Power Management API

Although Windows provides a great deal of power management functions that we can take advantage of, there is currently no way to do it using managed code in VB .NET. Therefore, we'll create a module that will contain the appropriate structures, functions, and API calls.

Let's take a quick look at some of the API functions we'll use:

SetActivePwrScheme: Sets active power scheme

CanUserWritePwrScheme: Determines whether the user can write to power scheme

GetActivePwrScheme: Finds the index of the active power scheme

GetSystemPowerStatus: Locates the power status of the system

ReadGlobalPwrPolicy: Gets the current global power policy settings

ReadProcessorPwrScheme: Gets the processor power policy settings for the specified power scheme

ReadPwrScheme: Gets the power policy settings that are unique to the specified power scheme

WriteProcessorPwrScheme: Writes processor power policy settings for the specified power scheme

We'll also use some of the structures:

GlobalMachinePowerPolicy: This structure contains global computer power policy settings that apply to all power schemes for all users. This structure is part of the next structure called GlobalPowerPolicy.

GlobalPowerPolicy: This structure contains global power policy settings that apply to all power schemes.

GlobalUserPowerPolicy: This is another structure that is part of the GlobalPowerPolicy structure and contains global user power policy settings that apply to all power schemes for a user.

MachinePowerPolicy: This structure, part of the PowerPolicy structure, contains computer power policy settings that are unique to each power scheme on the computer.

MachineProcessorPowerPolicy: This structure contains processor power policy settings that apply while the system is running on AC power or battery power.

PowerActionPolicy: This structure contains information used to set the system power state.

PowerPolicy: This structure contains power policy settings that are unique to each power scheme.

ProcessorPowerPolicy: This structure contains information about processor performance control and C-states.

ProcessorPowerPolicyInfo: This structure is part of ProcessorPowerPolicy and contains information about processor C-state policy settings.

SystemPowerLevel: This structure is part of GlobalUserPowerPolicy and contains information about system battery drain policy settings.

SystemPowerStatus: This structure contains information about the power status of the system.

UserPowerPolicy: This last structure is also part of PowerPolicy. It has power policy settings that are unique to each power scheme for a user.

We need to add these structures and functions to a project. Start a new Windows Forms application and add a new code module to the project called  Powerapi.vb . We can now add the code for the structures and functions we have talked about beginning with the structures:

