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Power-aware Computing n Dramatic increases in computer power consumption: » 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
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Case Study: Alpha Generations 1.2NA<2000125-15021464 1.5340>100010021364 2.23145759021264 3.32993005021164 3.32342003021064 V dd Die Size (mm 2 ) Freq. (Mhz) Power (Watts) Source Wilcox&Manne, Compaq
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Power Density Source: Fred Pollack, Intel, Micro32
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Fried Egg a la Athlon XP1500+ Source: The New York Times, 25 June 2002
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Motivation n Battery-powered applications: » 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 n Power dissipation is difficult » Energy density of modern processors > energy density of a hotplate » High temperatures accelerate failure rates
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Types of Power Consumption n Static Power » 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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n Dynamic Power » Is spent when devices switch state » Proportional to: n Switching rate n Square of the supply voltage n Load capacitance
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Ways to Save Energy n Switch off subunits when they are not needed » 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
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Voltage Scaling n As the supply voltage drops, » Power consumption drops rapidly » Circuit delays increase n The energy to execute a given workload decreases as the square of the supply voltage n Voltage Scaling Algorithms » Slow down the execution to reduce energy consumption, while managing to meet all deadlines
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Voltage Scaling (contd.) n Static Voltage Scaling » Based on the WCET of the various tasks, find a suitable voltage schedule n Dynamic Voltage Scaling » If a task finishes before its WCET, reclaim the released resources to run the system at a slower speed
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