1. Toward Energy-Aware Software-Based Fault Tolerance in Real-Time Systems Osman S. Unsal, Israel Koren, C. Mani Krishna Architecture and Real-Time Systems Laboratory Department of Electrical and Computer Engineering University of Massachusetts, Amherst

2. The Problem Real-Time (RT) systems are energy and thermal constrained. Many RT applications run on battery-powered platforms. RT systems require small form factor. Fault-Tolerance (FT) is an important design parameter in RT systems. Many RT applications are life-critical. Many RT systems operate in hostile (industrial, space) environments. FT ensures error-free operation in the face of faults.

3. Fault-Tolerance in RT Systems Hardware based fault tolerance Massive redundancy (duplex, TMR) Requires additional hardware for error checking mechanism Very power-inefficient Software based fault tolerance Application-Level Fault Tolerance (ALFT), an amalgam of time and software redundancy

4. ALFT Characteristics Tasks have a primary and secondary copy Secondaries might be exact copy of primaries, or they could be scaled-down Resolution reduction Precision reduction A secondary task may be aborted if primary successfully finishes execution

5. The System Model Distributed RT System Tasks are periodic, have deadlines Each primary has one secondary Primary and Secondaries assigned to separate processors Concentrating on scheduling, compare w.r.t. EDF Tasks with random periods, execution-time Six processor configuration

6. Energy Model The more a task executes, the more the energy consumed. Assumed to linearly scale with the increase in task execution Appropriate for COTS processors

7. Overlap

8. A Simple Energy Saving Heuristic : Shortest Execution-Time First (SEF) Power-Unaware Task Duplication Power-Aware (ALFT) Sec. Size Sec.Size 100% 50% EDF 100 96 77 SEF 83 77 61 Relative Energy Consumption

9. Another Heuristic: Secondary Execution Time Shifting (SETS)

10. Case study: Asymmetric Digital Subscriber Line Modem Application PeriodWCETPeriodWCET 128313212 963163 12836421 12821163 7682402048864 25696418 12860576285

11. Energy Savings for the ADSL Application Secondary Size (%)Energy Savings (%) 339.51 5013.38 10019.44

12. Energy Savings for Different Secondary Sizes

13. Overlap Reduction for Different Secondary Sizes (20 tasks)

14. Overlap Reduction for Different Secondary Sizes (50 tasks)

15. Effect of Task Granularity on Energy Savings (Secondary Size 80%)

16. Effect of Task Granularity on Overlap Reduction (Secondary Size 80%)

17. Summary An initial analysis into energy-efficiency of various fault-tolerance mechanisms has been made Power-aware scheduling heuristics for ALFT schemes developed Current activity: On-line scheduling heuristics Power-aware DVS for FT systems