difference between non preemptive and cooperative and rate-monotonic scheduler?

Question

I have Read about co-operative Scheduler which not let higher priority task run till lower priority task block itself. so if there is no delay in task the lower task will take the CPU forever is it correct? because I have thought the non preemptive is another name for cooperative but there is another article which has confused me which say in non preemptive higher task can interrupt lower task at sys tick not in the middle between ticks so what's correct ? is actually cooperative and non preemptive are the same?

and Rate monotonic is one type of preemptive scheduler right? it's priority didn't set manually the scheduler Algo decide priority based on execution time or deadline it is correct?

is it rate monotonic better than fixed priority preemptive kernel (the one which FreeRtos Used)?

Answer 1

Co-operative scheduling is non-preemptive, but "non-preemptive" might describe any scheduler that does not use preemption. It is a rather non-specific term.

The article you describe (without citation) however, seems confused. Context switching on a tick event is preemption if the interrupted task did not explicitly yield. Not everything you read in the Internet is true or authoritative; always check your sources to determine thier level of expertise. Enthusiastic amateurs abound.

A fully preemptive priority based scheduler can context switch on "scheduling events" which include not just the timer tick, but also whenever a running thread or interrupt handler triggers an IPC or synchronisation mechanism on which a higher-priority thread than the current thread is waiting.

What you describe as "non-preemptive" I would suggest is in fact a time triggered preemptive scheduler, where a context switch occurs only in a tick event and not asynchronously on say a message queue post or a semaphore give for example.

A rate-monotonic scheduler does not necessarily determine the priority automatically (in fact I have never come across one that did). Rather the priority is set (manually) according to rate-monotonic analysis of the tasks to be executed. It is "rate-monotonic" in the sense that it supports rate-monotonic scheduling. It is still possible for the system designer to apply entirely inappropriate priorities or partition tasks in such a way that they are insufficiently deterministic for RMS to actually occur.

Most RTOS schedulers support RMS, including FreeRTOS. Most RTOS also support variable task priority as both a priority inversion mitigation, and via an API. But to be honest if your application relies on either I would argue that it is a failed design.

Answer 2

These terms can never fully cover the range of possibilities that can exist. The truth is that people can write whatever kind of scheduler they like, and then other people try to put what is written into one or more categories.

Pre-emptive implies that an interrupt (eg: from a clock or peripheral) can cause a task switch to occur, as well as it can occur when a scheduling OS function is called (like a delay or taking or giving a semaphore).

Co-operative means that the task function must either return or else call an OS function to cause a task switch.

Some OS might have one specific timer interrupt which causes context switches. The ARM systick interrupt is suitable for this purpose. Because the tasks themselves don't have to call a scheduling function then this is one kind of pre-emption.

If a scheduler uses a timer to allow multiple tasks of equal priority to share processor time then one common name for this is a "round-robin scheduler". I have not heard the term "rate monotonic" but I assume it means something very similar.

It sounds like the article you have read describes a very simple pre-emptive scheduler, where tasks do have different priorities, but task switching can only occur when the timer interrupt runs.