OTHER. pragma Time_Slice (0.0) or pragma Task_Dispatching_Policy (FIFO_Within_Priorities) will set the scheduling algorithm to SCHED_FIFO. This requires running the application as root.
Note: I had tried setting SCHED_FIFO manually using chrt on the running process, which did not work.