Imports System.Runtime.InteropServices Module powerapi <StructLayout(LayoutKind.Sequential)> Private Structure GlobalMachinePowerPolicy Public Revision As Integer Public LidOpenWakeAc As Integer Public LidOpenWakeDc As Integer Public BroadcastCapacityResolution As Integer End Structure <StructLayout(LayoutKind.Sequential)> Private Structure GlobalPowerPolicy Public User As GlobalUserPowerPolicy Public Machine As GlobalMachinePowerPolicy End Structure <StructLayout(LayoutKind.Sequential)> Private Structure GlobalUserPowerPolicy Public Revision As Integer Public PowerButtonAc As PowerActionPolicy Public PowerButtonDc As PowerActionPolicy Public SleepButtonAc As PowerActionPolicy Public SleepButtonDc As PowerActionPolicy Public LidCloseAc As PowerActionPolicy Public LidCloseDc As PowerActionPolicy Public DischargePolicy0 As SystemPowerLevel Public DischargePolicy1 As SystemPowerLevel Public DischargePolicy2 As SystemPowerLevel Public DischargePolicy3 As SystemPowerLevel Public GlobalFlags As Integer End Structure <StructLayout(LayoutKind.Sequential)> Private Structure MachinePowerPolicy Public Revision As Integer Public MinSleepAc As Integer Public MinSleepDc As Integer Public ReducedLatencySleepAc As Integer Public ReducedLatencySleepDc As Integer Public DozeTimeoutAc As Integer Public DozeTimeoutDc As Integer Public DozeS4TimeoutAc As Integer Public DozeS4TimeoutDc As Integer Public MinThrottleAc As Byte Public MinThrottleDc As Byte Public Pad0 As Byte Public Pad1 As Byte Public OverThrottledAc As PowerActionPolicy Public OverThrottledDc As PowerActionPolicy End Structure <StructLayout(LayoutKind.Sequential)> Private Structure MachineProcessorPowerPolicy Public Revision As Integer Public ProcessorPolicyAc As ProcessorPowerPolicy Public ProcessorPolicyDc As ProcessorPowerPolicy End Structure <StructLayout(LayoutKind.Sequential)> Private Structure PowerActionPolicy Public PowerAction As Integer Public Flags As Integer Public EventCode As Integer End Structure <StructLayout(LayoutKind.Sequential)> Private Structure PowerPolicy Public User As UserPowerPolicy Public Machine As MachinePowerPolicy End Structure <StructLayout(LayoutKind.Sequential)> Private Structure ProcessorPowerPolicy Public Revision As Integer Public DynamicThrottle As Byte Public Spare0 As Byte Public Spare1 As Byte Public Spare2 As Byte Public Reserved As Integer Public PolicyCount As Integer Public Policy0 As ProcessorPowerPolicyInfo Public Policy1 As ProcessorPowerPolicyInfo Public Policy2 As ProcessorPowerPolicyInfo End Structure <StructLayout(LayoutKind.Sequential)> Private Structure ProcessorPowerPolicyInfo Public TimeCheck As Integer Public DemoteLimit As Integer Public PromoteLimit As Integer Public DemotePercent As Byte Public PromotePercent As Byte Public Spare0 As Byte Public Spare1 As Byte Public AllowBits As Integer End Structure <StructLayout(LayoutKind.Sequential)> Private Structure SystemPowerLevel Public Enable As Byte Public Spare0 As Byte Public Spare1 As Byte Public Spare2 As Byte Public BatteryLevel As Integer Public PowerPolicy As PowerActionPolicy Public MinSystemState As Integer End Structure <StructLayout(LayoutKind.Sequential)> Private Structure SystemPowerStatus Public ACLineStatus As Byte Public BatteryFlags As Byte Public BatteryLifePercent As Byte Public Reserved1 As Byte Public BatteryLifeTime As Integer Public BatteryFullLifeTime As Integer End Structure <StructLayout(LayoutKind.Sequential)> Private Structure UserPowerPolicy Public Revision As Integer Public IdleAc As PowerActionPolicy Public IdleDc As PowerActionPolicy Public IdleTimeoutAc As Integer Public IdleTimeoutDc As Integer Public IdleSensitivityAc As Byte Public IdleSensitivityDc As Byte Public ThrottlePolicyAc As Byte Public ThrottlePolicyDc As Byte Public MaxSleepAc As Integer Public MaxSleepDc As Integer Public Reserved0 As Integer Public Reserved1 As Integer Public VideoTimeoutAc As Integer Public VideoTimeoutDc As Integer Public SpindownTimeoutAc As Integer Public SpindownTimeoutDc As Integer Public OptimizeForPowerAc As Byte Public OptimizeForPowerDc As Byte Public FanThrottleToleranceAc As Byte Public FanThrottleToleranceDc As Byte Public ForcedThrottleAc As Byte Public ForcedThrottleDc As Byte End Structure Private Class Win32 Declare Auto Function ApplyPwrScheme Lib "powrprof.dll" Alias "SetActivePwrScheme" _ (ByVal SchemeId As Integer, Optional ByVal Unused As Integer = 0, _ Optional ByVal Unused2 As Integer = 0) As Byte Declare Auto Function CanUserWritePwrScheme Lib "powrprof.dll" () As Byte Declare Auto Function GetActivePwrScheme Lib "powrprof.dll" _ (ByRef SchemeId As Integer) As Byte Declare Auto Function GetSystemPowerStatus Lib "kernel32.dll" _ (ByRef Status As SystemPowerStatus) As Byte Declare Auto Function ReadGlobalPwrPolicy Lib "powrprof.dll" _ (ByRef GlobalPolicy As GlobalPowerPolicy) As Byte Declare Auto Function ReadProcessorPwrScheme Lib "powrprof.dll" _ (ByVal SchemeId As Integer, ByRef Policy As MachineProcessorPowerPolicy) As Byte Declare Auto Function ReadPwrPolicy Lib "powrprof.dll" Alias "ReadPwrScheme" _ (ByVal SchemeId As Integer, ByRef Policy As PowerPolicy) As Byte Declare Auto Function SetActivePwrScheme Lib "powrprof.dll" _ (ByVal SchemeId As Integer, ByRef GlobalPolicy As GlobalPowerPolicy, _ ByRef Policy As PowerPolicy) As Byte Declare Auto Function WriteProcessorPwrScheme Lib "powrprof.dll" _ (ByVal SchemeId As Integer, ByRef Policy As MachineProcessorPowerPolicy) As Byte End Class

With the API functions added, we can now focus on the functions and Sub procedures we need to create to take advantage of them. The following sections list the functions we will create and their overall objectives along with the code for each of them (the code can be added to the module after the Win32 Class).

