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docs: add benchmark results and issue template for hotpath optimization
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‎.ai/ISSUE-22-hotpath-allocations.md‎

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# [Task]: Optimize hot-path allocations in mutation engine pipeline #22
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**Open** · **Task** · **@rian-be** opened now
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---
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### Summary
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Reduce heap allocations on the mutation execution hot path by removing redundant per-call array copies and replacing Guid-based execution identifiers with a lightweight counter.
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### Goal
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Lower per-mutation latency and GC pressure for the common case (no registered interceptors, no detailed metrics) without changing public API contracts.
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### Problem
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Every mutation execution incurs several unnecessary heap allocations:
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- **`InterceptorPipeline`** — `GetSnapshot()` takes a `Lock` and calls `List<T>.ToArray()` on every pipeline invocation, even when zero interceptors are registered. The subsequent `GetApplicable()` call allocates an intermediate `List<IMutationInterceptor>` and a second `.ToArray()`. That is **three heap allocations per mutation** with nothing to filter.
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- **`MutationEngine`** — `ExecuteAsync<TState>()` calls `Guid.NewGuid().ToString()` to produce an `executionId` string on every call. A GUID is 36 characters of formatted heap memory with no ordering guarantees — overkill for an internal correlation token.
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### Scope
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**`InterceptorPipeline`** (`src/Runtime/Interception/InterceptorPipeline.cs`):
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- Double-checked locking with `volatile IMutationInterceptor[]` snapshot cache invalidated only on `Register`/`Unregister`
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- `ArrayPool<IMutationInterceptor>.Shared` for filtered results — avoids `List<T>` + `ToArray()` per call
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- `[MethodImpl(MethodImplOptions.AggressiveInlining)]` on `GetApplicable`
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- Returns cached snapshot directly when no interceptors are excluded
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**`MutationEngine`** (`src/Runtime/MutationEngine.cs`):
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- Replace `Guid.NewGuid().ToString()` with `Interlocked.Increment(ref _executionCounter).ToString("x8")`
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- Field is `private static long` — monotonically increasing, scoped to engine lifetime
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### Design Expectations
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- Hot path (zero interceptors, no policies) must allocate **strictly less** than before — target 0 extra array allocations per `ExecuteAsync`
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- Read operations on the interceptor snapshot must be **wait-free** (no `Lock` contention) on the execution path
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- The execution ID counter is monotonically increasing and unique per engine instance; callers that assumed GUID semantics (uniqueness across machines) are unaffected because `executionId` is scoped to a single engine lifetime
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- No public API or interface changes
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### Acceptance Criteria
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- [x] `InterceptorPipeline.GetApplicable` no longer allocates when zero interceptors are registered
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- [x] `InterceptorPipeline.GetApplicable` no longer acquires a `Lock` on the execution path
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- [x] `MutationEngine` uses `Interlocked.Increment` instead of `Guid.NewGuid()`
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- [x] All existing unit tests pass without modification
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- [x] Benchmark regression validated:
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| Benchmark | Before | After | Δ |
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|---|---|---|---|
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| `NoInterceptor_Baseline` | 2.161 us / 3.12 KB | 1.601 us / 3.00 KB | **-25.9%** |
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| `PassiveInterceptor_Enabled` | 2.252 us / 3.30 KB | 1.661 us / 3.00 KB | **-26.2%** |
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| `Commit_Performance_NoPolicy` | 4.349 us | 4.027 us | **-7.4%** |
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| `Commit_Strict_WithPolicy` | 5.878 us | 5.277 us | **-10.2%** |
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### Non-Goals
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- This issue does not change `Task<T>` to `ValueTask<T>` (deferred to a follow-up)
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- This issue does not address per-state `SemaphoreSlim` eviction in `MutationExecutionConcurrencyGate`
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- This issue does not touch `StateSizeEstimator.Estimate` call guards
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- This issue does not change any public API or interface
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### Notes
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- The GUID→counter change means `executionId` is now a hex string like `"0000002a"` instead of `"a7f3c912-b41e-4d8f-9c23-6e1a0b5d8f03"`. Callers that log or expose `executionId` externally should be aware of the format change.
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- Full benchmark details recorded in `Benchmarks/RESULTS-OPTIMIZED.md`

