- By William Stanek
Power Management Options
When working with power management, important characteristics to focus on include the following:
- Cooling modes
- Device states
- Processor states
ACPI defines active and passive cooling modes. These cooling modes are inversely related to each other:
- Passive cooling reduces system performance but is quieter because there’s less fan noise. With passive cooling, Windows lessens power consumption to reduce the operating temperature of the computer but at the cost of system performance. Here, Windows reduces the processor speed in an attempt to cool the computer before increasing fan speed, which would increase power consumption.
- Active cooling allows maximum system performance. With active cooling, Windows increases power consumption to reduce the temperature of the machine. Here, Windows increases fan speed to cool the computer before attempting to reduce processor speed.
Power policy includes an upper and lower limit for the processor state, referred to as the maximum processor state and the minimum processor state, respectively. These states are implemented by making use of a feature of ACPI 3.0 and later versions called processor throttling, and they determine the range of currently available processor performance states that Windows can use. By setting the maximum and minimum values, you define the bounds for the allowed performance states, or you can use the same value for each to force the system to remain in a specific performance state. Windows reduces power consumption by throttling the processor speed. For example, if the upper bound is 100 percent and the lower bound is 5 percent, Windows can throttle the processor within this range as workloads permit to reduce power consumption. In a computer with a 3-GHz processor, Windows would adjust the operating frequency of the processor between .15 GHz and 3.0 GHz.
Processor throttling and related performance states were introduced with Windows XP and are not new, but these early implementations were designed for computers with discrete-socketed processors and not for computers with processor cores. As a result, they are not effective in reducing the power consumption of computers with logical processors. Windows 7 and later releases of Windows reduce power consumption in computers with multicore processors by leveraging a feature of ACPI 4.0 called logical processor idling and by updating processor throttling features to work with processor cores.
Logical processor idling is designed to ensure that Windows uses the fewest number of processor cores for a given workload. Windows accomplishes this by consolidating workloads onto the fewest cores possible and suspending inactive processor cores. As additional processing power is required, Windows activates inactive processor cores. This idling functionality works in conjunction with management of process performance states at the core level.
ACPI defines processor performance states, referred to as p-states, and processor idle sleep states, referred to as c-states. Processor performance states include P0 (the processor/core uses its maximum performance capability and can consume maximum power), P1 (the processor/core is limited below its maximum and consumes less than maximum power), and Pn (where state n is a maximum number that is processor dependent, and the processor/core is at its minimal level and consumes minimal power while remaining in an active state).
Processor idle sleep states include C0 (the processor/core can execute instructions), C1 (the processor/core has the lowest latency and is in a nonexecuting power state), C2 (the processor/core has longer latency to improve power savings over the C1 state), and C3 (the processor/core has the longest latency to improve power savings over the C1 and C2 states).
Windows saves power by putting processor cores in and out of appropriate p-states and c-states. On a computer with four logical processors, Windows might use p-states 0 to 5, where P0 allows 100 percent usage, P1 allows 90 percent usage, P2 allows 80 percent usage, P3 allows 70 percent usage, P4 allows 60 percent usage, and P5 allows 50 percent usage. When the computer is active, logical processor 0 would likely be active with a p-state of 0 to 5, and the other processors would likely be at an appropriate p-state or in a sleep state. Figure 1-1 shows an example. Here, logical processor 1 is running at 90 percent, logical processor 2 is running at 80 percent, logical processor 3 is running at 50 percent, and logical processor 4 is in the sleep state.
Figure 1-1 Understanding processor states