How to Disable Dynamic Frequency Scaling?

Question

Hi, I would like to do some microbenchmarks, and try to do them right. Unfortunately dynamic frequency scaling makes benchmarking highly unreliable.

Is there a way to programmatically (C++, Windows) find out if dynamic frequency scaling is enabled? If, can this be disabled in a program?

Ive tried to just use a warmup phase that uses 100% CPU for a second before the actual benchmark takes place, but this turned out to be not reliable either.

UPDATE: Even when I disable SpeedStep in the BIOS, cpu-z shows that the frequency changes between 1995 and 2826 GHz

Answer 1

In Windows XP and later CPU speed is managed by power policy. Doesn't it turn off the scaling if you set "Max performance" mode in Windows power management dialog?

There're also some third party tools - SpeedSwitchXP for example.

Programmatically this could be done, I suppose, using CallNtPowerInformation function.

Answer 2

In general, you need to do the following steps:

• Call CallNtPowerInformation() and pass SystemPowerCapabilities to InformationLevel parameter, set lpInputBuffer and nInputBufferSize to NULL, then set lpOutputBuffer to SYSTEM_POWER_CAPABILITIES structure, and set nOutputBufferSize to the size of the structure. After this first call, SYSTEM_POWER_CAPABILITIES structure containing the current system power capabilities. To check whether the system supports processor throttling, read the value of ProcessorThrottle.

• There are other two members we are interested in, they are, ProcessorMinThrottle and ProcessorMaxThrottle; they represents the minimum and maximum level of system processor throttling supported, expressed as a percentage. If both members has already values 100%, this means CPU throttling is currently disabled, so you don't need to reconfigure it.

• To disable CPU throttling, you need to set ProcessorMinThrottle and ProcessorMaxThrottle to 100%. To do this, call CallNtPowerInformation() again and pass SystemPowerCapabilities to InformationLevel parameter; but now, set lpInputBuffer to the SYSTEM_POWER_CAPABILITIES structure in which the two members has been set to 100%. I'm sure you know what to do next.

In non-programmatic way, you can also get/set Windows Power Options using the Windows built-in command-line tools, that is, PowerCfg.

Further Reading

• Power Management
• Power Management Functions