1. Introduction to Real-Time Operating Systems
David Burnett
wtf?

2. Schedule RTOS Background Designing an RTOS Designing with an RTOS
Solving Problems
Real-Time Products

3. Background: What’s a “Real Time” System?
When correctness of results depend on content and time
Hard or Soft: indicates how forgiving the system is

4. What makes an OS Real-Time?
Predictable (possibly deterministic behavior), that’s all
Not necessarily fast
Byproduct: mediocre throughputs

5. How do they work?
Tasks are scheduled by OS according to fixed priority (typically)
Tasks can’t directly interact; they use messages or shared memory & semaphores to communicate

6. Designing an RTOS: Typical Features
Many POSIX Specs Exist
No virtual memory (swap file)
Shared memory capabilities
High-resolution timer(s)
Real-time signals/QoS

7. Designing an RTOS: End Goal
Have known switching & scheduling overhead
Avoid common problems like priority inversion and deadlock

8. Designing an RTOS: Common Problems
Priority inversion
High-level task stalled due to low-level using shared resources, then a medium-level task holding up the low-level one
Solution: Priority inheritance – give low-level task high-level priority
Mars Pathfinder had this problem

9. Designing an RTOS: Common Problems (con’t)
Deadlock
Two semaphores locked out of order by two tasks and circularly block each other
Solution: “Instant Inheritance” implementation of Priority Ceiling Protocol – semaphores possibly needed by higher processes become priority tokens

10. Designing with an RTOS: What do you need?
Task information
Priorities for each task
Worst-case runtime
Best-case period
Interference information
Deadline Monotonic Analysis (DMA) calculations

11. Designing with an RTOS: Interference via DMA
Finding maximum interference value requires several bits of information:
How often & for how long higher tasks will step in
How often & for how long lower tasks will use higher semaphores to step in
How often & for how long the OS scheduler will check on things (depends on number of tasks)
How long the task switcher takes

12. Designing with an RTOS: Interference via DMA(con’t)
Add everything up: acceptable?
Yes: that’s nice.
No: find a faster OS or CPU, clean your code, etc.

13. Solving Problems
Logs
Time Machines
Memory Conservation

14. Real-Time Products VxWorks Support for equal priority
Uses both priority-based preemption or round-robin scheduling
Basic set of task communication
No memory protection
Includes pSOS support

15. Real-Time Products QNX Small memory footprint
Uses S/R/R task messaging
Dynamically start & stop filesystems, network, serial, etc.
Bootable from ROM
VxWorks-compatible

16. Real-Time Products QNX4 QNX6 (aka Nucleus) Runs on x86 hardware
Support for tons of peripherals
QNX6 (aka Nucleus)
Runs on a variety of architectures, optionally without an MMU
Increasing peripheral support
Tiny memory footprint
“Photon” window manager

17. Real-Time Products Linux Is Linux Runs on anything, even toasters
Hardware support for anything
Isn’t technically real-time, but can be

18. Real-Time Products Compared to something like QNX:
Lacks easy ROM booting
Lacks driver priority control
Can emulate S/R/R message style
Patches can make kernel preemptible and improve timer resolution, so that’s nice

19. Real-Time Products
Lots of details about the following RTOSes available at:
Linux
RTLinux
RTAI
RTEMS
QNX
VxWorks
LynxOS
And that’s not all!

20. Conclusion
Buying an RTOS is worth it

21. As Always, There’s More Information Available
More RTOS Details
Embedded Design DMA Article

22. Other Sources