[CoreCLR/NativeAOT] Use a no-GC region during Android startup

[simonrozsival]

Rationale

CoreCLR and NativeAOT Android startup can incur managed GC work before the first frame is fully drawn. Reserve a short startup allocation budget so this work is deferred until the app reports that startup is complete.

Implementation

• Start a no-GC region at the beginning of managed runtime initialization for CoreCLR and NativeAOT with a 24 MiB budget and disallowFullBlockingGC: true.
• End the region once after the first managed Activity.ReportFullyDrawn() completes.
• Preserve the existing public, virtual, and JNI registration contract by removing only the generated method and providing the equivalent binding in the existing Activity partial class.
• Leave Mono startup unchanged.
• Bound apps that never call ReportFullyDrawn() with a 10-second one-shot timer.
• Treat start failure, allocation exhaustion, and an induced collection as expected best-effort outcomes.

Benchmark evidence

Samsung A16, five matched CoreCLR trimmable launches:

Metric	Baseline	Startup no-GC	Change
Fully drawn median	1,782 ms	1,705 ms	-77 ms (-4.3%)
Scheduler-observed startup wall	1,859.7 ms	1,774.6 ms	-4.6%
App-process CPU	1,907.8 ms	1,835.6 ms	-3.8%
Main-thread CPU	1,674.7 ms	1,642.0 ms	-2.0%
Managed pause before fully drawn	12.7 ms	0 ms	-12.7 ms
GC/finalizer CPU	9.4 ms	1.4 ms	-8.0 ms

The startup request itself performed a near-empty preparation collection at about 120 KiB allocated. The Android GC bridge's explicit ART collection moved until after fully drawn.

Risks and fallback

The region is process-wide by design and intentionally short-lived. It ends on the first managed fully-drawn report or after 10 seconds. The runtime automatically leaves the region if the 24 MiB budget is exhausted or a collection is induced. All start and end paths are best effort and one-shot.

Validation

• Mono.Android API 37 build passed.
• Microsoft.Android.Runtime.NativeAOT build passed.
• A .NET 11 NativeAOT-published probe successfully started and ended a 24 MiB no-GC region with full blocking GC disallowed.
• API 36 binding generation passed and confirmed the generated ReportFullyDrawn method is removed in favor of the manual binding.
• Built assembly inspection confirms Activity.ReportFullyDrawn() remains public, virtual, supported from Android 19, and registered as reportFullyDrawn, ()V, GetReportFullyDrawnHandler.
• The in-repo device test project could not be run locally because the local repo SDK is absent and the installed Android workload requires JDK 21; this machine only has JDK 23.

[simonrozsival]

/review

[github-actions]

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Generated by Android PR Reviewer for #12782

[Copilot]

Copilot review overview

🔵 Needs a closer look

Unresolved moderate API, dispatch, and no-GC ownership issues remain.

Review tier: Lite
Findings: None

What changed in this PR

Adds a CoreCLR-only Android startup no-GC region, ending after ReportFullyDrawn() or a timeout.

Changes:

• Adds bounded no-GC lifecycle management and startup initialization.
• Provides a manual ReportFullyDrawn() JNI binding.
• Updates metadata, project files, and lifecycle tests.

File	Summary
tests/​Mono.Android-Tests/​Mono.Android-Tests/​Mono.Android.NET-Tests.csproj	Registers the new test coverage.
tests/​Mono.Android-Tests/​Mono.Android-Tests/​Android.Runtime/​StartupNoGCRegionTest.cs	Tests lifecycle, timeout, collection, and registration behavior.
src/​Mono.Android/​Mono.Android.csproj	Includes the startup region implementation.
src/​Mono.Android/​metadata	Removes the generated ReportFullyDrawn binding.
src/​Mono.Android/​Android.Runtime/​StartupNoGCRegion.cs	Implements bounded startup region management; a concurrency race remains.
src/​Mono.Android/​Android.Runtime/​JNIEnvInit.cs	Starts the region during CoreCLR initialization.
src/​Mono.Android/​Android.App/​Activity.cs	Adds the manual binding and end hook; API metadata, virtual dispatch, and integration coverage issues remain.

[github-actions]

❌ Reject

Findings: 2 errors, 0 warnings, 0 suggestions.

The measured startup improvement is promising, and the manual Activity.ReportFullyDrawn() binding preserves the public/JNI shape. However, the implementation unconditionally takes ownership of the process-wide no-GC facility and its collection-count heuristic cannot guarantee that it will not end a customer replacement region. Both compatibility hazards need resolution before merge.

CI: Azure DevOps build 1595487 was still in progress when reviewed. The completed Android Tools/Java.Interop lanes and CLA check were green; no failures were reported yet.

Generated by Android PR Reviewer for #12782 · copilot · gpt56 · 186.2 AIC · ⌖ 11.3 AIC · ⊞ 21K
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[simonrozsival]

/review

[github-actions]

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Generated by Android PR Reviewer for #12782

[github-actions]

❌ Reject

Findings: 2 errors, 0 warnings, 0 suggestions.

The startup measurements are encouraging, and the manual Activity.ReportFullyDrawn() binding preserves the existing public/JNI shape. However, the current revision reintroduces two process-wide GC ownership hazards: it provides no compatibility escape hatch for applications that manage their own no-GC region, and a delayed completion can terminate a replacement region after ownership has been lost.

CI: Azure DevOps build 1595775 is still in progress. At review time, 17 checks had passed, 25 were running, 1 was queued, and dotnet-android (MAUI Tests MAUI Integration) was failing. The available integration could not retrieve the Azure failure details, so I could not determine whether that lane is related to this change.

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Generated by Android PR Reviewer for #12782 · copilot · gpt56 · 140.1 AIC · ⌖ 8.65 AIC · ⊞ 21.8K
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[github-actions]

🤖 ❌ Resource management — GC.EndNoGCRegion() has no ownership token. If this region is ended implicitly by budget exhaustion or an induced collection, application code can start a replacement region before this timeout/ReportFullyDrawn() path runs; this call will then terminate the application-owned region. The current lock only serializes this helper and cannot protect against external GC callers. Please use a design that cannot end a replacement region (or add runtime support for conditional ownership), and restore deterministic coverage for this interleaving.

Rule: Thread safety of shared state

[github-actions]

🤖 ❌ API design — This again unconditionally occupies the process-wide no-GC facility before RunStartupHooksIfNeeded(). A customer startup hook or early library that uses GC.TryStartNoGCRegion() will now fail until this region ends, and the opt-out introduced in the earlier revision has been removed. Please restore a documented runtime/MSBuild escape hatch (or make the optimization opt-in) so existing applications can preserve their prior GC behavior.

Rule: Behavior-safe defaults