Jacob R. Lorch and Alan Jay Smith University of California, Berkeley Reducing Processor Power Consumption by Improving Processor Time Management in a Single-User Operating System Jacob R. Lorch and Alan Jay Smith University of California, Berkeley

Welfare reform initiatives Overall problem: not enough money for welfare recipients Three possible initiatives stop overpaying people who have filed fraudulent claims overstating their worthiness stop overpaying people who have filed honest claims but who, through bureaucratic error, are being paid more than regulations require reduce all payments by a certain percentage Money : welfare recipient :: CPU time : process

Outline of presentation Motivation Background on MacOS A different strategy for power management Problems with this strategy and our solutions to these problems Results of simulations Conclusions

Motivation Power reduction is important on the desktop expense, heat, fan noise, contribution to pollution government regulation: Energy Star, Blue Angel Power reduction is critical in portable computers battery capacity per unit weight advancing slowly limited space and weight available in portables drives importance of power management Importance of processor energy reduction Accounts for large percentage of total power (18-34%) Watching the CPU turning on and off, we got the impression it was frequently on when it wasn’t needed

Background on MacOS Current power management (strategy “C”): inactivity timer after 2 seconds of no activity and 15 seconds of no disk activity, processor turns off until next interrupt Process scheduling: cooperative multitasking processes switch out voluntarily when a process switches out, it indicates a sleep period, the maximum amount of time it wants to remain unscheduled if the process is busy, it will give a sleep period of zero

The basic strategy Energy and time costs to turn on and off the CPU are low Window-based operating systems are event-driven We want the CPU off whenever it is idle A process handles an event, then blocks The basic strategy (strategy “B”): Turn off the CPU when all processes are blocked, waiting for their next events However, there are some problems with the basic strategy on MacOS...

The simple scheduling technique Problem: MacOS sometimes schedules processes when they do not need to be scheduled i.e. it wakes them up after too little time asleep Why? Well, it is a single-user OS; who would care? Technique “I”: never schedule a process whose sleep time has not yet expired

Sleep extension technique Why use sleep periods: to perform periodic tasks blinking the cursor, checking if time for backups Not a real-time system, so deadlines not critical in fact, current inactivity-based strategy freely violates these deadlines Therefore, perhaps it is reasonable to let processes sleep for longer Technique “S”: multiply all sleep periods by a multiplier s, set by user or OS to maximize energy savings while maintaining desired performance

The greediness technique How processes should behave receive event, process event, block for next event perhaps, while waiting, do periodic tasks How a process should not behave: “greediness” failing to block, i.e. using sleep period 0, when not busy happens often in MacOS Technique “G”: call a process greedy if it yields control g times without activity and without blocking block greedy processes, but only for a certain period f, in case our heuristic is wrong

Trace-driven simulation Trace collection (6 users): IdleTracer only collects data while running on battery power shuts off power management while tracing Simulators: ItmSim (current strategy) and AsmSim (basic strategy) Evaluation criterion 1: CPU energy savings Evaluation criterion 2: Performance impact estimated percent increase in workload completion time increase comes from skipping over useful work when processor is off: when processor comes back on, such work must take place heuristic: quantum is useful if it has activity or an event

Results 11.5% more battery lifetime than C Performance impact for C was 1.84%, so that was the impact for BIS and BIG. Performance impact for B and OPT was 0%. Performance impact for BI was 1.08%. 11.5% more battery lifetime than C 20.0% more battery lifetime than C

Problems with sleep extension Not as much energy savings per unit performance impact as greediness technique does not even work well as a supplement to greediness Does not eliminate an important problem with basic strategy: processes that fail to block this is why even BIS did not beat C for user 2 Extending real-time sleep factors violates the user interface, and may produce undesirable effects

Future work Implementation of strategies Collection of additional traces Adaptation of techniques to other OS’s Completely different energy-saving techniques for the CPU, to get closer to optimal (or even surpass it!)

Conclusions Basic strategy needs good CPU time management Our suggested ways to improve CPU time management are: never schedule processes that don’t want time identify and block processes that are hogging the CPU let processes sleep longer than they want to With the best of these techniques, the basic strategy far surpasses the inactivity timer strategy 53% less processor energy consumption 20% more battery lifetime