Real Time Operating Systems.md

title	date	lastmod
Real Time Operating Systems	2022-11-08	2022-11-21

Real Time OS

Systems whose correctness depends not only on logical aspects but also on the temporal aspects i.e. able to meet specific deadlines.

Real Time Process

Notice that C is often impossible to determine except for small specific applications.

Recurrent RTOS process

Usually executes some function or goes through a set of steps in a regular manner repeated over time. e.g. Collect data from sensors, execute control laws, send actuator commands

Periodic RTOS Process

Sporadic RTOS Process

Real Time Process Scheduling

Classical scheduling algorithms will fail to schedule RTOS processes as they do not take into account the deadlines of the processes.

Fixed priority scheduling

Priorities are fixed across instances of recurrent processes. Easy to implement with low time complexity. If all types of processes are known and small, a hash map is able to do this in constant time.

Rate Monotonic Scheduler

Since the scheduler is based on periodic time, it still does not prioritise based on deadlines. P3 misses the deadline if it is <20,2,14>: less time to complete the process in first appearance.

Deadline Monotonic Scheduler

A natural solution to RM scheduler:

Dynamic Priority Scheduling

Earliest Deadline First Scheduler

Priority is based on the instance level rather than the process level.

Comparisons

Practice Problems

a. False. Real time does not mean fast or very short time, but rather to guarantee response times within a pre-defined deadline in the worst case b. True. c. False. It is the host OS that does this. The guest OS interacts with the hypervisor and provides services to the applications running on it. d. False. Hypervisor manages interactions between H/W and guest OS e. True. Yes: Yes?