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This article applies to: ✔️ .NET Core 3.1 SDK and later versions
In this tutorial, you'll learn how to debug an excessive CPU usage scenario. Using the provided example ASP.NET Core web app source code repository, you can cause a deadlock intentionally. The endpoint will stop responding and experience thread accumulation. You'll learn how you can use various tools to diagnose this scenario with several key pieces of diagnostics data.
In this tutorial, you will:
The tutorial uses:
Before attempting to collect diagnostics data, you need to observe a high CPU condition. Run the sample application using the following command from the project root directory.
dotnet run
To find the process ID, use the following command:
dotnet-trace ps
Take note of the process ID from your command output. Our process ID was 22884, but yours will be different. To check the current CPU usage, use the dotnet-counters tool command:
dotnet-counters monitor --refresh-interval 1 -p 22884
The refresh-interval is the number of seconds between the counter polling CPU values. The output should be similar to the following:
Press p to pause, r to resume, q to quit.
Status: Running
[System.Runtime]
% Time in GC since last GC (%) 0
Allocation Rate / 1 sec (B) 0
CPU Usage (%) 0
Exception Count / 1 sec 0
GC Heap Size (MB) 4
Gen 0 GC Count / 60 sec 0
Gen 0 Size (B) 0
Gen 1 GC Count / 60 sec 0
Gen 1 Size (B) 0
Gen 2 GC Count / 60 sec 0
Gen 2 Size (B) 0
LOH Size (B) 0
Monitor Lock Contention Count / 1 sec 0
Number of Active Timers 1
Number of Assemblies Loaded 140
ThreadPool Completed Work Item Count / 1 sec 3
ThreadPool Queue Length 0
ThreadPool Thread Count 7
Working Set (MB) 63
With the web app running, immediately after startup, the CPU isn't being consumed at all and is reported at 0%. Navigate to the api/diagscenario/highcpu route with 60000 as the route parameter:
https://localhost:5001/api/diagscenario/highcpu/60000
Now, rerun the dotnet-counters command. If interested in monitoring just the cpu-usage counter, add '--counters System.Runtime[cpu-usage]` to the previous command. We are unsure if the CPU is being consumed, so we will monitor the same list of counters as above to verify counter values are within expected range for our application.
dotnet-counters monitor -p 22884 --refresh-interval 1
You should see an increase in CPU usage as shown below (depending on the host machine, expect varying CPU usage):
Press p to pause, r to resume, q to quit.
Status: Running
[System.Runtime]
% Time in GC since last GC (%) 0
Allocation Rate / 1 sec (B) 0
CPU Usage (%) 25
Exception Count / 1 sec 0
GC Heap Size (MB) 4
Gen 0 GC Count / 60 sec 0
Gen 0 Size (B) 0
Gen 1 GC Count / 60 sec 0
Gen 1 Size (B) 0
Gen 2 GC Count / 60 sec 0
Gen 2 Size (B) 0
LOH Size (B) 0
Monitor Lock Contention Count / 1 sec 0
Number of Active Timers 1
Number of Assemblies Loaded 140
ThreadPool Completed Work Item Count / 1 sec 3
ThreadPool Queue Length 0
ThreadPool Thread Count 7
Working Set (MB) 63
Throughout the duration of the request, the CPU usage will hover around the increased percentage.
Tip
To visualize an even higher CPU usage, you can exercise this endpoint in multiple browser tabs simultaneously.
At this point, you can safely say the CPU is running higher than you expect. Identifying the effects of a problem is key to finding the cause. We will use the effect of high CPU consumption in addition to diagnostic tools to find the cause of the problem.
When analyzing an app with high CPU usage, you need a diagnostics tool that can provide insights into what the code is doing. The usual choice is a profiler, and there are different profiler options to choose from. dotnet-trace can be used on all operating systems, however, its limitations of safe-point bias and managed-only callstacks result in more general information compared to a kernel-aware profiler like 'perf' for Linux or ETW for Windows. If your performance investigation involves only managed code, generally dotnet-trace will be sufficient.
On Windows, you can use the dotnet-trace tool as a profiler. Using the previous sample debug target, exercise the high CPU endpoint (https://localhost:5001/api/diagscenario/highcpu/60000) again. While it's running within the 1-minute request, use the collect command, with the providers option to specify the provider we want: Microsoft-DotNetCore-SampleProfiler, to collect a trace of the app as follows:
dotnet-trace collect -p 22884 --providers Microsoft-DotNETCore-SampleProfiler
Let dotnet-trace run for about 20-30 seconds, and then press the Enter to exit the collection. The result is a nettrace file located in the same folder. The nettrace files are a great way to use existing analysis tools on Windows.
Open the nettrace with PerfView by navigating to samples/core/diagnostics/DiagnosticScenarios/ and clicking on the arrow by the nettrace file. Open the 'Thread Time (with StartStop Activities) Stacks' and choose the 'CallTree' tab near the top. After checking the box to the left of one of the threads, your file should look similar to the one pictured below.
All *.nettrace files can be analyzed in Visual Studio. To analyze a Linux *.nettrace file in Visual Studio, transfer the *.nettrace file, in addition to the other necessary documents, to a Windows machine, and then open the *.nettrace file in Visual Studio. For more information, see Analyze CPU Usage Data.
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