hakmem/docs/archive/PHASE_6.6_SUMMARY.md

# Phase 6.6 Complete Summary

**Date**: 2025-10-21
**Status**: ✅ **COMPLETE**

---

## 🎯 Goal & Achievement

**Goal**: Fix ELO control flow bug that prevented batch madvise activation
**Result**: ✅ **Successfully fixed and verified** - Batch madvise now working correctly

---

## 🐛 Problem

After Phase 6.5 (Learning Lifecycle) integration:
- 2MB allocations were using `MALLOC` instead of `MMAP`
- BigCache eviction called `free()` instead of `hak_batch_add()`
- Batch madvise statistics showed **0 blocks batched** (completely inactive)

---

## 🔍 Root Cause (Diagnosed by Gemini Pro)

**Control flow ordering bug** in `hakmem.c:hak_alloc_at()`:

1. OLD policy decision (`infer_policy()`) executed FIRST → returned `POLICY_DEFAULT`
2. Allocation happened using old policy → `alloc_malloc()` called
3. ELO strategy selection executed TOO LATE → results completely ignored
4. ELO results only used for BigCache eligibility, not allocation method

**Key insight**: "The right answer computed at the wrong time is the wrong answer"

---

## ✅ Fix Applied

**Modified**: `hakmem.c` (lines 645-720)

**Before** (WRONG):
```c
void* hak_alloc_at(size_t size, ...) {
    // 1. Old policy (WRONG!)
    policy = POLICY_DEFAULT;

    // 2. Allocate (TOO EARLY!)
    ptr = allocate_with_policy(size, policy);  // Uses malloc

    // 3. ELO selection (TOO LATE!)
    strategy_id = hak_elo_select_strategy();   // Result not used!
    threshold = hak_elo_get_threshold(strategy_id);
}
```

**After** (CORRECT):
```c
void* hak_alloc_at(size_t size, ...) {
    // 1. ELO selection FIRST!
    strategy_id = hak_elo_select_strategy();
    threshold = hak_elo_get_threshold(strategy_id);

    // 2. BigCache check
    if (hak_bigcache_try_get(...)) return cached_ptr;

    // 3. Use ELO threshold to decide malloc vs mmap
    ptr = (size >= threshold) ? alloc_mmap(size) : alloc_malloc(size);
}
```

**Result**: 2MB allocations now correctly use `mmap`, enabling batch madvise.

---

## 📊 Benchmark Results

**Configuration**: `bench_runner.sh --warmup 2 --runs 10` (200 total runs)

### VM Scenario (2MB allocations)

| Allocator | Median (ns) | vs Phase 6.4 | vs mimalloc |
|-----------|-------------|--------------|-------------|
| mimalloc | 19,964 | +12.6% | baseline |
| jemalloc | 26,241 | -3.0% | +31.4% |
| **hakmem-evolving** | **37,602** | **+2.6%** | **+88.3%** |
| hakmem-baseline | 40,282 | +9.1% | +101.7% |
| system | 59,995 | -4.4% | +200.4% |

### Analysis

1. ✅ **No regression**: +2.6% difference vs Phase 6.4 is within measurement variance
2. ✅ **ELO working**: hakmem-evolving beats hakmem-baseline
3. ✅ **Batch madvise active**: Verified with debug logging
4. ⚠️ **Overhead gap**: Still 2× slower than mimalloc → Phase 6.7 investigation

**Note**: README.md claimed "16,125 ns" for Phase 6.4, but FINAL_RESULTS.md shows 36,647 ns (the correct baseline for comparison).

---

## 🧪 Verification

### Batch Madvise Activation Confirmed

```
[DEBUG] BigCache eviction: method=1 (MMAP), size=2097152  ✅
[DEBUG] Calling hak_batch_add(raw=0x..., size=2097152)    ✅

Batch Statistics:
  Total blocks added:       1                              ✅
  Flush operations:         1                              ✅
  Total bytes flushed:      2097152                        ✅
```

---

## 🎓 Lessons Learned

### Design Mistakes

1. **Control flow ordering**: Strategy selection must happen BEFORE usage
2. **Dead code accumulation**: Old `infer_policy()` logic left behind
3. **Silent failures**: ELO results computed but not used

### Detection Challenges

1. **High-level symptoms**: "Batch not activating" didn't point to control flow
2. **Required detailed tracing**: Had to add debug logging to discover MALLOC usage
3. **Multi-layer architecture**: Problem spanned ELO, allocation, BigCache, batch

### AI Collaboration Success

- **Gemini Pro**: Root cause diagnosis from logs + code analysis
- **Claude**: Applied fix, tested, documented
- **Synergy**: Gemini saw the forest (control flow), Claude fixed the trees (code)

---

## 📝 Bonus Findings

### BigCache Size Check Bug (Already Fixed)

Gemini Task 5cfad9 diagnosed a heap-buffer-overflow bug:
- **Problem**: BigCache returning undersized blocks without `actual_bytes >= requested_bytes` check
- **Impact**: cold-churn benchmark (varying sizes) triggers buffer overflow
- **Status**: ✅ **Already fixed** in previous session
- **Code**: `hakmem_bigcache.c:151` has size check with "Segfault fix!" comment

---

## 🚀 Next Steps (Phase 6.7)

### 1. Overhead Analysis

**Goal**: Identify why hakmem is 2× slower than mimalloc

**Candidates** (from OVERHEAD_ANALYSIS_PLAN.md):
- P0: BigCache lookup (~50-100 ns)
- P0: ELO strategy selection (~100-200 ns)
- P1: mmap/munmap syscalls (~1,000-5,000 ns) ← **Main suspect**
- P1: Page faults (~100-500 ns per page)

**Strategy**:
1. Feature isolation testing (environment variables)
2. `perf` profiling (hotspot identification)
3. `strace` syscall counting

### 2. Optimization Ideas

1. **FROZEN mode by default** (after learning) → -5% overhead
2. **BigCache direct indexing** (instead of linear search) → -5% overhead
3. **Pre-allocated arena** (Phase 7+) → -50% overhead target

**Realistic goal**: Reduce gap from +88% to +40% (Phase 7), then +20% (Phase 8)

**Limit**: Cannot beat mimalloc without slab allocator (industry standard, 10+ years optimization)

---

## 📁 Documentation Created

1. **PHASE_6.6_ELO_CONTROL_FLOW_FIX.md** (updated with benchmark results)
2. **OVERHEAD_ANALYSIS_PLAN.md** (Phase 6.7 preparation)
3. **PHASE_6.6_SUMMARY.md** (this file)
4. **GEMINI_BIGCACHE_ANALYSIS.md** (confirmed existing fix)

---

## 🏆 Final Status

**Phase 6.6**: ✅ **COMPLETE**

**Achievements**:
- ✅ ELO control flow bug fixed
- ✅ Batch madvise activation verified
- ✅ Performance parity with Phase 6.4 maintained (+2.6% variance)
- ✅ Comprehensive documentation created
- ✅ Phase 6.7 roadmap prepared

**Code quality**:
- Modified files: 1 (`hakmem.c`)
- Lines changed: ~75 lines (reordering + cleanup)
- Test coverage: VM scenario verified (200 runs)

**Time investment**: ~6 hours (diagnosis + fix + benchmarking + documentation)

---

**Ready for Phase 6.7: Overhead Analysis & Optimization** 🚀