By David Ott
Part 4: Operating System Power Management (OSPM)
A distinctive highlight of ACPI is the shift from BIOS-controlled power management in APM to operating system power management, or OSPM. Several reasons underlie this shift. First, the complexity requirements for power management have continued to grow and BIOS-based solutions increasingly pose integration difficulties with operating systems running at the software layer. Second, an OS-based solution allows the software management layer to evolve independently of hardware feature implementations. BIOS-based solutions are necessarily hardware-specific. Finally, an OS has a great deal of information on the applications and processes that run on the system. This information can be leveraged in making management decisions that anticipate usage patterns and respect system and user priorities.

Two key areas of OSPM management are, as the reader might guess, C-states and P-states. C-states, or “idle” states, dictate how much power savings can be achieved during a period of processor idleness. C-states can be applied on a per-core basis within today’s multi-core processors, lending an added degree of complexity to the management task. Typically, a series of stepwise timers are used to detect idleness and to advance the current C-state of an idle core through a sequence of deeper and deeper sleep states (C1, then C2, then C3, etc.) as idle time increases. Other algorithms are possible, and work is continually going on to develop more sophisticated control algorithms that jump to deeper sleep states earlier and generate better sleep state opportunities through OS task scheduling.

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