1. Real-Time Operating Systems
CSED191 | 컴퓨터공학소개 | November 2006
Real-Time Operating Systems
서광열, 류준호, 오병찬

2. What is a Real-Time OS? A RTOS(Real-Time Operating System)
An Operating System with the necessary features to support a Real-Time System
What is a Real-Time System?
A system where correctness depends not only on the correctness of the logical result of the computation, but also on the result delivery time
A system that responds in a timely, predictable way to unpredictable external stimuli arrivals
99% of all RTOS are for the embedded systems market

3. Examples of Real-Time Apps
GSM cell phone
One TDMA frame/4.6 ms
Maximum latency guarnatee < 500 usec
Video
HDTV 60 frames/sec -> 16 ms/frame
Audio
1-10 ms
Network Traffic Routing
Real-Time QOS(Quality of Service) contraints

4. Type of Real-Time Systems
Hard Real-Time
Missing a deadline has catastrophic results for the system
Firm Real-Time
Missing a deadline causes an unacceptable quality reduction
Soft Real-Time
Reduction in system quality is acceptable
Deadlines may be missed and can be recovered from

5. Characteristics of RTOS
Scheduling
Intertask communication and resource sharing
Interrupt handlers and the scheduler
Memory allocation
From Wikipedia

6. Scheduling
Minimize the worst-case length of time spent in the scheduler's critical section
Pre-emptive priority scheduling – higher priority tasks may interrupt lower priority tasks
Running
Task 1
Prio 7
Dispatch
Block
Task 2
Prio 5
Timer run-out
Blocked
Ready
Wakeup
Task 3
Prio 10
Choose the highest priority task

7. Intertask communication and resource sharing
Sharing data and hardware resources among multiple tasks
Unsafe for two tasks to access the same specific data or hardware resource simultaneously
Common approaches
Temporarily masking/disabling interrupts
Binary Semaphores
Message passing
RTOS
Priority Inversion Problem
Priority Inheritnace
Low Priority
High Priority
Thread 1
Thread 2
Waiting
Lock
Held
By 1
Shared
Resource

8. Interrupt handlers and the scheduler
Keep interrupt handlers as short as possible
Procedures
Acknowledge or disable the interrupt
The interrupt handler queues work to be done at a lower priority level, often by unblocking a driver task
Interrupts
Top Half
(Interrupt disabled)
Deferred
Process
Bottom Half
(Low priority)
Linux Interrupt Handling
Example

9. Memory Allocation Requirements
Problems
Speed of allocation: Scanning a linked list of indeterminate length is not acceptable
Memory fragmentation
Possible solution
Use simple fixed-size block allocation
Fragmented Memory

10. CASE STUDY
VxWorks

11. CASE: VxWorks Features
Modularized kernel
Many libraries and subroutines
High-speed interrupt process
Separated Task/Interrupt stacks
Shorter interrupt latency than Linux
Cross-compiling target software to run on various target CPU architectures

12. An application merges with OS kernel
CASE: VxWorks The pros
Multi-thread model OS
An application merges with OS kernel
It can access common workplace (memory) freely
The size of OS is small, good for small systems (easy to make)
Multi-process model은 이와 반대로 OS 커널과 Application이 독립적인 프로그램으로 동작하도록 설계되어 있고, 서로의 메모리가 보호되므로 모듈 단위의 Application 개발이나 모듈의 추가, 변경이 쉽고 안정적인 시스템 개발이 용이하다. 대신, OS의 크기가 상대적으로 크므로 작은 시스템 개발에는 오히려 부담이 될 수 있다.
비교적 작고, 복잡하지 않은 기능의 시스템 개발에는 Multi-thread 모델을 사용하고, 의료기기와 같은 대규모의 복잡한 시스템 개발에는 Multi-process 모델의 RTOS를 사용하는 것이 좋다고 알려져 있다.

13. A simple bug can spoil whole system Specialized to some vendors
CASE: VxWorks The cons
A simple bug can spoil whole system
Specialized to some vendors
Commercial software
High license cost, even running royalty exists
Plus, the market of linux-based OS is going bigger and bigger

14. Mars Pathfinder Lander fault protection
CASE: VxWorks Usages
Mars Pathfinder Lander fault protection
continuously monitors hundreds of parts of the spacecraft for a wide range of problems
takes action to fix the problem if it can, and if it can't, keeps the spacecraft safe while it waits for instructions from Earth.

15. CASE: VxWorks Usages (Continued)
Boeing 787 common core system
the backbone of the airplane's computers, networks and interfacing electronics
extremely complex device software with 80 to 100 applications running simultaneously

16. CASE STUDY
Windows CE

17. CASE: Windows CE Features
Preemptive kernel
Predictable synchronization mechanism
Predictable and bounded interrupt latency

18. CASE: Windows CE Preemptive Multitasking
Eight levels of thread priority
Real-time processing device drivers
Kernel threads and normal applications
Applications that always can be preempted
Does not age priorities
Does not mask interrupts

19. CASE: Windows CE Predictable synchronization mechanism
Correct interaction between threads
mutex, critical section, and event objects
"FIFO-by-priority" order
First come first served order
Different queue for each priority levels

20. CASE: Windows CE Interrupt latency
Amount of time that elapse
Arrives at the processor
Interrupt processing begins
Bounded and predictable
Possible to calculate worst-case latency
Does not support nested interrupts

21. CASE STUDY
Real-time Linux

22. CASE: Real-time Linux Linux is not a Real-Time OS
OOPS!
Linux
Linux
Process
Linux
Process
Linux
Process
Hardware
(CPU, Memory, Disks, etc)

23. CASE: Real-time Linux (1) FSMLab’s RTLinux
Process
Linux
Process
RTOS
Fixed Priority
Scheduler
Lowest
Priority RT
Thread RT
Thread RT
Thread
(Linux)
Hardware
(CPU, Memory, Disks, etc)

24. CASE: Real-time Linux (1) RTLinux Characteristics
RTLinux is nonpreemptible
Small size and limited operations guarantee predictable delay
Real-time tasks
Direct access to hardware
No virtual memory
Written as a special Linux module, dynamically loaded into memory
No Linux kernel modification

25. CASE: Real-time Linux (2) MontaVista’s Preemptive Kernel
2.4 Linux kernel is not preemptive
Higher priority task should be suspended

26. CASE: Real-time Linux (2) Preemptive Kernel
Others emhancements
BKL
Interrupt Threads
Read/write locks
Kernel lists/queues/fifos
Deadlock detection
Instrumentation
Preemptive kernel
Originate from SMP supports (spinlocks)
Improve latency
Lock breaking
Some spinlocks contains a long loop
Voluntary Preemption
Use might_sleep() debugging macro
Release all held spinlocks and reacquire when awakened
Priority Inheritance Mutexes
The PI mutex patch replaces the spinlocks with priority inheritance mutexes

27. The End
extremely complex device software with 80 to 100 applications running simultaneously