Preemption Handling and Scalability of Feedback DVS-EDF

1. Preemption Handling and Scalability of Feedback DVS-EDF Yifan Zhu , Frank Mueller mueller@cs.ncsu.edu Center for Embedded Systems Research / Department of Computer Science North Carolina State University

2. Background • Dynamic voltage scaling • Reduce energy consumption by lowering the supply voltage and operating frequency • Energy consumption scales • lineally with frequency • quadratically with voltage E ~ f  V² • Hard real-time system • Actual exec. time  Worst case exec. time (WCET) • Most practical system: utilization<1 NC State University

3. DVS-EDF • Original EDF: • DVS-EDF: , =scaling level • Greedily scale current task • Capitalize on early completion NC State University

4. slack Slack Generation • Two opportunities for slack generation: 1. Early completion of tasks 2. Idle slots WCET c1j T1 T1 T2 T3 NC State University

5. Idle Slots --> Slack • Introducing idle task into the task set, to convert idle time slots into slack • We choose so that there is at least one idle time slot lying between any actual task’s invocation (actual exec. time=0) NC State University

6. C1 (WCET) slack c1j T1 T2 t2 t4 t5 t8 t9 Slack Passing • Slack generated by one task will usually not be exhausted when the task completes • Initial slack capable to be passed on = C1-c1j S=S+C1-c1j, Note: it’s possible that c1j > C1, as long as the total slack is not over- exploited d1 d2 NC State University

7. r2 r2 r2 d1 d1 d1 T1 T1 T1 T2 T2 T2 t1 t1 t1 Deadline Restriction • Initial slack can’t be passed in full to the next task. Slack beyond the next task’s release time and deadline cannot be used. • Assume: T1’s deadline is d1, T2 releases at r2 • T2 can use: all the slack part of the slack none of the slack r2<t1 t1  r2<d1 d1 t2 NC State University

8. d1 d2 C1 (WCET) slack c1j idle T1 T2 t2 t4 t5 t8 t9 Adding Idle Slots • Total slack can be further increased if idle task exists between T1 and T2’s deadline slack = slack + idle(d1…d2) NC State University

9. Preemption Handling • Preempted task relinquishes its remaining slack, passing it on to the next task • Two differences here • Preempted task itself cannot generate any slack • Remaining exec. time of the preempted task must be reserved in advance NC State University

10. idle idle idle idle idle idle idle Forward Sweep Scheme • Remaining exec. time is reserved by stealing slack from the next task • Assume: = 3 slots • Forward sweep: zero slack for T2 and T3 (in the worst case, if P2>>P1, T2 is likely to get no slack always) T1 preempted T1 cont. T3 T2 t1 t2 d2 d3 d1 NC State University

11. idle idle idle idle idle idle idle Backward Sweep Scheme • Remaining exec. time is reserved as late as possible ( but no later than the preempted task’s deadline ). Assume: = 3 slots Backward sweep: 2 idle slots for T2, 1 idle slots for T3 • More greedy than forward sweep scheme T1 preempted T1 cont. T3 T2 t1 t2 d2 d3 d1 NC State University

12. Feedback Scheme • Split task k: Ck = Ca + Cb • The average actual exec. time of Ck over past is used to anticipate future Ca portions. 100%  Ck Ca Cb • the 1st instance of Ck: • later instances: NC State University

13. Experiments • Parameters: 3-task sets and 10-tasks sets • Vary: utilization and cc/WCET • Compare • Look-ahead DVS [Pillai et al.] • Our feedback DVS • Optimal: ideal scaling, disregard deadline • All energy values are relative to Look-ahead DVS : • Look-ahead DVS : 0% • Ours: additional savings over Look-ahead DVS NC State University

14. Scalability ( lowest workload) • cc=25%WCET Look-ahead DVS NC State University

15. Look ahead DVS Scalability ( medium workload) • cc=50% WCET NC State University

16. Look ahead DVS Scalability ( higher workload) • cc= 75% WCET NC State University

17. Look ahead DVS major savings minor losses Scalability ( maximum workload) • cc= 100% WCET NC State University

18. Feedback Effect • Varying actual exec. time of task set • Actual exec. time (cc) always equals to 50% WCET • Actual exec. Time = sin(t), in [0.1WCET, 0.9 WCET] WCET cc 50% t WCET cc 50% t NC State University

19. Feedback Effect • Not much difference  good! • Feedback scheme compensates for fluctuations NC State University

20. Conclusion • Feedback-EDF DVS for hard real-time systems • Slack generation and passing • Slots reservation at preemption points • Up to 37% additional energy savings over the best prior work • Scalability: 3-tasks and 10-tasks show savings. NC State University

21. Future Work • Time penalty for voltage changes • Cost depends on architecture (getting smaller) • Experiments with embedded device • Handling of dependent tasks NC State University

22. Related Work • Pillai et al. [SOSP’01] • Assuming same scaling factor for all real-time tasks • Look ahead DVS • F. Gruian [ISLPED’01] • Deriving dual-voltage schedule from the ideal case with stochastic information • D. Mosse et al. [RTSS’01] • Dynamically exploiting early completion of tasks’ future executions NC State University

23. Slack Passing (Over-Scaling) • Over-scaling: • actual exec. time > WCET • is feasible if the total slack is not exceeded as well Slack=C1-c1j<0 T2 can still be scaled without any deadline misses as long as S=S+C1-c1j >0 C1 (WCET) c1j T2 T1 t2 t4 t5 d1 d2 NC State University