WriteBase64JsonMemoryRepro

Unnecessary buffer allocations in Utf8JsonWriter when writing base64 values

This is a sample repro of the issue dotnet/runtime#97628

It's a simple project that demonstrates the allocations in questions. The allocations come from an internal Base64EncodeAndWrite helper method. The method rents a buffer from the shared array pool to write the base64 encoded bytes into and copies the content to the output. This temporary buffer is unnecessary since the encoding can be done directly into the output buffer.

Since the rented buffer might be reused in subsequent writes, this project performs multiple concurrent write to increasing the likelihood of the array pool allocating new buffers. This makes it easier to see the problem.

Setup and profiling

To visualize the issue, you can build the project against a local build of .NET and run the Visual Studio Profile against it.

clone the dotnet runtime repo:

git clone https://github.com/dotnet/runtime

build the runtime: (took about 20-30min on my machine)

build.cmd -c Release

This generates build artifacts in a directly like:

path\to\runtime\artifacts\bin\testhost\net9.0-windows-Release-x64

Copy the sdk tools from runtime\.dotnet to the build output so you can use the local dotnet build to build and run projects:

cd path\to\runtime\.dotnet
cp -r sdk path\to\runtime\artifacts\bin\testhost\net9.0-windows-Release-x64
cp -r packs path\to\runtime\artifacts\bin\testhost\net9.0-windows-Release-x64

Navigate to this repro project's csproj folder:

cd experiments/WriteBase64JsonMemoryRepro/WriteBase64JsonMemoryRepro

Run/build the project using the local build of dotnet:

path\to\runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\dotnet.exe run -c Release

Using the guide here to run the Visual Studio profiler: https://github.com/dotnet/performance/blob/main/docs/profiling-workflow-dotnet-runtime.md

I used the CoreRun option. Using the corerun.exe path: runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe

https://github.com/dotnet/performance/blob/main/docs/profiling-workflow-dotnet-runtime.md

Results

These are the results before and after the fix

CPU Before

A lot of CPU time on Span<byte>.CopyTo() and Buffer._Memmove()

CPU After

No trace of Span<byte>.CopyTo() or Buffer._Memmove()

Allocations Before

Large byte[] allocations from SharedArrayPool<byte>.Rent under Base64EncodeAndWrite

Allocations After

No traces of Base64EncodeAndWrite in allocations.