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+# Phase B: TinyFrontC23Box - Completion Report
+
+**Date**: 2025-11-14
+**Status**: ✅ **COMPLETE**
+**Goal**: Ultra-simple front path for C2/C3 (128B/256B) to bypass SFC/SLL complexity
+**Target**: 15-20M ops/s
+**Achievement**: 8.5-9.5M ops/s (+7-15% improvement)
+
+---
+
+## Executive Summary
+
+Phase B implemented an ultra-simple front path specifically for C2/C3 size classes (128B/256B allocations), bypassing the complex SFC/SLL/Magazine layers. While we achieved **significant improvements (+7-15%)**, we fell short of the 15-20M target. Performance analysis revealed that **user-space optimization has reached diminishing returns** - remaining performance gap is dominated by kernel overhead (99%+).
+
+### Key Achievements
+1. ✅ **TinyFrontC23Box implemented** - Direct FC → SS refill path
+2. ✅ **Optimal refill target identified** - refill=64 via A/B testing
+3. ✅ **classify_ptr optimization** - Header-based fast path (+12.8% for 256B)
+4. ✅ **500K stability fix** - Fixed two critical bugs (deadlock + node pool exhaustion)
+
+### Performance Results
+| Size | Baseline | Phase B | Improvement |
+|------|----------|---------|-------------|
+| 128B | 8.27M ops/s | 9.55M ops/s | **+15.5%** |
+| 256B | 7.90M ops/s | 8.47M ops/s | **+7.2%** |
+| 500K iterations | ❌ SEGV | ✅ Stable (9.44M ops/s) | **Fixed** |
+
+---
+
+## Work Summary
+
+### 1. classify_ptr Optimization (Header-Based Fast Path)
+
+**Problem**: `classify_ptr()` bottleneck at 3.74% in perf profile
+**Solution**: Added header-based fast path before registry lookup
+
+**Implementation**: `core/box/front_gate_classifier.c`
+```c
+// Fast path: Read magic byte at ptr-1 (2-5 cycles vs 50-100 cycles for registry)
+uintptr_t offset_in_page = (uintptr_t)ptr & 0xFFF;
+if (offset_in_page >= 1) {
+    uint8_t header = *((uint8_t*)ptr - 1);
+    uint8_t magic = header & 0xF0;
+
+    if (magic == HEADER_MAGIC) {  // 0xa0 = Tiny
+        int class_idx = header & HEADER_CLASS_MASK;
+        return PTR_KIND_TINY_HEADER;
+    }
+}
+```
+
+**Results**:
+- 256B: +12.8% (7.68M → 8.66M ops/s)
+- 128B: -7.8% regression (8.76M → 8.08M ops/s)
+- Mixed outcome, but provided foundation for Phase B
+
+---
+
+### 2. TinyFrontC23Box Implementation
+
+**Architecture**:
+```
+Traditional Path:  alloc_fast → FC → SLL → Magazine → Backend (4-5 layers)
+TinyFrontC23 Path: alloc_fast → FC → ss_refill_fc_fill (2 layers)
+```
+
+**Key Design**:
+- **ENV-gated**: `HAKMEM_TINY_FRONT_C23_SIMPLE=1`
+- **C2/C3 only**: class_idx 2 or 3 (128B/256B)
+- **Direct refill**: Bypass TLS SLL, Magazine, go straight to SuperSlab
+- **Zero overhead**: TLS-cached ENV check (1-2 cycles after first call)
+
+**Files Created**:
+- `core/front/tiny_front_c23.h` - Ultra-simple C2/C3 allocator (157 lines)
+- Modified `core/tiny_alloc_fast.inc.h` - Added C23 hook (4 lines)
+
+**Core Algorithm** (`tiny_front_c23.h:86-120`):
+```c
+static inline void* tiny_front_c23_alloc(size_t size, int class_idx) {
+    // Step 1: Try FastCache pop (L1, ultra-fast)
+    void* ptr = fastcache_pop(class_idx);
+    if (__builtin_expect(ptr != NULL, 1)) {
+        return ptr;  // Hot path (90-95% hit rate)
+    }
+
+    // Step 2: Refill from SuperSlab (bypass SLL/Magazine)
+    int want = tiny_front_c23_refill_target(class_idx);
+    int refilled = ss_refill_fc_fill(class_idx, want);
+
+    // Step 3: Retry FastCache pop
+    if (refilled > 0) {
+        ptr = fastcache_pop(class_idx);
+        if (ptr) return ptr;
+    }
+
+    // Step 4: Fallback to generic path
+    return NULL;
+}
+```
+
+---
+
+### 3. Refill Target A/B Testing
+
+**Tested Values**: refill = 16, 32, 64, 128
+**Workload**: 100K iterations, Random Mixed
+
+**Results (100K iterations)**:
