1. courseware Scheduling Uniprocessor Real-Time Systems Jan Madsen Informatics and Mathematical Modelling Technical University of Denmark Richard Petersens Plads, Building 321 DK2800 Lyngby, Denmark Jan@imm.dtu.dk

2. SoC-MOBINET coursewareJan Madsen2 Rate-monotonic scheduling  Real-time scheduling with provable properties [Liu and Layland 73]  Each process is independent and given a unique priority  A process may be preempted by a higher-priority process  All processes have a deadline equal to their period and a fixed WCET  Context switching overhead is ignored

3. SoC-MOBINET coursewareJan Madsen3 RMS The critical instance for a process occurs when the process and all higher-priority processes are released simultaneously sisi cici aiai didi = T i Must guarantee that all processes meet their deadlines, independent of order of releases pipi pipi

4. SoC-MOBINET coursewareJan Madsen4 RMS example 50 40 30 p3p3 p2p2 p1p1 0203040506010 TiTi CiCi p1p1 5012 p2p2 4010 p3p3 3010

5. SoC-MOBINET coursewareJan Madsen5 Rate-monotonic priority assignment  Each process is given a unique priority where 1 is the highest  Prioritize according to T i ’s, with shortest-period process given highest priority  Fixed-priority scheme  Independent of c i ’s  This priority assignment is optimal No fixed priority scheme does it better!

6. SoC-MOBINET coursewareJan Madsen6 RMS example (cont.) TiTi CiCi priority p1p1 50123 p2p2 40102 p3p3 30101

7. SoC-MOBINET coursewareJan Madsen7 CPU utilization under RMS  Given N independent processes, utilization U    N i i i T c U 1 12 1  N NU ( ) %3.69)12((lim 1   N N N Least upper bound for utilization

8. SoC-MOBINET coursewareJan Madsen8 RMS example (cont.) TiTi CiCi priorityU p1p1 50121 p2p2 40102 p3p3 30103 12 1  3 3U ( ) = 0.78 0.24 0.25 0.33 0.82

9. SoC-MOBINET coursewareJan Madsen9 RMS example (cont.) p3p3 p2p2 p1p1 50 40 30 0203040506010

10. SoC-MOBINET coursewareJan Madsen10 RMS  The utilization-based schedulability test is sufficient but not necessary  If the test is passed it is schedulable  If the test is not passed, it may be schedulable  If all processes meet their first deadline then they will meet all future one

11. SoC-MOBINET coursewareJan Madsen11 Response time analysis  Highest-priority process r = c  Other processes will suffer interference from higher-priority processes r i = c i + i i  Maximum interference occur at a critical instance

12. SoC-MOBINET coursewareJan Madsen12 Response time analysis         cj cj TjTj RiRi cici RiRi )(ihpj  Processes with higher-priority than p i number of releases of process p j during [0, R i ) each release will impose an interference of c j cjcj cici ihpj    TjTj wiwi n         )( wiwi n   1

13. SoC-MOBINET coursewareJan Madsen13 Response time analysis example TiTi CiCi priority p1p1 731 p2p2 1232 p3p3 2053 w1w1 0  3 w2w2 0  3 7 3         w2w2 1  3 + 3 = 6 7 6         w2w2 2  R 1 =3 R 2 =6

14. SoC-MOBINET coursewareJan Madsen14 Earliest-Deadline-First Scheduling  Dynamic priority scheme  changes priorities during execution based on initiation times  can achieve higher CPU utilization than RMS, i.e. 100%  Priority policy:  process with closest deadline gets highest-priority

15. SoC-MOBINET coursewareJan Madsen15 EDF example TiTi CiCi p1p1 31 p2p2 41 p3p3 52 p1p1 p2p2 p3p3 0123456789101213141516 123123 312312 231231 231231 123123 213213 123123

16. SoC-MOBINET coursewareJan Madsen16 RMS example 50 40 30 p3p3 p2p2 p1p1 0203040506010 TiTi CiCi p1p1 5012 p2p2 4010 p3p3 3010

17. SoC-MOBINET coursewareJan Madsen17 RMS example scheduled using EDF 40 30 50 p3p3 p2p2 p1p1 0203040506010 809010011070 TiTi CiCi p1p1 5012 p2p2 4010 p3p3 3010

18. SoC-MOBINET coursewareJan Madsen18 Comparing RMS and EDF  Higher CPU utilization with EDF  Easier to ensure that deadlines will be satisfied with RMS  RMS easier to implement

19. SoC-MOBINET coursewareJan Madsen19 Overload conditions  When not all processes can meet their deadlines  EDF may result in domino effect  First process missing its deadline may cause all subsequent processes to miss their deadlines  Accociate value w i of importance with each process  Use T i /w i as priority (similar to shortest-process-first)

20. SoC-MOBINET coursewareJan Madsen20 More realistic model assumptions  Shared resources  Data dependencies  Context switching  Cache effects  Power reduction

21. SoC-MOBINET coursewareJan Madsen21 Shared resources  Scheduling processes without considering resource requirements may cause priority inversion  example, p 1 highest-priority process p 1 {r 1 } p 2 {r 1 } deadlock!

22. SoC-MOBINET coursewareJan Madsen22 Shared resources  When there are mutual exclusion constraints, it is impossible to find a totally on-line optimal runtime scheduler [Mok83]  Priority ceiling protocol (PCP)  a way to bound priority inversion  blocking of processes that may cause them  temporary priority increase of process  PCP prevents deadlocks

23. SoC-MOBINET coursewareJan Madsen23 t1t1 t2t2 Data dependencies p1p1 p3p3 p2p2 didi t1t1 10 t2t2 8 cici p1p1 2 p2p2 1 p3p3 4 p1p1 p2p2 p3p3

24. SoC-MOBINET coursewareJan Madsen24 Context switching cici didi p1p1 35 p2p2 210 p2p2 p1p1 0468 2 p2p2 p1p1 0468 2

25. SoC-MOBINET coursewareJan Madsen25 Cache effects cici average c i p1p1 21.5 p2p2 10.75 p3p3 1 p3p3 p2p2 p1p1 p1p1 p2p2 p1p1 p3p3 p2p2 p1p1 p3p3 p2p2 p1p1 p3p3 p2p2 LRU cache policy: least-recently-used

26. SoC-MOBINET coursewareJan Madsen26 Cache effects cici average c i p1p1 21.5 p2p2 10.75 p3p3 1 p1p1 p2p2 p1p1 p3p3 p1p1 p3p3 p1p1 p2p2 p1p1 p3p3 p1p1 p3p3 p2p2 p1p1 cache policy: keep p 1 in cache

27. SoC-MOBINET coursewareJan Madsen27 Power reduction  Power management policy  Takes time to power-down/up  Techniques:  event/request based  predictive  probabilistic

28. SoC-MOBINET coursewareJan Madsen28 Power reduction p3p3 p2p2 p1p1 r1r1 r1r1 r1r1 r1r1 p3p3 p2p2 p1p1 r1r1 r1r1 r 1 idle