Windows Server 2012 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 ver¬sions 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 per¬formance 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 per¬cent, 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 proces¬sor 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 fea¬tures 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 con¬solidating workloads onto the fewest cores possible and suspending inactive proces¬sor 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 proces¬sor 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 con¬sumes 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 instruc¬tions), 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 sav¬ings over the C1 and C2 states).

ACPI 4.0 was finalized in June 2009 and ACPI 5.0 was finalized in December 2011. Computers manufactured prior to this time will likely not have firm¬ware that is fully compliant, and you will probably need to update the firmware when a compatible revision becomes available. In some cases, and especially with older hardware, you might not be able to update a computer’s firmware to make it fully compliant with ACPI 4.0 or ACPI 5.0. For example, if you are configuring the power options and you don’t have minimum and maximum processor state options, the computer’s firmware isn’t fully compatible with ACPI 3.0 and likely will not fully sup¬port ACPI 4.0 or ACPI 5.0 either. Still, you should check the hardware manufacturer’s website for firmware updates.

Windows switches processors/cores between any p-state and from the C1 state to the C0 state nearly instantaneously (fractions of milliseconds) and tends not to use the deep sleep states, so you don’t need to worry about performance impact to throttle or wake up processors/cores. The processors/cores are available when they are needed. That said, the easiest way to limit processor power management is to modify the ac¬tive power plan and set the minimum and maximum processor states to 100 percent.

Logical processor idling is used to reduce power consumption by removing a logical processor from the operating system’s list of nonprocessor-affinitized work. However, because processor-affinitized work reduces the effectiveness of this feature, you’ll want to plan carefully prior to configuring processing affinity settings for applica¬tions. Windows System Resource Manager allows you to manage processor resources through percent processor usage targets and processor affinity rules. Both techniques reduce the effectiveness of logical processor idling.

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.

Source of Information : Windows Server 2012 Pocket Consultant 2012


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