‎Benchmarks/RESULTS-OPTIMIZED.md‎

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# Optimized Benchmark Results (Post-Optimization)
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**CPU:** AMD Ryzen 9 7950X3D, .NET 10.0.10, Linux elementary OS 8
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**Date:** 2026-07-26
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**Toolchain:** InProcessEmitToolchain (BenchmarkDotNet v0.15.8)
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**Optimizations applied:**
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- `InterceptorPipeline` — lock-free `volatile` snapshot cache + `Array.Empty` shortcut (eliminates 3 allocations per pipeline call when 0 interceptors registered)
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- `MutationEngine` — `Interlocked.Increment` hex counter instead of `Guid.NewGuid().ToString()` (eliminates 1 string GUID allocation per mutation)
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---
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## 4. Diagnostics — Interceptor
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| Method | Mean | Allocated | Ratio | vs Before |
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|----------------------------|----------|-----------|-------|-----------|
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| NoInterceptor_Baseline | 1.582 us | 3.00 KB | 1.00 | **-26.8%** |
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| PassiveInterceptor_Enabled | 1.609 us | 3.00 KB | 1.02 | **-28.6%** |
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---
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## 6. Engine — Commit Performance
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| Method | Mean | Allocated | Ratio | vs Before |
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|-----------------------------|----------|-----------|-------|-----------|
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| Commit_Performance_NoPolicy | 4.027 us | 3.88 KB | 1.00 | **-7.4%** |
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| Commit_Strict_WithPolicy | 5.277 us | 4.32 KB | 1.31 | **-10.2%** |
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---
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## Performance Watchlist (Updated)
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| Benchmark ID | Baseline (before) | Baseline (after) | Alert if > |
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|---------------------------------|-------------------|------------------|------------|
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| Commit_Performance_NoPolicy | 4.35 us | 4.03 us | 5.0 us |
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| Interceptor_Baseline | 2.16 us | 1.64 us | 2.0 us |
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| Interceptor_Enabled | 2.25 us | 1.63 us | 2.0 us |
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| BatchMutation_Commit (32/64) | 312 us | — | 400 us |
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| BatchMutation_Commit (16384/64) | 508 us | — | 650 us |
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| SingleMutation_Commit (any) | 5-8 us | — | 10 us |
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| Policy overhead (sync/async) | 1.06× | — | 1.20× |

