Processor Clocking Control in Windows Server 2008 R2
Opteron and Xeon processors can be throttled and under-clocked to save power. ProLiant servers can do this automatically (using the Dynamic setting of of Power Regulator). Alternatively, operating systems can do this in drivers.
Controlling processor frequency with 2008 R2
Under-clocking is running a processor at less than its rated frequency. Xeon and Opteron processors can be switched between about a half-dozen settings, each called a processor performance state, or "p-state". The smarts inside Windows Server 2008 to switch the processors between p-states is part of Processor Power Management (PPM).
With 2008 R2, Microsoft added additional power features, including Core Parking, Enhanced Power Management, and Processor Clocking Control, which broadens the capability of PPM.
Processor Clocking Control
Since both Windows 2008 and ProLiant hardware can both control processor frequency, which should you use? Windows understands workload demands, while hardware knows environmental and architectural constraints. Ideally, they could work together.
So tucked in 2008 R2 is a feature that HP and Microsoft jointly developed to do just that. Processor Clocking Control (PCC) is an extension of PPM that lets software and hardware coordinate p-state changes.
Uses ACPI Interface
Under PCC, system firmware passes to the OS a range of hardware-supported processor frequencies. While running, the OS submits requests to firmware to raise or lower the current frequency, based on its anticipated performance needs. System firmware either makes the requested changes, or -- based on its knowledge of hardware and environmental conditions -- rejects them.
If the OS requests a higher frequency and the firmware can't deliver, an error message is sent back. Under Windows, that message shows up in an Event log entry:
Enabling and Disabling PCC
The default settings on ProLiant G6 and G7 servers and Windows Server 2008 R2 enable PCC.
You can change this default behavior by changing BIOS settings. Here are the settings to use in RBSU:
| Control Method | BIOS settings |
| 1. Maximum Frequency / No p-state usage | Power Profile="Custom"; Power Regulator="Static High"; Collaborative Mode="Disabled" |
| 2. OS control mode | Power Profile="Custom"; Power Regulator = "OS control" |
| 3. Hardware control mode | Power Profile="Custom"; Power Regulator="Dynamic"; Collaborative Mode="Disabled" |
| 4. PCC mode | Power Regulator = any setting except "OS Control"; Collaborative Mode="Enabled" |
EPA Test Results
Here's a study from the EPA that highlights the power efficiency improvements in recent ProLiant hardware and Windows Server. It specifically calls out the use of PCC. The paper shows that the hardwarwe and software combine for a power savings of about 25% over Server 2003 on older servers.