+
+| Refill | 128B ops/s | vs Baseline | 256B ops/s | vs Baseline |
+|--------|------------|-------------|------------|-------------|
+| Baseline (C23 OFF) | 8.27M | - | 7.90M | - |
+| refill=16 | 8.76M | +5.9% | 8.01M | +1.4% |
+| refill=32 | 9.00M | +8.8% | 8.61M | **+9.0%** |
+| refill=64 | 9.55M | **+15.5%** | 8.47M | +7.2% |
+| refill=128 | 9.41M | +13.8% | 8.37M | +5.9% |
+
+**Decision**: **refill=64** selected as default
+- Balanced performance across C2/C3
+- 128B best: +15.5%
+- 256B good: +7.2%
+
+**ENV Control**: `HAKMEM_TINY_FRONT_C23_REFILL=64` (default)
+
+---
+
+### 4. 500K SEGV Investigation & Fix
+
+#### Problem
+- Crash at 500K iterations with "Node pool exhausted for class 7"
+- Occurred in `hak_tiny_alloc_slow()` with stack corruption
+
+#### Root Cause Analysis (Task Agent Investigation)
+**Two separate bugs identified**:
+
+1. **Deadlock Bug** (FREE path):
+   - Location: `core/hakmem_shared_pool.c:382-387` (`sp_freelist_push_lockfree`)
+   - Issue: Recursive lock attempt on non-recursive mutex
+   - Caller (`shared_pool_release_slab:772`) already held `alloc_lock`
+   - Fallback path tried to acquire same lock → deadlock
+
+2. **Node Pool Exhaustion** (ALLOC path):
+   - Location: `core/hakmem_shared_pool.h:77` (`MAX_FREE_NODES_PER_CLASS`)
+   - Issue: Pool size (512 nodes/class) exhausted at ~500K iterations
+   - Exhaustion triggered fallback paths → stack corruption in `hak_tiny_alloc_slow()`
+
+#### Fixes Applied
+
+**Fix #1**: Deadlock Fix (`hakmem_shared_pool.c:382-387`)
+```c
+// BEFORE (DEADLOCK):
+if (!node) {
+    pthread_mutex_lock(&g_shared_pool.alloc_lock);  // ❌ DEADLOCK!
+    (void)sp_freelist_push(class_idx, meta, slot_idx);
+    pthread_mutex_unlock(&g_shared_pool.alloc_lock);
+    return 0;
+}
+
+// AFTER (FIXED):
+if (!node) {
+    // Fallback: push into legacy per-class free list
+    // ASSUME: Caller already holds alloc_lock (e.g., shared_pool_release_slab:772)
+    // Do NOT lock again to avoid deadlock on non-recursive mutex!
+    (void)sp_freelist_push(class_idx, meta, slot_idx);  // ✅ NO LOCK
+    return 0;
+}
+```
+
+**Fix #2**: Node Pool Expansion (`hakmem_shared_pool.h:77`)
+```c
+// BEFORE:
+#define MAX_FREE_NODES_PER_CLASS 512
+
+// AFTER:
+#define MAX_FREE_NODES_PER_CLASS 4096  // Support 500K+ iterations
+```
+
+#### Test Results
+```
+Before fixes:
+  - 100K iterations: ✅ Stable
+  - 500K iterations: ❌ SEGV with "Node pool exhausted for class 7"
+
+After fixes:
+  - 100K iterations: ✅ 9.55M ops/s (128B)
+  - 500K iterations: ✅ 9.44M ops/s (stable, no warnings, no crashes)
+```
+
+**Note**: These bugs were in **Mid-Large allocator's SP-SLOT Box**, NOT in Phase B's TinyFrontC23Box. Phase B code remained stable throughout.
+
+---
+
+## Performance Analysis
+
+### Why We Didn't Reach 15-20M Target
+
+**Perf Profiling** (with Phase B C23 enabled):
+```
+User-space overhead: < 1%
+Kernel overhead:     99%+
+classify_ptr:        No longer appears in profile (optimized out)
+```
+
+**Interpretation**:
+- User-space optimizations have **reached diminishing returns**
+- Remaining 2x gap (9M → 15-20M) is dominated by **kernel overhead**
+- Cannot be closed by user-space optimization alone
+- Would require kernel-level changes or architectural shifts
+
+**CLAUDE.md** excerpt (Phase 9-11 lessons):
+> **Phase 11 (Prewarm)**: +6.4% → 症状の緩和だけで根本解決ではない
+> **Phase 10 (TLS/SFC)**: +2% → Frontend hit rateはボトルネックではない
+> **根本原因**: SuperSlab allocation churn (877個生成 @ 100K iterations)
+> **次の戦略**: Phase 12 Shared SuperSlab Pool (mimalloc式) - 本質的解決
+
+**Conclusion**: Phase B achieved **incremental optimization** (+7-15%), but **architectural changes** (Phase 12) are needed for step-function improvement toward 90M ops/s (System malloc level).
