hakmem/docs/archive/PHASE_6.15_SUMMARY.md

# Phase 6.15: Multi-threaded Safety - Quick Summary

**Full Plan**: [PHASE_6.15_PLAN.md](PHASE_6.15_PLAN.md)

---

## 🎯 **3-Step Approach**

### **Step 1: Documentation Updates** (1 hour)
- Fix PHASE_6.14_COMPLETION_REPORT.md (67.9M measurement issue)
- Update CURRENT_TASK.md (Phase 6.15 status)
- Create this plan document ✅

### **Step 2: P0 Safety Lock** (2-3 hours)
**Goal**: Correctness first (no performance improvement expected)

**Implementation** (30 min):
```c
// Add global lock
static pthread_mutex_t g_hakmem_lock = PTHREAD_MUTEX_INITIALIZER;

// Wrap malloc/free
void* hak_alloc_at(size_t size, uintptr_t site_id) {
    pthread_mutex_lock(&g_hakmem_lock);
    void* ptr = hak_alloc_at_internal(size, site_id);
    pthread_mutex_unlock(&g_hakmem_lock);
    return ptr;
}
```

**Testing** (1.5 hours):
1. larson 1T/4T benchmark
2. Helgrind race detection (expect: 0 errors)
3. Stability test (10 consecutive runs)

**Expected Results**:
- 1T: 13-15M ops/sec (lock overhead 0-15%)
- 4T: 13-15M ops/sec (same as 1T, safe but no scalability)
- **Critical**: Zero crashes, zero data races ✅

---

### **Step 3: TLS Performance** (8-10 hours)

#### **P1: Tiny Pool TLS** (2 hours)
```c
static __thread TinySlab* tls_tiny_cache[8];  // Per-thread cache
```
**Expected**: 4T = 12-15M ops/sec (+264-355%)

#### **P2: L2 Pool TLS** (3 hours)
```c
static __thread L2Block* tls_l2_cache[5];
```
**Expected**: 4T = 15-18M ops/sec

#### **P3: L2.5 Pool TLS Expansion** (3 hours)
**Existing**: `hakmem_l25_pool.c:26` already has TLS declaration
**Missing**: Refill/eviction logic
**Expected**: 4T = 18-22M ops/sec (+445-567%)

---

## 📊 **Performance Expectations**

| Phase | 1-thread | 4-thread | vs Baseline (3.3M) | Notes |
|-------|----------|----------|-------------------|-------|
| **Before** | 15.1M | **3.3M** | baseline (-78%) | UNSAFE |
| **P0 (Lock)** | 13-15M | 13-15M | +294-355% | Safe, no scaling |
| **P0+P1 (Tiny TLS)** | 13-15M | 12-15M | +264-355% | 95%+ TLS hit |
| **P0+P1+P2 (L2 TLS)** | 13-15M | 15-18M | +355-445% | 90%+ TLS hit |
| **P0+P1+P2+P3 (All)** | 13-15M | 18-22M | +445-567% | Full TLS |
| **Phase 6.13 Actual** | 17.8M | **15.9M** | **+381%** ✅ | **PROVEN** |

**Validation**: Phase 6.13 already proved TLS achieves **15.9M ops/sec** at 4 threads ✅

---

## 🎯 **Success Criteria**

### **Minimum (P0 only)**
- ✅ 4T ≥ 13M ops/sec (safe, from 3.3M)
- ✅ Zero race conditions (Helgrind)
- ✅ 10/10 stability runs

### **Target (P0+P1+P2)**
- ✅ 4T ≥ 15M ops/sec (+355%)
- ✅ TLS hit rate ≥ 90%
- ✅ No 1T regression (≤15%)

### **Stretch (All Phases)**
- ✅ 4T ≥ 18M ops/sec (+445%)
- ✅ 16T ≥ 11.6M ops/sec (match system)

---

## 📋 **Quick Checklist**

**Step 1**: Documentation ✅
- [ ] Fix PHASE_6.14_COMPLETION_REPORT.md
- [ ] Update CURRENT_TASK.md
- [ ] Verify with grep commands

**Step 2**: P0 Safety Lock
- [ ] Add pthread.h + global lock
- [ ] Wrap hak_alloc_at/hak_free_at
- [ ] Test: larson 1T/4T
- [ ] Test: Helgrind (expect: 0 errors)
- [ ] Test: 10 stability runs
- [ ] Document: PHASE_6.15_P0_RESULTS.md

**Step 3**: TLS Performance
- [ ] P1: Tiny Pool TLS (2h) → 12-15M ops/sec
- [ ] P2: L2 Pool TLS (3h) → 15-18M ops/sec
- [ ] P3: L2.5 Pool TLS (3h) → 18-22M ops/sec
- [ ] Final: Validation + completion report

---

## ⏱️ **Timeline**

**Day 1**: Step 1 (1h) + Step 2 (2-3h)
**Day 2**: P1 Tiny TLS (2h)
**Day 3-4**: P2 L2 TLS (3h)
**Day 5**: P3 L2.5 TLS (3h)
**Day 6**: Final validation (1h)

**Total**: 12-13 hours over 6 days

---

## 🔗 **Key References**

- **Full Plan**: [PHASE_6.15_PLAN.md](PHASE_6.15_PLAN.md) - Detailed implementation guide
- **Thread Safety Analysis**: [THREAD_SAFETY_SOLUTION.md](THREAD_SAFETY_SOLUTION.md) - Option A/B/C comparison
- **TLS Validation**: [PHASE_6.13_INITIAL_RESULTS.md](PHASE_6.13_INITIAL_RESULTS.md) - Proof that TLS works (+147%)
- **Current Issue**: [PHASE_6.14_COMPLETION_REPORT.md](PHASE_6.14_COMPLETION_REPORT.md) - 4-thread collapse discovery

---

## 💡 **Key Insights**

1. **67.9M was measurement error** - Actual performance is 15.1M (1T)
2. **4-thread collapse is NOT a feature** - It's complete thread-unsafety (3.3M vs 15.1M baseline)
3. **TLS is proven to work** - Phase 6.13 achieved 15.9M at 4T (+381%)
4. **P0 is safety net** - Even if TLS fails, we have working thread-safe allocator
5. **Gradual rollout is key** - P0 → P1 → P2 → P3 (each validated independently)

---

**Status**: ✅ **Ready to Execute**
**Next Action**: Start with Step 1 (Documentation, 1 hour)