How to Disable Dynamic Frequency Scaling?
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
In general, you need to do the following steps:
Call
CallNtPowerInformation()and pass SystemPowerCapabilities to InformationLevel parameter, set lpInputBuffer and nInputBufferSize toNULL, then set lpOutputBuffer to SYSTEM_POWER_CAPABILITIES structure, and set nOutputBufferSize to the size of the structure. After this first call,SYSTEM_POWER_CAPABILITIESstructure 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, callCallNtPowerInformation()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 to100%. 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.