[NativeAOT] Initialize GC threshold much earlier#10488
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jonathanpeppers merged 1 commit intomainfrom Sep 15, 2025
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The threshold value is used by the global reference creation
routine to determine whether to [start GC collection][0]:
```csharp
if (gc >= JNIEnvInit.gref_gc_threshold) {
Logger.Log (LogLevel.Debug, "monodroid-gc", gc + " outstanding GREFs. Performing a full GC!");
System.GC.Collect ();
}
```
However, the threshold value was being initialized after we've already invoked some non-trivial
managed code in the managed [NativeAOT runtime][1], too late to avoid at least a handful of
`System.GC.Collect ()` calls:
09-15 14:37:25.923 10761 10761 D monodroid-gc: 1 outstanding GREFs. Performing a full GC!
09-15 14:37:25.923 10761 10761 D monodroid-gc: 2 outstanding GREFs. Performing a full GC!
09-15 14:37:25.923 10761 10761 D monodroid-gc: 3 outstanding GREFs. Performing a full GC!
09-15 14:37:25.924 10761 10761 D monodroid-gc: 3 outstanding GREFs. Performing a full GC!
Fix this by splitting the GC threshold value initialization into a separate method which is
invoked as soon as possible from the `JNI_OnLoad` function exported by the NativeAOT application.
[0]: https://github.com/dotnet/android/blob/e72bf3626024fd1e27dd9cd26d7f8439bc088e19/src/Mono.Android/Android.Runtime/AndroidRuntime.cs#L178-L195
[1]: https://github.com/dotnet/android/blob/e72bf3626024fd1e27dd9cd26d7f8439bc088e19/src/Microsoft.Android.Runtime.NativeAOT/Android.Runtime.NativeAOT/JavaInteropRuntime.cs#L60
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Pull Request Overview
This PR fixes a timing issue in NativeAOT where the GC threshold was being initialized too late, causing unnecessary garbage collection calls during early runtime initialization.
- Splits GC threshold initialization into a separate method called earlier in the process
- Adds explicit early initialization call in NativeAOT's JNI_OnLoad function
- Removes redundant GC threshold initialization from the main InitializeJniRuntime method
Reviewed Changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| src/Mono.Android/Android.Runtime/JNIEnvInit.cs | Extracts GC threshold initialization into new NativeAotInitializeMaxGrefGet method and removes it from InitializeJniRuntime |
| src/Microsoft.Android.Runtime.NativeAOT/Android.Runtime.NativeAOT/JavaInteropRuntime.cs | Calls the new GC threshold initialization method early in JNI_OnLoad before other initialization |
src/Microsoft.Android.Runtime.NativeAOT/Android.Runtime.NativeAOT/JavaInteropRuntime.cs
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filipnavara
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Sep 15, 2025
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filipnavara
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Tested on our app, the inital 4 GCs with "outstanding GREFs. Performing a full GC!" message are gone.
jonathanpeppers
approved these changes
Sep 15, 2025
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I'm rerunning these, but if they end up passing, this looks good:
There are some other test failures caused by Android 36.1 changes, hopefully I can fix shortly.
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The threshold value is used by the global reference creation routine to determine whether to start GC collection:
However, the threshold value was being initialized after we've already invoked some non-trivial managed code in the managed NativeAOT runtime, too late to avoid at least a handful of
System.GC.Collect ()calls:Fix this by splitting the GC threshold value initialization into a separate method which is invoked as soon as possible from the
JNI_OnLoadfunction exported by the NativeAOT application.