Yifan Zhu, Frank Mueller North Carolina State University Center for Efficient, Secure and Reliable Computing DVSleak: Combining Leakage Reduction and Voltage Scaling in Feedback EDF Scheduling 2 Background Dyn. Voltage scaling (DVS): lowers dyn. power Power ~ Sleep: lowers leakage (static) power Dynamic power was dominating Leakage becoming dominant 3 Real-Time Systems Hard real-time systems periodic, preemptive, independent tasks [Liu, Layland] w/ known worst-case execution time (WCET) jobs: periodically released instances of a task WCET: measured at the max. freq., w/o DVS most practical system: U t th before DVS but < t th afterwards no DVS 2. idle1+idle2 < t th no delay 3. If idle1 < C B no delay 4. Otherwise delay Still guaranteed to meet deadline proof in paper idle1 idle2 T1T2T3 t (i) Consider Schedule WCET idle1 sleep T1T2T3 t (ii) No Delay WCET idle2 sleep T1T2T3 t (iii) Delay CBCB WCET threshold for sleep 13 Scaling below the Critical Speed Pure DVS: should never scale frequency below critical speed DVS combined with sleeping: sleep if threshold t th > idle slot If idle slot is too short (< t th ), scale below critical speed No other work to do (in contrast to non real-time) Lower frequency/voltage power savings 14 Experimental Framework Scheduling simulator Accurately reflects energy trends [Zhu05] PPC405LP Use the same power model as [Jejurikar04] Critical speed, wakeup cost Assume four discrete frequency levels: 25%, 50%, 75%, 100% of f max Compare energy in hyperperiod (const. amount of work) for Pure Feedback-DVS DVS+sleep: Feedback-DVS w/ sleep policy (no delay policy) DSR-DP: dyn. procrastination+slack reclamation [Jejurikar05] DVSleak: feedback-DVS w/ sleep & delay now/later policies Lower-bound schedule: best frequency + sleep for max. idle 15 3 Tasks, Const. Execution, 25% WCET Significant savings w/ sleep, more for low utilizations DVSleak: Delay most impact for medium to high utilizations Close to lower bound 16 3 Tasks, Const. Execution, 75% WCET All schemes: resilient to actual/WCET ratio DVSleak never worse than other schemes, savings: 50% over pure, 20% over DVS+sleep, 8.5% over DSR-DP 17 3 Tasks, Var. Execution (pat1), 75% WCET DVSleak: more resilient to fluctuating exec. times (unchanged) feedback helps! All others: 5-10% more energy consumption than for const. exec. 18 10 Tasks, Const. Execution, 25% WCET More tasks 5-10% higher energy cost (switching) DVSleak still best of all (~ same margin) 19 Length of Task Periods U=60%, E normalized to hyperperiod task set 2, c=50% WCET Harmonic (1) vs. non-harmonic (2): 10-27% more energy for non-harmonic cannot fold jobs released at same time more uncertainty Longer (2) vs. shorter (3) periods for non-harmonic: 2-28% more energy for shorter periods more job releases, less sleep time DVSleak ~ 15% lower energy than DSR-DP Feedback more important for shorter periods 20 Conclusion DVSleak: Novel Feedback DVS + leakage (sleep), benefits for fluctuating execution times shorter task periods can scale below critical speed medium utilizations (most common) sleep policy by itself enough for high/low utilizations (always sleep/never sleep) DVSleak energy over other schemes: avg. 50% over DVS-only avg. 20% more over DVS+sleep Avg. 8.5% more over [ Jejurikar05] Sleep now/later important when actual exec.