Massive allocations of Func<Type, JsonTypeInfo> in System.Text.Json.JsonSerializerOptions+CachingContext.GetOrAddJsonTypeInfo(Type)

[cmeeren]

Create a plain F# console app targeting .NET 7, with this minimal repro code:

open System.Text.Json

let data = Array.init 1_000_000 (fun _ -> [box 1])
let json = JsonSerializer.Serialize(data)

This initializes an array with one million elements, each being an F# list with a boxed integer. It then serializes the data.

When I run a memory profiler (dotMemory), the top hitter in terms of allocation size is Func<Type, JsonTypeInfo> in System.Text.Json, specifically in GetOrAddJsonTypeInfo. It is allocated almost 900.000 times:

Allocated type : System.Func<Type, JsonTypeInfo>
  Objects : 896629
  Bytes   : 57384256

Allocated by
   100%  GetOrAddJsonTypeInfo • 54,73 MB / 54,73 MB • System.Text.Json.JsonSerializerOptions+CachingContext.GetOrAddJsonTypeInfo(Type)
     100%  GetTypeInfoInternal • 54,73 MB / - • System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type, Boolean, Boolean)
      ►  95,0%  ResolvePolymorphicConverter • 51,99 MB / - • System.Text.Json.Serialization.JsonConverter.ResolvePolymorphicConverter(Object, JsonTypeInfo, JsonSerializerOptions, WriteStack)
      ►  5,00%  InitializePolymorphicReEntry • 2,74 MB / - • System.Text.Json.WriteStackFrame.InitializePolymorphicReEntry(Type, JsonSerializerOptions)
      ► <0,01%  get_ElementTypeInfo • 128 B / - • System.Text.Json.Serialization.Metadata.JsonTypeInfo.get_ElementTypeInfo()
      ► <0,01%  GetTypeInfoForRootType • 64 B / - • System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type)

I assume the reason is that there is a lambda or similar somewhere that is not being optimized/cached by the C# compiler.

You may want to adjust your code to prevent these allocations.

Maybe related: dotnet/roslyn#5835 and linked issues

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
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Issue Details

---

Create a plain F# console app targeting .NET 7, with this minimal repro code:

open System.Text.Json

let data = Array.init 1_000_000 (fun _ -> [box 1])
let json = JsonSerializer.Serialize(data)

This initializes an array with one million elements, each being an F# list with a boxed integer. It then serializes the data.

When I run a memory profiler (dotMemory), the top hitter in terms of allocation size is Func<Type, JsonTypeInfo> in System.Text.Json, specifically in GetOrAddJsonTypeInfo. It is allocated almost 900.000 times:

Allocated type : System.Func<Type, JsonTypeInfo>
  Objects : 896629
  Bytes   : 57384256

Allocated by
   100%  GetOrAddJsonTypeInfo • 54,73 MB / 54,73 MB • System.Text.Json.JsonSerializerOptions+CachingContext.GetOrAddJsonTypeInfo(Type)
     100%  GetTypeInfoInternal • 54,73 MB / - • System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type, Boolean, Boolean)
      ►  95,0%  ResolvePolymorphicConverter • 51,99 MB / - • System.Text.Json.Serialization.JsonConverter.ResolvePolymorphicConverter(Object, JsonTypeInfo, JsonSerializerOptions, WriteStack)
      ►  5,00%  InitializePolymorphicReEntry • 2,74 MB / - • System.Text.Json.WriteStackFrame.InitializePolymorphicReEntry(Type, JsonSerializerOptions)
      ► <0,01%  get_ElementTypeInfo • 128 B / - • System.Text.Json.Serialization.Metadata.JsonTypeInfo.get_ElementTypeInfo()
      ► <0,01%  GetTypeInfoForRootType • 64 B / - • System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type)

I assume the reason is that there is a lambda or similar somewhere that is not being optimized/cached by the C# compiler.

You may want to adjust your code to prevent these allocations.

Maybe related: dotnet/roslyn#5835 and linked issues

Author:	cmeeren
Assignees:	-
Labels:	area-System.Text.Json, tenet-performance
Milestone:	-

[eiriktsarpalis]

🙀 I was today years old when I realized these delegates aren't being cached.

[eiriktsarpalis]

Even though this will mostly impact collections with polymorphic elements, we should probably service this in .NET 7.

[cmeeren]

🙀 I was today years old when I realized these delegates aren't being cached.

We're of the same age, then 😅 (Based on my very recent experience, seems F# doesn't cache these, either. @dsyme, is that something to consider, or is there no point in raising an F# issue about that?)

[vzarytovskii]

🙀 I was today years old when I realized these delegates aren't being cached.

We're of the same age, then 😅 (Based on my very recent experience, seems F# doesn't cache these, either. @dsyme Don Syme FTE, is that something to consider, or is there no point in raising an F# issue about that?)

Please create (or search for) a suggestion at https://github.com/fsharp/fslang-suggestions/issues, we can follow-up there.