CprE 458/558: Real-Time Systems
Energy-aware QoS packet
scheduling
CprE 458/558: Real-Time Systems (G. Manimaran)
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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)
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Energy Consumption
Motivation
QoS / Real-time guarantees
CprE 458/558: Real-Time Systems (G. Manimaran)
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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)
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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)
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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)
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DMS: Energy-Delay tradeoffs
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Problem - 1
• To assign modulation levels to the
incoming traffic flows while guaranteeing
delay bounds within the WFQ
framework.
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E2WFQ scheduler
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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
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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)
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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)
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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)
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A typical inflow rate profile
Rate
Peak rate
Guaranteed rate (gi)
Average rate (λi)
time
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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)
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The instantaneous output rate
The output rate Rout,i for a particular flow “i”
Guaranteed
rate
The total output rate of the link
Maximum of
the output rates
required by all
the queued
packets
CprE 458/558: Real-Time Systems (G. Manimaran)
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The instantaneous modulation level
The modulation level for the link with a capacity “C” is given by
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Maximum delay expressions
Theorem: The maximum packet of delay of stream “i” ,
under the E2WFQ scheme is given by:
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Energy aware real-time packet scheduling
Sensor nodes send real-time (periodic) multimedia streams to
the aggregation node G.
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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 preempted.
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Periodic RT Messages
The Real-Time DMS packet Scheduler
Admission
Controller
CprE 458/558: Real-Time Systems (G. Manimaran)
RT-DMS
Scheduler
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Admission test
The following time completion test is employed
for admission of periodic streams
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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)
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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
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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.
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RT-DMS example with 3 periodic streams
No DMS
Static DMS
Run-time
Dynamic
DMS
Stretch DMS
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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.
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Thank You!!
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