JsonSerializer.Serialize<T>(Utf8JsonWriter, T) regression in .NET 6

[stephentoub]

Benchmark:

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.IO;
using System.Text.Json;

[MemoryDiagnoser]
public class Program
{
    public static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private Data _data = new Data { Name = "Stephen", Address = "Here" };
    private MemoryStream _stream = new MemoryStream();
    private Utf8JsonWriter _writer;

    [GlobalSetup]
    public void Setup()
    {
        _writer = new Utf8JsonWriter(_stream);
    }

    [Benchmark(OperationsPerInvoke = 1000)]
    public void Serialize()
    {
        for (int i = 0; i < 1000; i++)
        {
            _stream.Position = 0;
            _writer.Reset();
            JsonSerializer.Serialize(_writer, _data);
        }
    }

    internal class Data
    {
        public string Name;
        public string Address;
    }
}

On my machine, running this with:

dotnet run -c Release -f net5.0 --filter ** --join --runtimes net5.0 net6.0

with .NET 5.0.8 and .NET 6.0.0 (6.0.21.40103), I get these results:

Method	Job	Runtime	Toolchain	Mean	Error	StdDev	Ratio
Serialize	Job-DDTUUM	.NET 5.0	net5.0	85.54 ns	0.487 ns	0.407 ns	1.00
Serialize	Job-EGSUAL	.NET 6.0	net6.0	106.80 ns	0.328 ns	0.273 ns	1.25

For me at least, that 25% regression is very consistent.

[ghost]

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Issue Details

---

Benchmark:

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.IO;
using System.Text.Json;

[MemoryDiagnoser]
public class Program
{
    public static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private Data _data = new Data { Name = "Stephen", Address = "Here" };
    private MemoryStream _stream = new MemoryStream();
    private Utf8JsonWriter _writer;

    [GlobalSetup]
    public void Setup()
    {
        _writer = new Utf8JsonWriter(_stream);
    }

    [Benchmark(OperationsPerInvoke = 1000)]
    public void Serialize()
    {
        for (int i = 0; i < 1000; i++)
        {
            _stream.Position = 0;
            _writer.Reset();
            JsonSerializer.Serialize(_writer, _data);
        }
    }

    internal class Data
    {
        public string Name;
        public string Address;
    }
}

On my machine, running this with:

dotnet run -c Release -f net5.0 --filter ** --join --runtimes net5.0 net6.0

with .NET 5.0.8 and .NET 6.0.0 (6.0.21.40103), I get these results:

Method	Job	Runtime	Toolchain	Mean	Error	StdDev	Ratio
Serialize	Job-DDTUUM	.NET 5.0	net5.0	85.54 ns	0.487 ns	0.407 ns	1.00
Serialize	Job-EGSUAL	.NET 6.0	net6.0	106.80 ns	0.328 ns	0.273 ns	1.25

For me at least, that 25% regression is very consistent.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[steveharter]

I can repro this; this is a regression mostly from refactoring done in #53212.

However, the sample POCO above:

  internal class Data
  {
      public string Name;
      public string Address;
  }

only measures serializer overhead, since fields are not serialized by default. [JsonInclude] must be used for fields.

[steveharter]

Please re-verify based on #57327. Thanks