How do Real Time Operating Systems work?
I mean how and why are realtime OSes able to meet deadlines without ever missing them? Or is this just a myth (that they do not miss deadlines)? How are they different from any regular OS and what prevents a regular OS from being an RTOS?
Answer
Meeting deadlines is a function of the application you write. The RTOS simply provides facilities that help you with meeting deadlines. You could also program on "bare metal" (w/o a RTOS) in a big main loop and meet you deadlines.
Also keep in mind that unlike a more general purpose OS, an RTOS has a very limited set of tasks and processes running.
Some of the facilities an RTOS provide:
- Priority-based Scheduler
- System Clock interrupt routine
- Deterministic behavior
Priority-based Scheduler
Most RTOS have between 32 and 256 possible priorities for individual tasks/processes. The scheduler will run the task with the highest priority. When a running task gives up the CPU, the next highest priority task runs, and so on...
The highest priority task in the system will have the CPU until:
- it runs to completion (i.e. it voluntarily give up the CPU)
- a higher priority task is made ready, in which case the original task is pre-empted by the new (higher priority) task.
As a developer, it is your job to assign the task priorities such that your deadlines will be met.
System Clock Interrupt routines
The RTOS will typically provide some sort of system clock (anywhere from 500 uS to 100ms) that allows you to perform time-sensitive operations. If you have a 1ms system clock, and you need to do a task every 50ms, there is usually an API that allows you to say "In 50ms, wake me up". At that point, the task would be sleeping until the RTOS wakes it up.
Note that just being woken up does not insure you will run exactly at that time. It depends on the priority. If a task with a higher priority is currently running, you could be delayed.
Deterministic Behavior
The RTOS goes to great length to ensure that whether you have 10 tasks, or 100 tasks, it does not take any longer to switch context, determine what the next highest priority task is, etc...
In general, the RTOS operation tries to be O(1).
One of the prime areas for deterministic behavior in an RTOS is the interrupt handling. When an interrupt line is signaled, the RTOS immediately switches to the correct Interrupt Service Routine and handles the interrupt without delay (regardless of the priority of any task currently running).
Note that most hardware-specific ISRs would be written by the developers on the project. The RTOS might already provide ISRs for serial ports, system clock, maybe networking hardware but anything specialized (pacemaker signals, actuators, etc...) would not be part of the RTOS.
This is a gross generalization and as with everything else, there is a large variety of RTOS implementations. Some RTOS do things differently, but the description above should be applicable to a large portion of existing RTOSes.