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

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 * V 2 = CC * F 2 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 R s 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 E 2 WFQ scheduler

CprE 458/558: Real-Time Systems (G. Manimaran)10 The WFQ scheduler bounds Traffic flow model: Leaky bucket regulated flow If a flow A i (σ i, λ i ) is guaranteed a rate of g i, then the maximum delay D i under GPS is given by –D i ≤ σ i / g i The maximum delay D i under WFQ is given by –D i ≤ σ i / g i + L max / C –where L max is the maximum packet size –Where C is the link capacity

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

CprE 458/558: Real-Time Systems (G. Manimaran)12 E 2 WFQ 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 Guaranteed rate (g i ) Average rate (λ i ) time Rate

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 (P 1, P 2 … P m ) in the queue arrived at times (A 1, A 2 … A m ) respectively. Let, A m = T = current time and further assume each packet of low “i” is of size L i PmPm PkPk P1P1 What should be the output rate ( r i,k ) of the flow “i” to guarantee the ∆ delay constraint to a packet P k ? K * L i

CprE 458/558: Real-Time Systems (G. Manimaran)16 The instantaneous output rate Maximum of the output rates required by all the queued packets Guaranteed rate The output rate R out,i for a particular flow “i” 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 E 2 WFQ 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 RT-DMS Scheduler Admission Controller 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 Dynamic DMS Run-time Stretch DMS

CprE 458/558: Real-Time Systems (G. Manimaran)27 Some References [1] V. Raghunathan et al, “E 2 WFQ: An energy efficient fair scheduling policy for wireless systems”, ISPLED [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!!