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POWMI, negative CPU usage value and Timestamp_Sys100NS in past
 

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  1. POWMI, negative CPU usage value and Timestamp_Sys100NS in past
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    <p>I am monitoring some machines using WMI, using .NET's <code>System.Management</code> stuff. The query I am using is this:</p> <pre><code>SELECT Timestamp_Sys100NS, PercentProcessorTime FROM Win32_PerfRawData_PerfOS_Processor WHERE Name='_Total' </code></pre> <p>From that I compute the CPU usage % using the well known formula:</p> <pre><code>double cpu_usage = (1 - (double)delta_cpu / delta_time) * 100; </code></pre> <p>It works very well every machine but one (so far).</p> <p>The problem is that for one machine, which is Windows 2003 server (with hyper-threading enabled, if it matters), I am sometimes getting negative CPU usage values. In other words, the <code>(double)delta_cpu / delta_time</code> expression yields number <code>&gt; 1</code>. I did search the web for hints as to why this could be happening but I found nothing.</p> <p>Is this Windows 2003 server specific? Or is it hyper-threading related problem? Or is it just expected and I should just clamp the CPU usage value or the <code>cpu_delta</code> value into some range?</p> <p><em>EDIT:</em> The second weird thing I am observing with this one machine is that the <code>Timestamp_Sys100NS</code> value does not indicate <code>FILETIME</code> like date (ticks since epoch January 1, 1600) but instead it looks like ticks since boot time.</p> <p><em>EDIT 2</em>: I have now verified that this problem is across a lot of Windows 2003 servers. And I am apparently <a href="http://www.ureader.com/msg/14773088.aspx" rel="nofollow">not the only one with the same problem</a>.</p> <p><em>EDIT 3</em>: I have solved the time stamp issue by querying <code>LastBootUpTime</code> from <code>Win32_OperatingSystem</code> and adding that to the <code>Timestamp_Sys100NS</code> when the value of <code>Timestamp_Sys100NS</code> is too far in the past. That seems to give correct date and time. The code manipulating the date after it is retrieved from <code>Win32_OperatingSystem</code> looks like this:</p> <pre><code>WbemScripting.SWbemDateTime swbem_time = new WbemScripting.SWbemDateTime(); swbem_time.Value = date_str; string time_as_file_time_str = swbem_time.GetFileTime(true); return new DateTimeOffset(epoch.Ticks + long.Parse(time_as_file_time_str), swbem_time.UTCSpecified ? TimeSpan.FromMinutes(swbem_time.UTC) : TimeSpan.Zero); </code></pre> <p>...then adjust to UTC...</p> <pre><code>boot_time = boot_time.UtcDateTime; </code></pre> <p>...then is <code>boot_time</code> simply added to the time stamp (<code>current</code>) returned in by WMI in the <code>Timestamp_Sys100NS</code> field... </p> <pre><code>if (time.Year &lt; 2000) time = boot_time + current; </code></pre> <p><em>EDIT 4</em>: It appears that there are 3 classes of system with respect to <code>Timestamp_Sys100NS</code>:</p> <ol> <li>First are Vista+ system where the <code>Timestamp_Sys100NS</code> is time in ticks since epoch in UTC.</li> <li>Second are some Windows 2003 systems where the <code>Timestamp_Sys100NS</code> needs to be added to <code>Win32_OperatingSystem.LastBootUpTime</code> to get reasonable time.</li> <li>Third class are systems where doing the above addition still results in a date days off of the right date and time.</li> </ol> <p><em>EDIT 5</em>: Some of the affected machines might have been VMs but not all of them. </p>
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    1. COIt means the CPU is using YOU :)
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    2. COHow did you add LastBootUpTime to TimeStamp_Sys100NS? What kind of conversion did you do?
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    3. CO@raz3r: I am using the `System.DateTime(Int64)` ctor (takes ticks) and I add it to the LastBootUpTime. I will only be able to check the exact detail tomorrow.
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