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
Case Study: Alpha Generations 1.2NA< > V dd Die Size (mm 2 ) Freq. (Mhz) Power (Watts) Source Wilcox&Manne, Compaq
Power Density Source: Fred Pollack, Intel, Micro32
Fried Egg a la Athlon XP1500+ Source: The New York Times, 25 June 2002
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
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.
n Dynamic Power » Is spent when devices switch state » Proportional to: n Switching rate n Square of the supply voltage n Load capacitance
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
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
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