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CprE 458/558: Real-Time Systems

Learn about key hardware techniques for energy savings, such as dynamic voltage scaling and dynamic modulation scaling, in real-time systems. Explore the E2WFQ scheduler and how it can improve energy efficiency while guaranteeing quality of service.

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CprE 458/558: Real-Time Systems

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  1. CprE 458/558: Real-Time Systems Energy-aware QoS packet scheduling CprE 458/558: Real-Time Systems (G. Manimaran)

  2. Overview • Motivation • Key hardware techniques for energy savings • Energy-aware weighted fair queuing • Energy-aware real-time packet scheduling CprE 458/558: Real-Time Systems (G. Manimaran)

  3. Motivation Energy Consumption QoS / Real-time guarantees CprE 458/558: Real-Time Systems (G. Manimaran)

  4. Key hardware techniques • Dynamic Voltage Scaling (DVS) for processor energy savings • Dynamically vary the operating voltage & frequency of the processor to reduce energy consumption • Dynamic Modulation Scaling (DMS) for wireless radio energy savings CprE 458/558: Real-Time Systems (G. Manimaran)

  5. Dynamic Voltage Scaling • Energy consumption of task with “cc” number of computation cycles operated at a voltage V and a corresponding frequency “f” is given by • E = CC * V2 = CC * F2 • Time taken to complete the task is given by • T = CC / F • Therefore we can run a task at a lower frequency and reduce energy consumption. However, you will need relatively more time to complete the task. CprE 458/558: Real-Time Systems (G. Manimaran)

  6. Dynamic Modulation Scaling (DMS) The energy consumption of the radio in transmitting a bit at a modulation level “b” is given by: The transmission time a bit at a modulation level “b” (number of bits per symbol) is given by: Where Rs is the number of symbols sent over the channel per sec. CprE 458/558: Real-Time Systems (G. Manimaran)

  7. DMS: Energy-Delay tradeoffs CprE 458/558: Real-Time Systems (G. Manimaran)

  8. Problem - 1 • To assign modulation levels to the incoming traffic flows while guaranteeing delay bounds within the WFQ framework. CprE 458/558: Real-Time Systems (G. Manimaran)

  9. E2WFQ scheduler CprE 458/558: Real-Time Systems (G. Manimaran)

  10. The WFQ scheduler bounds • Traffic flow model: Leaky bucket regulated flow • If a flow Ai (σi, λi) is guaranteed a rate of gi, then the maximum delay Di under GPS is given by • Di ≤ σi / gi • The maximum delay Di under WFQ is given by • Di ≤ σi / gi + Lmax / C • where Lmax is the maximum packet size • Where C is the link capacity CprE 458/558: Real-Time Systems (G. Manimaran)

  11. Important Observation • λi , the input rate of an input stream is much lower than its guaranteed rate gi • Therefore, operating at the link transmission at the instantaneous rate will result in energy savings CprE 458/558: Real-Time Systems (G. Manimaran)

  12. E2WFQ scheduler: basic idea • Monitor the instantaneous input rate • Adapt the transmission rate to the input rate subject to the delay constraints CprE 458/558: Real-Time Systems (G. Manimaran)

  13. Monitoring the input rate • Instantaneous queue size (number of packets) is a good indicator of the instantaneous input arrival rate • If input rate is greater than the output rate the queue size increases • On the other hand, if the input rate is lesser than the output rate the queue size decreases • This where we can apply DMS to reduce energy consumption CprE 458/558: Real-Time Systems (G. Manimaran)

  14. A typical inflow rate profile Peak rate Rate Guaranteed rate (gi) Average rate (λi) time CprE 458/558: Real-Time Systems (G. Manimaran)

  15. Delay constraints • Let ∆ be the desired time from the packet’s arrival at the end of the queue to its departure from the head of the queue • Let there be “m” packets (P1, P2… Pm ) in the queue arrived at times (A1, A2… Am ) respectively. Let, Am = T = current time and further assume each packet of low “i” is of size Li Pm Pk P1 K * Li • What should be the output rate ( ri,k ) of the flow “i” to guarantee the ∆ delay constraint to a packet Pk ? CprE 458/558: Real-Time Systems (G. Manimaran)

  16. The instantaneous output rate The output rate Rout,i for a particular flow “i” Guaranteed rate Maximum of the output rates required by all the queued packets The total output rate of the link CprE 458/558: Real-Time Systems (G. Manimaran)

  17. The instantaneous modulation level The modulation level for the link with a capacity “C” is given by CprE 458/558: Real-Time Systems (G. Manimaran)

  18. Maximum delay expressions Theorem: The maximum packet of delay of stream “i” , under the E2WFQ scheme is given by: CprE 458/558: Real-Time Systems (G. Manimaran)

  19. Energy aware real-time packet scheduling Sensor nodes send real-time (periodic) multimedia streams to the aggregation node G. CprE 458/558: Real-Time Systems (G. Manimaran)

  20. Problem • Assign modulation levels to each of the packets to reduce energy consumption subject to the real-time deadline constraints. • This is very similar to the DVS scheduling of periodic tasks at the processor. • Unlike the tasks on a processor, the messages on the communication link cannot be pre-empted. CprE 458/558: Real-Time Systems (G. Manimaran)

  21. The Real-Time DMS packet Scheduler Admission Controller RT-DMS Scheduler Periodic RT Messages CprE 458/558: Real-Time Systems (G. Manimaran)

  22. Admission test The following time completion test is employed for admission of periodic streams CprE 458/558: Real-Time Systems (G. Manimaran)

  23. Static DMS • Assuming maximum packet size for all the admitted packets, find the least modulation level which ensures all the deadlines • This can be accomplished an iterative approach trying each modulation level for all the packets CprE 458/558: Real-Time Systems (G. Manimaran)

  24. Dynamic DMS • The packet sizes exhibit variations, the exact packet size is known before the transmission. • The idea behind dynamic DMS is to reduce the modulation level of a smaller packet so that it takes as much time as the maximum sized packet would have taken CprE 458/558: Real-Time Systems (G. Manimaran)

  25. Stretch DMS • If the finish time of the current packet transmission and the arrival time of the next packet transmission are unequal. Some amount of slack will be left unused or the link will idling during that slack. • We can further reduce the modulation level to exploit the entire slack. CprE 458/558: Real-Time Systems (G. Manimaran)

  26. RT-DMS example with 3 periodic streams No DMS Static DMS Run-time Dynamic DMS Stretch DMS CprE 458/558: Real-Time Systems (G. Manimaran)

  27. Some References • [1] V. Raghunathan et al, “E2WFQ: An energy efficient fair scheduling policy for wireless systems”, ISPLED 2002. • [2] C. Schurgers et al., “Modulation scaling for real-time energy aware packet scheduling”, GLOBECOM’ 2001. CprE 458/558: Real-Time Systems (G. Manimaran)

  28. Thank You!! CprE 458/558: Real-Time Systems (G. Manimaran)

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