Collecting memory dumps for .NET Core on Kubernetes

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In the world of Kubernetes and microservices, diagnosing and debugging issues can be a challenging task. One powerful tool in your troubleshooting arsenal is memory dump analysis. Memory dumps capture the state of an application’s memory at a particular point in time, providing insights into potential issues, bottlenecks, and crashes. In this blog post, I’ll walk you through the process of collecting a memory dump from a .NET Core application running on Kubernetes.

Step 1: Getting Started 🔗︎

If you don’t already have it, you’ll need to install kubectl, the command-line tool for interacting with Kubernetes clusters. You can follow the installation instructions provided on the official Kubernetes documentation or use the winget command if you’re on Windows:

winget install Kubernetes.kubectl

Having the kubectl tool installed, begin by identifying the relevant pods using a label selector. For instance, you can list the pods with the label app.kubernetes.io/instance=YOUR_APP_NAME in the YOUR_NAMESPACE_NAME namespace:

kubectl get pods --selector=app.kubernetes.io/instance=YOUR_APP_NAME --namespace YOUR_NAMESPACE_NAME -o=name --kubeconfig "path_to_kubeconfig.yaml"

The --kubeconfig parameter is used to specify the path to the configuration file for authenticating and interacting with a Kubernetes cluster. This file contains information about the cluster’s API server, authentication credentials, and context. You can obtain the kubeconfig file from your Kubernetes administrator or generate it yourself if you have access to the cluster. If you are using Rancher, follow this guideline to obtain your kubeconfig file.

Step 2: Accessing the Pod 🔗︎

To access the pod and execute commands within it, you can use the kubectl exec command. This example assumes you’re accessing the pod with the name YOUR_POD_NAME:

kubectl exec -it "pod/YOUR_POD_NAME" --kubeconfig "path_to_kubeconfig.yaml" --namespace YOUR_NAMESPACE_NAME -- sh

You will need to download dotnet-dump. However, you might have limited permission regarding saving data to disk in container so it’s good to execute the following steps from /tmp directory. Navigate to the /tmp directory within the pod:

cd /tmp

Step 3: Creating a Memory Dump 🔗︎

Now, let’s proceed to collect the memory dump for the .NET Core application. Start by downloading the dotnet-dump tool:

curl -L -o dotnet-dump https://aka.ms/dotnet-dump/linux-x64

Give the necessary permissions to the downloaded tool:

chmod 777 ./dotnet-dump

Specify an extraction directory for the tool:

export DOTNET_BUNDLE_EXTRACT_BASE_DIR="/tmp/bundle_extract"

Now, initiate the memory dump collection. Replace 1 with the appropriate process ID of your .NET Core application:

./dotnet-dump collect -p 1

The collected memory dump will be saved as core_ in the /tmp directory.

Step 4: Archiving the Memory Dump 🔗︎

Once the memory dump is generated, you can compress it for easier transfer and analysis:

gzip core_<timestamp>

Step 5: Downloading the Memory Dump 🔗︎

Now that the memory dump is ready, you can copy it from the pod to your local machine using the kubectl cp command.

First you need to exit the pod console:

exit

Being back on your workstation console, execute the following command to download the memory file to your machine:

kubectl cp "YOUR_POD_NAME:/tmp/core_<timestamp>.gz" ./core_<timestamp>.gz --kubeconfig "path_to_kubeconfig.yaml" --namespace YOUR_NAMESPACE_NAME

Step 5: Unpacking the Memory Dump 🔗︎

Now you need to unpack the memory dump file. You can do that with Total Commander or using the PowerShell script that I found here Unzip GZ files using Powershell

Now you are ready to start a memory analysis. You can do that with VisualStudio, WindDBG or with dotMemory.

Everything Everywhere All at Once 🔗︎

All the steps described above can be compiled into a simple PowerShell script to bring the whole process to a single command execution.

param (
  [Parameter(Mandatory=$true)][string] $PodName,
  [Parameter(Mandatory=$true)][string] $Namespace,
  [Parameter(Mandatory=$true)][string] $KubeconfigFile,
  [Parameter(Mandatory=$true)][string] $OutputDirectory
)


Write-Host "Preparing dump file"

$linuxDumpScript = @"
cd /tmp && \
curl -L -o dotnet-dump https://aka.ms/dotnet-dump/linux-x64 && \
chmod 777 ./dotnet-dump && \
export DOTNET_BUNDLE_EXTRACT_BASE_DIR='/tmp/bundle_extract' && \
./dotnet-dump collect -p 1
"@ -replace "`r`n","`n"

$dumpLog = kubectl exec -it "pod/$PodName" --kubeconfig $KubeconfigFile --namespace "$Namespace" -- sh -c $linuxDumpScript.Trim() 

Write-Host $dumpLog

$pattern = "Writing full to (.*?)Complete"
$matches = [Regex]::Matches($dumpLog, $pattern)

if ($matches.Count -eq 0) {
    Write-Error "Cannot find dump file name"
    return
}

$dumpFile = $matches[0].Groups[1].Value.Trim()

Write-Host "Dump file $($matches[0].Groups[1].Value)"
Write-Host "Packing dump file"

kubectl exec -it "pod/$PodName" --kubeconfig $KubeconfigFile --namespace "$Namespace" -- sh -c "gzip $dumpFile" | Out-Host

$fileName = [System.IO.Path]::GetFileName($dumpFile)
$archiveFileName = "$fileName.gz"

if ([string]::IsNullOrWhiteSpace($OutputDirectory)) {
    $OutputDirectory = "."
}

# Relative path is required by `kubectl cp`
$directoryRelativePath = Resolve-Path -Relative $OutputDirectory
$outputFile = Join-Path $directoryRelativePath $archiveFileName
Write-Host "Downloading dump file to $outputFile"

kubectl cp "$PodName`:$dumpFile`.gz" $outputFile --kubeconfig $KubeconfigFile --namespace $Namespace

function DeGZip-File {
    param (
        $infile,
        $outfile = ($infile -replace '\.gz$', '')
    )

    $input = [System.IO.File]::OpenRead($inFile)
    $output = [System.IO.File]::Create($outFile)
    $gzipStream = [System.IO.Compression.GzipStream]::new($input, [System.IO.Compression.CompressionMode]::Decompress)

    $buffer = [byte[]]::new(1024)
    while ($true) {
        $read = $gzipStream.Read($buffer, 0, 1024)
        if ($read -le 0) { break }
        $output.Write($buffer, 0, $read)
    }

    $gzipStream.Close()
    $output.Close()
    $input.Close()
}

Write-Host "Unpacking dump file"

DeGZip-File (Join-Path $OutputDirectory $archiveFileName) (Join-Path $OutputDirectory $fileName)

Save the script as MemoryDump.ps1 file and enjoy creating memory dumps with this single line:

./MemoryDump.ps1 -PodName 'YOUR_POD_NAME' -Namespace 'YOUR_NAMESPACE' -KubeconfigFile './YOUR_KUBECONFIG.yaml'

Conclusion 🔗︎

Collecting memory dumps from applications running on Kubernetes can provide valuable insights into their state during critical moments. Armed with the information from memory dumps, you can more effectively troubleshoot and address issues within your .NET Core applications. Remember that memory dump analysis requires familiarity with debugging tools and techniques, but it’s a skill that can significantly enhance your ability to maintain and improve your applications' performance and reliability.