/// call GetPerformanceInfo.</summary>

        /// To calculate the system-wide available commit value, call GetPerformanceInfo

static extern bool GetPerformanceInfo(out PERFORMANCE_INFORMATION pPerformanceInformation, uint cb);

Declare Function GetPerformanceInfo Lib "psapi.dll" (TODO) As Boolean

    private static extern bool GetPerformanceInfo([Out] out PsApiPerformanceInformation PerformanceInformation, [In] int Size);

    public static PerfomanceInfoData GetPerformanceInfo()

      if (GetPerformanceInfo(out perfInfo, Marshal.SizeOf(perfInfo)))

    [DllImport( "psapi.dll", CharSet = CharSet.Auto, EntryPoint = "GetPerformanceInfo", SetLastError = true )]

    static extern bool _GetPerformanceInfo( ref PerformanceInformation pi, uint cb );

    /// <summary>Wrapper for native GetPerformanceInfo function - returns performance values</summary>

    public static bool GetPerformanceInfo( ref PerformanceInformation pi )

        var _ret = _GetPerformanceInfo( ref pi, pi.cb );

http://msdn.microsoft.com/en-us/library/windows/desktop/ms683210%28v=vs.85%29.aspx (GetPerformanceInfo)