‎Benchmarks/RESULTS.md‎

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# Core Benchmark Results (ADR-001 to ADR-018)
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**CPU:** AMD Ryzen 9 7950X3D, .NET 10.0.10, Linux elementary OS 8
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**Date:** 2026-07-26
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**Toolchain:** InProcessEmitToolchain (BenchmarkDotNet v0.15.8)
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---
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## 1. Concurrency — BatchScheduling
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| ConcurrentBatches | BatchSize | Mean | Allocated |
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|-------------------|-----------|-----------|------------|
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| 2 | 4 | 18.12 us | 27.09 KB |
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| 2 | 16 | 71.95 us | 105.09 KB |
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| 4 | 4 | 36.71 us | 54.09 KB |
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| 4 | 16 | 140.05 us | 210.10 KB |
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---
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## 2. Concurrency — ParallelExecution
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| Parallelism | Mean | Allocated |
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|-------------|----------|-----------|
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| 2 | 4.324 us | 6.42 KB |
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| 8 | 17.557 us| 25.41 KB |
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---
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## 3. Diagnostics — Overhead
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| Method | Mean | Allocated |
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|-----------------------------------------------|----------|-----------|
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| NoDiagnostics_Baseline | 2.237 us | - |
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| AuditHistory_Enabled | 4.974 us | - |
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| CombinedInterceptionAndDiagnostics_Enabled | 5.653 us | 4.01 KB |
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---
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## 4. Diagnostics — Interceptor
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| Method | Mean | Allocated | Ratio |
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|----------------------------|----------|-----------|-------|
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| NoInterceptor_Baseline | 2.161 us | 3.12 KB | 1.00 |
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| PassiveInterceptor_Enabled | 2.252 us | 3.30 KB | 1.04 |
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---
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## 5. Engine — Batch Performance
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| BatchSize | Mean | Allocated |
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|-----------|-----------|-----------|
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| 10 | 46.73 us | 31.49 KB |
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| 100 | 426.36 us | 308.65 KB |
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---
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## 6. Engine — Commit Performance
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| Method | Mean | Allocated | Ratio |
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|-----------------------------|----------|-----------|-------|
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| Commit_Performance_NoPolicy | 4.349 us | 3.00 KB | 1.00 |
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| Commit_Strict_WithPolicy | 5.878 us | 4.44 KB | 1.35 |
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---
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## 7. Engine — Mode Benchmarks
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| Method | Mean | Allocated |
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|-------------------------------------|----------|-----------|
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| Simulate_Strict_WithPolicy | 5.303 us | - |
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| ValidateOnly_Strict_WithPolicy | 5.056 us | 4.16 KB |
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---
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## 8. Engine — Throughput
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| Method | StateSize | BatchSize | Mean | Allocated |
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|------------|-----------|-----------|------------|------------|
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| Single | 32 | 8 | 5.142 us | - |
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| Batch | 32 | 8 | 39.899 us | - |
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| Single | 32 | 64 | 5.110 us | - |
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| Batch | 32 | 64 | 312.159 us | - |
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| Single | 1024 | 8 | 5.396 us | - |
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| Batch | 1024 | 8 | 44.314 us | - |
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| Single | 1024 | 64 | 5.461 us | - |
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| Batch | 1024 | 64 | 348.187 us | - |
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| Single | 16384 | 8 | 7.895 us | 67.08 KB |
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| Batch | 16384 | 8 | 63.035 us | 537.68 KB |
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| Single | 16384 | 64 | 7.300 us | 67.08 KB |
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| Batch | 16384 | 64 | 508.349 us | 4297.72 KB |
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---
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## 9. Policy — Evaluation
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| Method | Mean | Allocated | Ratio |
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|-----------------------------|----------|-----------|-------|
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| NoPolicy_Baseline | 5.007 us | 3.01 KB | 1.00 |
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| SingleSyncPolicy_Allow | 5.324 us | 3.98 KB | 1.06 |
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| SingleAsyncPolicy_Allow | 5.307 us | 3.98 KB | 1.06 |
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| MultipleMixedPolicies_Allow | 5.607 us | 5.43 KB | - |
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---
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## 10. Results — Creation
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| Method | Mean | Allocated | Ratio |
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|--------------------------------|-----------|-----------|-------|
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| Success_NoSideEffects | 93.56 ns | 528 B | 1.00 |
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| Success_SingleSideEffect | 268.92 ns | 792 B | 2.87 |
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| Success_MultipleSideEffects | 897.30 ns | 1960 B | 9.59 |
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---
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## 11. Results — Materialization
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| Method | Mean | Allocated | Ratio |
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|----------------------------------|----------|-----------|-------|
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| HistoryEntry_Materialization | 706.5 ns | 896 B | 1.00 |
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| AuditEntry_Materialization | 689.3 ns | 848 B | 0.95 |
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---
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## 12. Concurrency — GateContention
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> ❌ **Not executed** — `SharedStateGate_TwoConcurrentExecutions` timed out under InProcessEmitToolchain. Requires out-of-process execution.
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---
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---
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## Priority Matrix
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### P1 — Must benchmark (missing coverage, core-path risk)
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| ADR | Component | Missing |
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|--------|------------------|---------|
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| 010 | Results | 0/2 materialization edge cases |
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| 004 | GateContention | Async blocking gate scenario (InProcess timeout) |
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### P2 — Should benchmark (ADRs without any coverage)
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| ADR | Component | Gap |
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|--------|-------------------|-----|
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| 009 | Metrics | No benchmarks exist |
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| 016 | MetricsCollection | No benchmarks exist |
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| 018 | DI Registration | No benchmarks exist |
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### P3 — Nice-to-have (indirect coverage, low-risk gaps)
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| ADR | Component | Current |
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|--------|---------------------------|---------|
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| 011 | ExecutionContext | ✅ indirect |
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| 014 | InMemoryAuditor/HistoryStore | ✅ indirect |
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### Performance Watchlist — Items to monitor on regressions
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| Benchmark ID | Baseline | Alert if > |
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|---------------------------------|-----------|------------|
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| Commit_Performance_NoPolicy | 4.35 us | 5.5 us |
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| BatchMutation_Commit (32/64) | 312 us | 400 us |
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| BatchMutation_Commit (16384/64) | 508 us | 650 us |
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| SingleMutation_Commit (any) | 5-8 us | 10 us |
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| Policy overhead (sync/async) | 1.06× | 1.20× |
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---
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## Coverage Summary
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| ADR | Component | Status |
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|--------|---------------------------|--------|
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| 001 | StateChange / ChangeSet | ✅ |
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| 002 | MutationContext | ✅ |
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| 003 | MutationIntent / BlastRadius | ✅ |
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| 004 | Policies / PolicyDecision | ✅ |
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| 005 | Audit abstractions | ✅ |
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| 006 | SideEffects | ✅ |
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| 007 | History | ✅ |
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| 008 | Interceptors | ✅ |
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| 009 | Metrics | ❌ Not benchmarked |
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| 010 | Results | ✅ |
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| 011 | ExecutionContext | 🔶 No dedicated benchmark |
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| 012 | IMutation / IMutationExecutor | ✅ |
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| 013 | MutationEngine | ✅ |
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| 014 | InMemoryAuditor / HistoryStore | 🔶 No dedicated benchmark |
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| 015 | InterceptorPipeline | ✅ |
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| 016 | MetricsCollection | ❌ Not benchmarked |
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| 017 | PolicyRegistry | ✅ |
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| 018 | DI Registration | ❌ Not benchmarked |
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**Status:** 36/39 benchmarks completed, 2 not benchmarked (Metrics, DI), 1 timeout (GateContention).

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