1. For a good summary, visit:
Real-Time Scheduling
Module 3.2
For a good summary, visit:

2. Characteristics of Real-Time Operating Systems
A key characteristic is the variability or jitter.
i.e., level of its consistency concerning the amount of time it takes to accept and complete an application's task
A hard real-time operating system has less jitter than a soft real-time operating system.
The chief design goal is not high throughput, but rather a guarantee of a soft or hard performance category.
A real-time OS that can usually or generally meet a deadline is a soft real-time OS, but if can meet a deadline deterministically it is a hard real-time OS.

3. Characteristics of Real-Time Operating Systems
Responsiveness/Predictability
How long, after acknowledgment, it takes the operating system to service the interrupt
Includes amount of time to begin execution of the interrupt
Includes the amount of time to perform the interrupt
The effect of interrupt nesting will introduce more delays
Keep interrupt handling and context switching to minimal.
In some RTOS, disable interrupts altogether.

4. Characteristics of Real-Time Operating Systems
User Control
User control in non-RTOS has no or limited control (say scheduling function and priority)
User control in RTOS may include:
User specifies priority
User task priority can be higher than priority of system tasks
Specify paging or not for a process
What processes must always reside in main memory
Disks algorithms to use
Rights of processes

5. Characteristics of Real-Time Operating Systems
Reliability
Rebooting is unacceptable way of handling failures.
Degradation of performance may have catastrophic consequences (financial loss or loss of life.)
When there is a failure, a core dump is not acceptable. RTOS attempt either to correct the problem or minimize its effects while continuing to run
Most critical, high priority tasks must always be met. Stability issue.

6. Features of Real-Time Operating Systems
Fast context switch
Small size with minimal functionality
Ability to respond to external interrupts quickly
Multitasking with IPC primitives such as semaphores, signals, and events
Preemptive scheduling base on priority
Minimization of intervals during which interrupts are disabled
Delay tasks for fixed amount of time

7. RT Scheduler The heart of RTOS is the short term scheduler.
What is important is meeting the deadline of hard RT tasks and the completion of as many soft RT tasks as possible.
Issues like fairness, minimizing turnaround time, etc are not that important.
Military aircraft requires time constraints in the range of 10 to 100 microseconds.

8. RT Scheduling
RTOS schedulers must be able to guarantee that a process will complete by its required deadline. Failure leads to catastrophe; late execution is no better than no execution. One criteria is that the execution time of all processes (including operating system processes) be predictable.
When a new task arrives, the system can decide whether or not it can be guaranteed. Scheduling algorithms can use static or dynamic techniques. When combined with traditional scheduling notions of preemption and priority, several classes of algorithms result

9. Scheduling of a Real-Time Process

10. Scheduling of a Real-Time Process

11. Scheduling of a Real-Time Process

12. Scheduling of a Real-Time Process

13. Real-Time Scheduling Approaches
Static table-driven
can be developed when tasks are periodic, and have a known duration. Execution schedule can be planned off-line.
Of course such systems have little or no ability to respond to unexpected events.
Predictable but inflexible (change to any task means redoing the schedule.)
EDF is an example
Static priority-driven preemptive
assigns static priorities off-line and then applies them in some form of preemptive priority scheduler.
Traditional priority-driven scheduler is used
Priority is based on time constraints
Rate monotonic scheduling (RMS) is an example.
Dynamic planning-based
tries to build achievable schedules on the fly, i.e. feasibility analysis.
When a new task arrives, the system tries to fit it into the existing schedule. If this can be done, and all deadlines can still be met, the task will be accepted.
Dynamic best effort
No feasibility analysis. Admit every process
Commercially available, it is easy to implement
System assigns priority to the task when it arrives
Does not guarantee meeting deadlines

14. Deadline Scheduling
Real-time applications are not concerned with speed but with completing or starting tasks
Scheduling tasks with the earliest deadline minimized the fraction of tasks that miss their deadlines

15. Deadline Scheduling Information used Ready time
Periodic times are examples
Starting deadline
Completion deadline
Usually either starting or completion deadline
Processing time
Either supplied or exponentially estimated
Resource requirements
Priority
Hard vs. soft RT task
Subtask scheduler
Some tasks have a mandatory and optional subtasks. Only mandatory subtask possesses a hard deadline.

16. Two Tasks A runs every 20ms for 10ms, and B runs every 50ms for 25ms.
The system can do a scheduling decision every 10ms.

18. Execution time of each task is 20ms.

19. Rate Monotonic Scheduling
Assigns priorities to tasks on the basis of their periods
Highest-priority task is the one with the shortest period

20. Periodic Task Timing Diagram

22. Features of RMS Widely adopted for use in industrial applications.
Utilization as high as 90% is often achieved.
Adjustment of priority is easily achieved by making the task periods shorter. This is done whenever task starts missing its deadline.