The PAST algorithm is a dynamic voltage scaling algorithm, which works on the principle of idle time. It predicts the next interval to have the same load as the previous interval, and sets the frequency and voltage accordingly. If the system is near busy (utilization > 0.7), the system runs at maximum frequency and voltage. If the system has more idle time ( utilization < 0.7), the system runs at lower frequency and voltage, in steps, depending on the utilization, with lower utlization using a lower value of voltage and frequency, and high utlization using a higher value of voltage and frequency.
Since the prediction is based on only on the previous interval, a long interval could mean lower power, but also may not reflect the system state accurately. On the other hand, a shorter interval will give better performance, but also consume higher power.
The execution time is given as a range. The user must enter the upper and lower bounds of the execution time, or can use the default values provided. The algorithm picks a random execution time from within that boundary, and applies the appropriate voltage and frequency scaling to it. There are 4 different voltage and frequency levels provided that the user can modify. They must be entered in steps in ascending order.
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