Power Management

Why does an OS slightly shift and align the wake-up times of many programs' timers (timer coalescing)?

  1. ATo improve timer accuracy
  2. BTo reduce the number of programs
  3. CTo reduce how often the CPU wakes and lengthen each idle period, making deep power-saving states easier to enter
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Answer: C. To reduce how often the CPU wakes and lengthen each idle period, making deep power-saving states easier to enter

If many programs set timers at scattered times, the CPU must wake for each, idle periods stay short and deep power-saving states are out of reach. The OS groups timer expirations that can tolerate a little delay into nearby times and handles them together on one wake-up. Idle periods lengthen beyond the break-even time, allowing deep sleep and greatly reducing standby power.

  • A. Accuracy actually drops slightly.
  • B. The number of programs does not change.
  • C. Correct. It accepts small delays to process work together.
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More Power Management questions

  1. What is the idea behind the tickless kernel used in Linux and other OSes?
  2. What is the Arm WFI (Wait For Interrupt) instruction used for?
  3. What is commonly used to wake a battery-powered device from sleep at a set time?
  4. If only the supply voltage is lowered by 10% (all else equal), by roughly what percentage does dynamic power fall?
  5. In the CMOS dynamic power formula P = αCV²f, what does α represent?