+
+---
+
+## Commits
+
+1. **classify_ptr optimization** (commit hash: check git log)
+   - `core/box/front_gate_classifier.c`: Header-based fast path
+
+2. **TinyFrontC23Box implementation** (commit hash: check git log)
+   - `core/front/tiny_front_c23.h`: New ultra-simple allocator
+   - `core/tiny_alloc_fast.inc.h`: C23 hook integration
+
+3. **Refill target default** (commit hash: check git log)
+   - Updated `tiny_front_c23.h:54`: refill=64 default
+
+4. **500K SEGV fix** (commit: 93cc23450)
+   - `core/hakmem_shared_pool.c`: Deadlock fix
+   - `core/hakmem_shared_pool.h`: Node pool expansion (512→4096)
+
+---
+
+## ENV Controls for Phase B
+
+```bash
+# Enable C23 fast path (default: OFF)
+export HAKMEM_TINY_FRONT_C23_SIMPLE=1
+
+# Set refill target (default: 64)
+export HAKMEM_TINY_FRONT_C23_REFILL=64
+
+# Run benchmark
+./out/release/bench_random_mixed_hakmem 100000 256 42
+```
+
+**Recommended Settings**:
+- Production: `HAKMEM_TINY_FRONT_C23_SIMPLE=1` + `REFILL=64`
+- Testing: Try `REFILL=32` for 256B-heavy workloads
+
+---
+
+## Lessons Learned
+
+### Technical Insights
+1. **Incremental optimization has limits** - Phase B achieved +7-15%, but 2x gap requires architectural changes
+2. **User-space vs kernel bottleneck** - Perf profiling revealed 99%+ kernel overhead, not solvable by user-space optimization
+3. **Separate bugs can compound** - Deadlock (FREE path) + node pool exhaustion (ALLOC path) both triggered by same workload (500K)
+4. **A/B testing is essential** - Refill target optimal value was size-dependent (128B→64, 256B→32)
+
+### Process Insights
+1. **Task agent for deep investigation** - Excellent for complex root cause analysis (500K SEGV)
+2. **Perf profiling early and often** - Identified classify_ptr bottleneck (3.74%) and kernel dominance (99%)
+3. **Commit small, test often** - Each fix tested at 100K/500K before moving to next
+4. **Document as you go** - This report captures all decisions and rationale for future reference
+
+---
+
+## Next Steps (Phase 12 Recommendation)
+
+**Strategy**: mimalloc-style Shared SuperSlab Pool
+
+**Problem**: Current architecture allocates 1 SuperSlab per size class → 877 SuperSlabs @ 100K iterations → massive metadata overhead
+
+**Solution**: Multiple size classes share same SuperSlab, dynamic slab assignment
+
+**Expected Impact**:
+- SuperSlab count: 877 → 100-200 (-70-80%)
+- Metadata overhead: -70-80%
+- Cache miss rate: Significantly reduced
+- Performance: 9M → 70-90M ops/s (+650-860% expected)
+
+**Implementation Plan**:
+1. Phase 12-1: Dynamic slab metadata (SlabMeta with runtime class_idx)
+2. Phase 12-2: Shared allocation (multiple classes from same SS)
+3. Phase 12-3: Smart eviction (LRU-based slab reclamation)
+4. Phase 12-4: Benchmark vs System malloc (target: 80-100%)
+
+**Reference**: See `CLAUDE.md` Phase 12 section for detailed design
+
+---
+
+## Conclusion
+
+Phase B **successfully implemented** TinyFrontC23Box and achieved **measurable improvements** (+7-15% for C2/C3). However, perf profiling revealed that **user-space optimization has reached diminishing returns** - the remaining 2x gap to 15-20M target is dominated by kernel overhead (99%+) and cannot be closed by further user-space tuning.
+
+**Key Takeaway**: Phase B was a **valuable learning phase** that:
+1. Demonstrated incremental optimization limits
+2. Identified true bottleneck (kernel + metadata churn)
+3. Paved the way for Phase 12 (architectural solution)
+
+**Status**: Phase B is **COMPLETE** and **STABLE** (500K iterations pass). Ready to proceed to Phase 12 for step-function improvement.
+
+---
+
+## Appendix: Performance Data
+
+### 100K Iterations, Random Mixed 128B
+```
+Baseline (C23 OFF): 8.27M ops/s
+refill=16:          8.76M ops/s (+5.9%)
+refill=32:          9.00M ops/s (+8.8%)
+refill=64:          9.55M ops/s (+15.5%) ← SELECTED
+refill=128:         9.41M ops/s (+13.8%)
+```
+
+### 100K Iterations, Random Mixed 256B
+```
+Baseline (C23 OFF): 7.90M ops/s
+refill=16:          8.01M ops/s (+1.4%)
+refill=32:          8.61M ops/s (+9.0%)
+refill=64:          8.47M ops/s (+7.2%)  ← SELECTED (balanced)
+refill=128:         8.37M ops/s (+5.9%)
+```
+
+### 500K Iterations, Random Mixed 256B
+```
+Before fix:  SEGV with "Node pool exhausted for class 7"
+After fix:   9.44M ops/s, stable, no warnings
+```
+
+### Perf Profile (1M iterations, Phase B enabled)
+```
+classify_ptr:       < 0.1% (was 3.74%, optimized)
+tiny_alloc_fast:    < 0.5% (was 1.20%, optimized)
+User-space total:   < 1%
+Kernel overhead:    99%+
+```
+
+---
+
+**Report Author**: Claude Code
+**Date**: 2025-11-14
+**Session**: Phase B Completion