# Phase 6-3 Fast Path: Quick Fix Summary

## Root Cause (TL;DR)

Fast Path implementation creates a **double-layered allocation path** that ALWAYS fails due to SuperSlab OOM:

```
Fast Path → tiny_fast_refill() → hak_tiny_alloc_slow() → OOM (NULL)
  ↓
Fallback → Box Refactor path → ALSO OOM → crash
```

**Result:** -20% regression (4.19M → 3.35M ops/s) + 45 GB memory leak

---

## 3 Fix Options (Ranked)

### ⭐⭐⭐⭐⭐ Fix #1: Disable Fast Path (IMMEDIATE)

**Time:** 1 minute
**Confidence:** 100%
**Target:** 4.19M ops/s (restore baseline)

```bash
make clean
make BOX_REFACTOR_DEFAULT=1 TINY_FAST_PATH_DEFAULT=0 larson_hakmem
./larson_hakmem 10 8 128 1024 1 12345 4
```

**Why this works:** Reverts to proven Box Refactor path (Phase 6-2.2)

---

### ⭐⭐⭐⭐ Fix #2: Integrate Fast Path with Box Refactor (2-4 hours)

**Confidence:** 80%
**Target:** 5.0-6.0M ops/s (20-40% improvement)

**Change 1:** Make `tiny_fast_refill()` use Box Refactor backend

```c
// File: core/tiny_fastcache.c:tiny_fast_refill()
void* tiny_fast_refill(int class_idx) {
    // OLD: void* ptr = hak_tiny_alloc_slow(size, class_idx);  // OOM!
    // NEW: Use proven Box Refactor path
    void* ptr = hak_tiny_alloc(size);  // ← This works!

    // Rest of refill logic stays the same...
}
```

**Change 2:** Remove Fast Path from `hak_alloc_at()` (avoid double-layer)

```c
// File: core/hakmem.c:hak_alloc_at()
// Comment out lines 682-697 (Fast Path check)
// Keep ONLY in malloc() wrapper (lines 1294-1309)
```

**Why this works:**
- Box Refactor path is proven (4.19M ops/s)
- Fast Path gets actual cache refills
- Subsequent allocations hit 3-4 instruction fast path
- No OOM because Box Refactor handles allocation correctly

---

### ⭐⭐ Fix #3: Fix SuperSlab OOM (1-2 weeks)

**Confidence:** 60%
**Effort:** High (deep architectural change)

Only needed if Fix #2 still has OOM issues. See full analysis for details.

---

## Recommended Sequence

1. **Now:** Run Fix #1 (restore baseline)
2. **Today:** Implement Fix #2 (integrate with Box Refactor)
3. **Test:** A/B compare Fix #1 vs Fix #2
4. **Decision:**
   - If Fix #2 > 4.5M ops/s → Ship it! ✅
   - If Fix #2 still has OOM → Need Fix #3 (long-term)

---

## Expected Outcomes

| Fix | Time | Score | Status |
|-----|------|-------|--------|
| #1 (Disable) | 1 min | 4.19M ops/s | ✅ Safe baseline |
| #2 (Integrate) | 2-4 hrs | 5.0-6.0M ops/s | 🎯 Target |
| #3 (Root cause) | 1-2 weeks | Unknown | ⚠️ High risk |

---

## Why Statistics Don't Show

`HAKMEM_TINY_FAST_STATS=1` produces no output because:

1. **No shutdown hook** - `tiny_fast_print_stats()` never called
2. **Thread-local counters** - Lost when threads exit
3. **Early crash** - OOM kills benchmark before stats printed

**Fix:** Add to `hak_flush_tiny_exit()` in `hakmem.c`:
```c
// Line ~206
extern void tiny_fast_print_stats(void);
tiny_fast_print_stats();
```

---

**Full analysis:** `PHASE6_3_REGRESSION_ULTRATHINK.md`