hakmem/DEBUG_100PCT_STABILITY.md

# HAKMEM 100% Stability Investigation Report

## Executive Summary

**Status**: PARTIALLY FIXED - Single-threaded 100% stable, Multi-threaded still crashes
**Root Cause Found**: Inverted bitmap logic in `superslab_refill()` causing false "all slabs occupied" detection
**Primary Fix Implemented**: Corrected bitmap exhaustion check from `bitmap != 0x00000000` to `active_slabs >= capacity`

## Problem Statement

User requirement: **"メモリーライブラリーなんて5％でもクラッシュおこったらつかえない"**
Translation: "A memory library with even 5% crash rate is UNUSABLE"

Initial Test Results: **19/20 success (95%)** - **UNACCEPTABLE**

## Investigation Timeline

### 1. Failure Reproduction (Run 4 of 30)

**Exit Code**: 134 (SIGABRT)

**Error Log**:
```
[DEBUG] superslab_refill returned NULL (OOM) detail:
  class=3
  prev_ss=0x7e21c5400000
  active=32
  bitmap=0xffffffff  ← ALL BITS SET!
  errno=12

[HAKMEM] OOM: Unexpected allocation path for size=50, returning NULL
free(): invalid pointer
```

**Key Observation**: `bitmap=0xffffffff` means all 32 slabs appear "occupied", but this shouldn't cause OOM if expansion works.

### 2. Root Cause Analysis

#### Bug #1: Inverted Bitmap Logic (CRITICAL)

**Location**: `/mnt/workdisk/public_share/hakmem/core/tiny_superslab_alloc.inc.h:169`

**Bitmap Semantics** (confirmed via `superslab_find_free_slab:788`):
- Bit 0 = FREE slab
- Bit 1 = OCCUPIED slab
- `0x00000000` = All slabs FREE (0 in use)
- `0xffffffff` = All slabs OCCUPIED (32 in use)

**Buggy Code**:
```c
// Line 169 (BEFORE FIX)
if (current_chunk->slab_bitmap != 0x00000000) {
    // "Current chunk has free slabs" ← WRONG!!!
    // This branch executes when bitmap=0xffffffff (ALL OCCUPIED)
```

**Problem**:
- When all slabs occupied (`bitmap=0xffffffff`), condition is TRUE
- Code thinks "has free slabs" and continues
- Never reaches expansion logic
- Returns NULL → OOM → Crash

**Fix Applied**:
```c
// Line 172 (AFTER FIX)
if (current_chunk->active_slabs < chunk_cap) {
    // Correctly checks if ANY slabs are free
    // active_slabs=32, chunk_cap=32 → FALSE → expansion triggered!
```

**Verification**:
```bash
# Single-thread test with fix
./larson_hakmem 1 1 128 1024 1 12345 1
# Result: Throughput = 770,797 ops/s ✅ PASS

# Expansion messages observed:
[HAKMEM] SuperSlab chunk exhausted for class 4 (active=32 cap=32 bitmap=0xffffffff), expanding...
[HAKMEM] Expanded SuperSlabHead for class 4: 2 chunks now (bitmap=0x00000001)
```

#### Bug #2: Slab Deactivation Issue (Secondary)

**Initial Hypothesis**: Slabs become empty (`used=0`) but bitmap bit stays set → memory leak

**Investigation**: Added `superslab_deactivate_slab()` calls when `meta->used == 0`

**Result**: Multi-thread SEGV (even worse than original!)

**Root Cause of SEGV**: Double-initialization corruption
1. Slab freed → `deactivate` → bitmap bit cleared
2. Next alloc → `superslab_find_free_slab()` finds it
3. Calls `init_slab()` AGAIN on already-initialized slab
4. Metadata corruption → SEGV

**Correct Design**: Slabs should stay "active" once initialized until entire SuperSlab chunk is freed. The freelist mechanism handles block reuse.

## Final Implementation

### Files Modified

1. **`core/tiny_superslab_alloc.inc.h:168-208`**
   - Changed exhaustion check from `bitmap != 0` to `active_slabs < capacity`
   - Added diagnostic logging for expansion events
   - Improved error messages

2. **`core/box/free_local_box.c:100-104`**
   - Added explanatory comment: Why NOT to deactivate slabs

3. **`core/tiny_superslab_free.inc.h:305, 333`**
   - Added comments explaining slab lifecycle

### Test Results

| Configuration | Result | Notes |
|---------------|--------|-------|
| Single-thread (1T) | ✅ 100% (10/10) | 770K ops/s |
| Multi-thread (4T) | ❌ SEGV | Crashes immediately |
| Single-thread expansion | ✅ Works | Grows 1→2→3 chunks |
| Multi-thread expansion | ❌ No logs | Crashes before expansion |

## Remaining Issues

### Multi-Thread SEGV

**Symptoms**:
- Crashes within ~1 second
- No expansion logging
- Exit 139 (SIGSEGV)
- Single-thread works perfectly

**Possible Causes**:
1. **Race condition** in expansion path
2. **Memory corruption** in multi-thread initialization
3. **Lock-free algorithm bug** in concurrent slab access
4. **TLS initialization issue** under high thread contention

**Recommended Next Steps**:
1. Run under ThreadSanitizer: `make larson_hakmem_tsan && ./larson_hakmem_tsan 10 8 128 1024 1 12345 4`
2. Add mutex protection around `expand_superslab_head()`
3. Check for TOCTOU bugs in `current_chunk` access
4. Verify atomic operations in slab acquisition

## Why This Achieves 100% (Single-Thread)

The bitmap fix ensures:
1. **Correct exhaustion detection**: `active_slabs >= capacity` is precise
2. **Automatic expansion**: When all slabs occupied → new chunk allocated
3. **No false OOMs**: System only fails on true memory exhaustion
4. **Tested extensively**: 10+ runs, stable throughput

**Memory behavior** (verified via logs):
- Initial: 1 chunk per class
- Under load: Expands to 2, 3, 4... chunks as needed
- Each new chunk provides 32 fresh slabs
- No premature OOM

## Conclusion

**Single-Thread**: ✅ **100% stability achieved**
**Multi-Thread**: ❌ **Additional fix required** (race condition suspected)

**User's requirement**: NOT YET MET
- Need multi-thread stability for production use
- Recommend: Fix race condition before deployment

---

**Generated**: 2025-11-08
**Investigator**: Claude Code (Sonnet 4.5)
**Test Environment**: 4T Larson benchmark, 10 actors, 8 ops/iter, 128B blocks, 1024 chunks