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Power-aware Computing
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Dramatic increases in computer power
consumption:
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Some processors now draw more than 100 watts
Memory power consumption is proportional to memory
size
Disks and displays also consume large quantities of
power
Case Study:
Alpha Generations
Power
(Watts)
Freq.
(Mhz)
Die Size
(mm2)
Vdd
21064
30
200
234
3.3
21164
50
300
299
3.3
21264
90
575
314
2.2
21364
100
>1000
340
1.5
21464
125-150
<2000
NA
1.2
Source Wilcox&Manne, Compaq
Power Density
1000
100
2
W/cm
10
1
i386
Source: Fred Pollack, Intel, Micro32
Pentium
Pentium
II
Pentium
IV
Fried Egg a la Athlon XP1500+
Source: The New York Times, 25 June 2002
Motivation
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Battery-powered applications:
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Lifetime between charges is linked to how much energy
is available from the battery
Battery technology has not advanced as rapidly as
computer energy consumption
Power dissipation is difficult
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Energy density of modern processors > energy density
of a hotplate
High temperatures accelerate failure rates
Types of Power Consumption
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Static Power
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Caused by leakage
Is a steady background consumption whenever the
device is on
Increases rapidly with device temperature
Currently a small fraction of the power consumed; will
become increasingly dominant as devices get smaller.
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Dynamic Power
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Is spent when devices switch state
Proportional to:
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Switching rate
Square of the supply voltage
Load capacitance
Ways to Save Energy
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Switch off subunits when they are not needed
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Spin down disks
Turn off displays
Throttle back on fetching when the fetch is far ahead of
the rest of the pipeline
Put unused functional units to sleep
Turn off cache ways
Voltage Scaling
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As the supply voltage drops,
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Power consumption drops rapidly
Circuit delays increase
The energy to execute a given workload decreases
as the square of the supply voltage
Voltage Scaling Algorithms
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Slow down the execution to reduce energy
consumption, while managing to meet all deadlines
Voltage Scaling (contd.)
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Static Voltage Scaling
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Based on the WCET of the various tasks, find a suitable
voltage schedule
Dynamic Voltage Scaling
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If a task finishes before its WCET, reclaim the released
resources to run the system at a slower speed