WMI, negative CPU usage value and Timestamp_Sys100NS in past

Question

I am monitoring some machines using WMI, using .NET\'s System.Management stuff. The query I am using is this:

SELECT Timestamp_Sys100NS, PercentProc         


        

Answer 1

The formula you are specifically talking about is PERF_100NSEC_TIMER_INV http://msdn.microsoft.com/en-us/library/ms803963.aspx

I have not personally dealt with this problem because I have never seen values below zero.

This is all I have been doing:

/// 

/// PERF_100NSEC_TIMER_INV algorithm.
/// 
/// 
/// 
/// 
/// 
/// 
public static int CalculatePerf100NsecTimerInv(long n2, UInt64 d2,
                                               long n1, UInt64 d1)
{
    int usage = 0;
    try
    {
        double dataDiff = (n2 - n1);
        double timeDiff = (d2 - d1);
        double dUsage = (1 - (dataDiff / timeDiff)) * 100;

        // Evaluate
        usage = (dUsage >= 0.5) ? Convert.ToInt32(Math.Ceiling(dUsage)) : 0;
    }
    catch { }

    // Return
    return usage;
}

Usage:

// Calculate
int cpuTime =
    MSPerformanceAlgorithms.CalculatePerf100NsecTimerInv(
        current.PercentProcessorTime, current.TimestampSys100Ns,
        previous.PercentProcessorTime, previous.TimestampSys100Ns);

If I watch the CPU usage on the task manager for the 2003 box, it matches fine. I think you can disregard anything less than zero as long as you are calculating with these values.