How does powercfg -energy detect the requested timer resolution

Question

How does "powercfg -energy" command detect the requested timer resolution of each individual program running at the time?

I imagine one way would be injecting into each of running processes and inside that process calling timeEndPeriod for each increased resolution (values 1-15) in a loop over these resolutions and checking whether the timeEndPeriod call for a current resolution returns TIMERR_NOCANDO or TIMERR_NOERROR (note: these return values are NOT correspondingly false and true). And if it returns TIMERR_NOERROR then concluding that the program is using that frequency, and then calling again timeBeginPeriod. But this method seems cumbersome. Moreover, it is a bit intrusive since it modifies the state of the process, and also assumes that powercfg is able to inject into all processes.

Is there instead some documented or undocumented system API that enables requesting that information for foreign processes? At least I would like to know how it works, even if that API remains a secret...

A related (but not the main) question is, how to get the maximum timer resolution (minimum interval) that is currently in effect in the system in case I am not interested in the requests of individual applications? I imagine that maybe GetSystemTimeAdjustment()'s lpTimeIncrement helps with that, but I am not too sure. Please confirm me or propose an alternate method.

Metacenter

Answer 1

You may use the undocumented API of NtQueryTimerResolution().

NTSTATUS NtQueryTimerResolution(OUT PULONGMinimumResolution, 
                                OUT LONGMaximumResolution, 
                                OUT PULONGActualResolution);

See this post for an example on how to use it. But it will only give the ActualResolution as configured by any process. It does not give the information which process has configured which multimedia timer resolution.

You wouldn't want to change each processes mutimedia setting to find out which process has aquired which resolution. It would only be needed to obtain information on which process has requested ActualResolution.

Note: powercfg /energy is only available from Windows 7 / Windows Server 2008 R2 upwards.

Gerlachovka