GetCpuDynamicThrottling

The first function we look at determines both the AC and DC CPU dynamic throttling values. Here is its code:

Public Enum Powermode modedc modeac End Enum Public Enum ThrottlingMode ThrottleNone = 0 'NONE ThrottleConstant = 1 'CONSTANT ThrottleDegrade = 2 'DEGRADE ThrottleAdaptive = 3 'ADAPTIVE End Enum Private Sub GetCpuDynamicThrottling(ByRef throttleAc As Integer, _ ByRef throttleDc As Integer) Dim schemeId As Integer Dim currentPolicy As MachineProcessorPowerPolicy Dim result As Byte result = Win32.GetActivePwrScheme(schemeId) If result <> 1 Then Throw New System.Exception("Unable to determine the active power scheme") End If result = Win32.ReadProcessorPwrScheme(schemeId, currentPolicy) If result <> 1 Then Throw New System.Exception("Unable to read the current CPU power scheme") End If throttleAc = currentPolicy.ProcessorPolicyAc.DynamicThrottle throttleDc = currentPolicy.ProcessorPolicyDc.DynamicThrottle End Sub

GetCpuThrottlingAc

The AC throttling power policy is handled in this function. Here is this function's code:

Public Function GetCpuThrottlingAc() As ThrottlingMode

Dim throttleDc As Integer GetCpuDynamicThrottling(throttleAc, throttleDc) Return throttleAc End Function

GetCpuThrottlingDc

This is similar to the previous function, but this one handles the DC throttling power policy. Here is the code:

Public Function GetCpuThrottlingDc() As ThrottlingMode Dim throttleAc As Integer Dim throttleDc As Integer GetCpuDynamicThrottling(throttleAc, throttleDc) Return throttleDc End Function

GetPowerMode

We need a way to tell if the system is using AC or DC power, which is what we handle with the following code:

Public Function GetPowerMode() As Powermode Dim pwrStatus As SystemPowerStatus Dim result As Byte result = Win32.GetSystemPowerStatus(pwrStatus) If result <> 1 Then Throw New System.Exception("Cannot determine system power status") If pwrStatus.ACLineStatus = 0 Then Return Powermode.modedc Return Powermode.modeac End Function

SetCpuDynamicThrottling

This Sub procedure allows us to change the CPU throttling policies for both the AC and DC power modes:

Private Sub SetCpuDynamicThrottling(ByVal throttleAc As Integer, ByVal throttleDc As Integer) Dim schemeId As Integer Dim currentPolicy As MachineProcessorPowerPolicy Dim newPolicy As MachineProcessorPowerPolicy Dim result As Byte result = Win32.CanUserWritePwrScheme() If result <> 1 Then Throw New System.Exception("Cannot get CPU power information") End If result = Win32.GetActivePwrScheme(schemeId) If result <> 1 Then Throw New System.Exception("Cannot determine the active power scheme") End If result = Win32.ReadProcessorPwrScheme(schemeId, currentPolicy) If result <> 1 Then Throw New System.Exception("Cannot read the current CPU power scheme") End If newPolicy = currentPolicy If throttleAc >= 0 Then newPolicy.ProcessorPolicyAc.DynamicThrottle = throttleAc If throttleDc >= 0 Then newPolicy.ProcessorPolicyDc.DynamicThrottle = throttleDc result = Win32.WriteProcessorPwrScheme(schemeId, newPolicy) If result <> 1 Then Throw New System.Exception("Cannot change the current CPU power scheme") End If result = Win32.ApplyPwrScheme(schemeId) If result <> 1 Then Win32.WriteProcessorPwrScheme(schemeId, currentPolicy) Throw New System.Exception("Cannot apply the changes made to the power scheme") End If

SetCpuThrottling

This Sub procedure changes the CPU throttling power for the current power mode:

Public Sub SetCpuThrottling(ByVal throttle As ThrottlingMode) Dim pwrMode As Powermode pwrMode = GetPowerMode() If pwrMode = Powermode.modeac Then SetCpuThrottlingAc(throttle) Else SetCpuThrottlingDc(

throttle

) End If End Sub

SetCpuThrottlingAc

The AC power policy can be changed with this Sub procedure:

Public Sub SetCpuThrottlingAc(ByVal throttle As ThrottlingMode) SetCpuDynamicThrottling(throttle, -1) End Sub

SetCpuThrottlingDc

This is the same as the previous Sub , but for DC power:

Public Sub SetCpuThrottlingDc(ByVal throttle As ThrottlingMode) SetCpuDynamicThrottling(-1, throttle) End Sub