Course Outline. Embedded Real Time Operating System Chin Fu Kuo National University of Kaohsiung 2009 Fall

Transcription

1 Course Outline Embedded Real Time Operating System Chin Fu Kuo National University of Kaohsiung 2009 Fall

2 Information Instructor: Office: M624 TA:?

3 Course Materials Website Download syllabus from the website before classes Reference book Jane W. S. Liu, Real-Time Systems, Prentice Hall, 2000.

4 Prologue In 1996, NASA launched spacecraft Pathfinder onto Mars It successfully landed at 1997 Behind the huge success of the project, an extraordinary landing remains untold to the public

5 Pathfinder, protected by airbags The Pathfinder **Pictures courtesy of NASA

6 Prologue Pathfinder periodically experiences system hardreset Maybe software or hardware glitches? The problem persists, and frequent loss of data is caused Obviously the problem is not simple Fully interactive debugging and testing is now impossible! What really happened on Mars?

7 Course Objectives To deliver real time system theory Scheduling algorithms When we refer to real time, we usually mean resource scheduling CPU, I/O, semaphores, etc. To have solid experiences on real time scheduler implementation Exercises on embedded platforms, with real time embedded operating systems

8 Course Objectives Real time scheduling theory, however, usually can t be directly applied to realistic systems Then why should we lean theoretical matters? That s to avoid choosing bad designs, and, of course, to come to a good design For example, a frequently overloaded system should provide stable prioritization rather than high resource utilization Pathfinder itself is really another instance

9 Concepts for Real Time Systems What does time mean to computing systems? Is it finish a batch of tasks with a reduced period of time, users feel that tasks runs smoothly, or tasks deliver stable and predictable performance?

10 Concepts for Real Time Systems What s different in embedded systems? Embedded systems are dedicated to specific purposes, equipped with special hardware Resources are very limited due to cost consideration Usually working under timing pressure Ex: ABS, Avionics systems, Radars, and Multimedia systems

11 Concepts for Real Time Systems Any system works under (explicit or implicit) timing constraints is a real time system Tasks must be completed in time In other words, predictability is much more important than speed!

12 Concepts for Real Time Systems Guarantee, guarantee, and guarantee Real time systems are not high performance systems Let s use better algorithms to provide performance guarantee! Let s use more powerful hardware to improve performance! How to provide performance guarantees with low hardware cost? To answer this question, you have to know how to analyze your system

13 Example Periodic tasks Servers/reso urces τ 1 τ 2 τ 3 Emergency 50 msec 5 msec d:6 msec Aperiodic tasks Event handling task 40 msec 2 msec Desired response 4 msec on average

14 Topics Priority driven and share driven scheduling algorithms: Fundamentals of real time scheduling algorithms Static priority scheduling deadline monotonic scheduling, rate monotonic scheduling Dynamic priority scheduling Least slack first, earliest deadline first Share driven scheduling General Processor Sharing (GPS/EGPS)

15 Topics Scheduling algorithms for aperiodic and sporadic jobs How hardware events could be handled with bounded response time Background scheduling, priority exchange scheduling, sporadic servers, total bandwidth servers, constant utilization servers

16 Topics Resource synchronization protocols Management of resource contentions with bounded delay and fairness Priority ceiling protocol, priority inheritance protocol, stack resource protocol

17 Topics Overload management Management of system resource with transient workload bursts Firm real time scheduling, soft real time scheduling, window constrained scheduling Advanced topics Topics for real time scheduling beyond single processor Multi processor scheduling, end to end scheduling

18 Projects Use embedded development boards Samsung S3C2440A Projects: Implement EDF scheduler in uc/os II Lego NXT.

19 Grading Policy Midterm and final 25% and 25% respectively Two hands on projects 15% and 15% Paper presentation 15% Lecturer s flexibility 5%

20 Text Books Jane W.S. Liu, Real-Time Systems, Prentice Hall ISBN: Jean Labrosse, MicroC/OS II: The Real Time Kernel, 2/e CMPBooks ISBN:

21 Epilogue So what s really happened on Mars? Priority inversion Due to interference from low priority tasks, a high priority task must wait for resource becomes available If the waiting time is unbounded or exceeds a tolerable duration of time, trouble might happen E.g., watchdogs reset the whole system

22 T1 preempts T3 T1 High priority CPU T1 fails to lock R1 T2 Mid priority CPU T2 preempts T3 T3 Low priority CPU R1 R1 T1 can be blocked by Resource confliction of R1 from T3 It can easily be identified and accounted Execution of T2 But T1 and T2 have no resource contention?

23 I bet that you already have many thoughts on this problem.

24 See you next week!