How Do You Monitor The CPU Utilization of a Process Using PowerShell?

Question

I'm trying to write a PowerShell script to monitor % CPU utilization of a SQL Server process. I'd like to record snapshots of this number every day so we can monitor it over time and watch for trends.

My research online said this WMI query should give me what I want:

Get-WmiObject -Query "SELECT PercentProcessorTime FROM win32_PerfFormattedData_PerfProc_Process WHERE Name='SqlServr'"

When I run the WMI query I usually get a value somewhere between 30-50%

However, when I watch the process in Resource Monitor it usually averages at less than 1% CPU usage

I know the WMI query is simply returning snapshot of CPU usage rather than an average over a long period of time so I know the two aren't directly comparable. Even so, I think the snapshot should usually be less than 1% since the Resource Monitor average is less than 1%.

Does anybody have any ideas on why there is such a large discrepancy? And how I can get an accurate measurement of the CPU usage for the process?

Answer 1

Everything I've learned about WMI and performance counters over the last couple of days.

WMI stands for Windows Management Instrumentation. WMI is a collection of classes registered with the WMI system and the Windows COM subsystem. These classes are known as providers and have any number of public properties that return dynamic data when queried.

Windows comes pre-installed with a large number of WMI providers that give you information about the Windows environment. For this question we are concerned with the Win32_PerfRawData* providers and the two wrappers that build off of it.

If you query any Win32_PerfRawData* provider directly you'll notice the numbers it returns are scary looking. That's because these providers give the raw data you can use to calculate whatever you want.

To make it easier to work with the Win32_PerfRawData* providers Microsoft has provided two wrappers that return nicer answers when queried, PerfMon and Win32_PerfFormattedData* providers.

Ok, so how do we get a process's % CPU utilization? We have three options:

1. Get a nicely formatted number from the Win32_PerfFormattedData_PerfProc_Process provider
2. Get a nicely formatted number from PerfMon
3. Calculate the % CPU utilization for ourselves using Win32_PerfRawData_PerfProc_Process

We will see that there is a bug with option 1 so that it doesn't work in all cases even though this is the answer usually given on the internet.

If you want to get this value from Win32_PerfFormattedData_PerfProc_Process you can use the query mentioned in the question. This will give you the sum of the PercentProcessorTime value for all of this process's threads. The problem is that this sum can be >100 if there is more than 1 core but this property maxes out at 100. So, as long as the sum of all this process's threads is less than 100 you can get your answer by dividing the process's PercentProcessorTime property by the core count of the machine.

If you want to get this value from PerfMon in PowerShell you can use Get-Counter "\Process(SqlServr)\% Processor Time". This will return a number between 0 - (CoreCount * 100).

If you want to calculate this value for yourself the PercentProcessorTime property on the Win32_PerfRawData_PerfProc_Process provider returns the CPU time this process has used. So, you'll need to take two snapshots we'll call them s1 and s2. We'll then do (s2.PercentProcessorTime - s1.PercentProcessorTime) / (s2.TimeStamp_Sys100NS - s1.TimeStamp_Sys100NS).

And that is the final word. Hope it helps you.