hakmem/PHASE9_2_BENCHMARK_REPORT.md

# Phase 9-2 Benchmark Report: WS8192 Performance Analysis

**Date**: 2025-11-30
**Test Configuration**: WS8192 (Working Set = 8192 allocations)
**Benchmark**: bench_random_mixed_hakmem 10000000 8192
**Status**: Baseline measurements complete, optimization not yet implemented

---

## Executive Summary

WS8192ベンチマークを正しいパラメータで測定しました。結果：

1. **SuperSlab OFF vs ON**: ほぼ同じ性能（16.23M vs 16.15M ops/s、-0.51%）
2. **期待値とのギャップ**: Phase 9-2の期待値は25-30M ops/s (+50-80%)、実測は改善なし
3. **根本原因**: Phase 9-2の修正（EMPTY→Freelist recycling）が**未実装**であることが判明
4. **次のステップ**: Phase 9-2 Option Aの実装が必要

---

## 1. Benchmark Results

### 1.1 SuperSlab OFF (Baseline)

```bash
HAKMEM_TINY_USE_SUPERSLAB=0 ./bench_random_mixed_hakmem 10000000 8192
```

| Run | Throughput (ops/s) | Time (s) |
|-----|-------------------|----------|
| 1   | 16,468,918        | 0.607    |
| 2   | 16,192,733        | 0.618    |
| 3   | 16,035,542        | 0.624    |
| **Average** | **16,232,398** | **0.616** |
| **Std Dev** | 178,517 (±1.1%) | 0.007 |

**Key Observations**:
- Consistent performance (±1.1% variance)
- 4x `[SS_BACKEND] shared_fail→legacy cls=7` warnings
- TLS_SLL errors present (header corruption warnings)

### 1.2 SuperSlab ON (Current State)

```bash
HAKMEM_TINY_USE_SUPERSLAB=1 ./bench_random_mixed_hakmem 10000000 8192
```

| Run | Throughput (ops/s) | Time (s) |
|-----|-------------------|----------|
| 1   | 16,231,848        | 0.616    |
| 2   | 16,305,843        | 0.613    |
| 3   | 15,910,918        | 0.628    |
| **Average** | **16,149,536** | **0.619** |
| **Std Dev** | 171,766 (±1.1%) | 0.007 |

**Key Observations**:
- **No performance improvement** (-0.51% vs baseline)
- Same `shared_fail→legacy` warnings (4x Class 7 fallbacks)
- Same TLS_SLL errors
- SuperSlab enabled but not providing benefits

### 1.3 Improvement Analysis

```
Baseline (SuperSlab OFF): 16.23 M ops/s
Current (SuperSlab ON):   16.15 M ops/s
Improvement:              -0.51% (REGRESSION, within noise)

Expected (Phase 9-2):     25-30 M ops/s
Gap:                      -8.85 to -13.85 M ops/s (-35% to -46%)
```

**Verdict**: SuperSlab is enabled but **not functional** due to missing EMPTY recycling.

---

## 2. Problem Analysis

### 2.1 Why SuperSlab Has No Effect

From PHASE9_2_SUPERSLAB_BACKEND_INVESTIGATION.md investigation:

**Root Cause**: Shared pool Stage 3 soft cap blocks new SuperSlab allocation, but **EMPTY slabs are not recycled** to Stage 1 freelist.

**Flow**:
```
1. Benchmark allocates ~820 Class 7 blocks (10% of WS=8192)
2. Shared pool allocates 2 SuperSlabs (512KB each = 1022 blocks total)
3. class_active_slots[7] = 2 (soft cap reached)
4. Next allocation request:
   - Stage 0.5 (EMPTY scan): Finds nothing (only 2 SS, both ACTIVE)
   - Stage 1 (freelist): Empty (no EMPTY→ACTIVE transitions)
   - Stage 2 (UNUSED claim): Exhausted (first pass only)
   - Stage 3 (new SS alloc): FAIL (soft cap: current=2 >= limit=2)
5. shared_pool_acquire_slab() returns -1
6. Falls back to legacy backend
7. Legacy backend uses system malloc → kernel overhead
```

**Result**: SuperSlab backend is **bypassed 4 times** during benchmark → falls back to legacy system malloc.

### 2.2 Observable Evidence

**Log Snippet**:
```
[SS_BACKEND] shared_fail→legacy cls=7  ← SuperSlab failed, using legacy
[SS_BACKEND] shared_fail→legacy cls=7
[SS_BACKEND] shared_fail→legacy cls=7
[SS_BACKEND] shared_fail→legacy cls=7
```

**What This Means**:
- SuperSlab attempted allocation → hit soft cap → failed
- Fell back to `hak_tiny_alloc_superslab_backend_legacy()`
- Legacy backend uses **system malloc** (not SuperSlab)
- Kernel overhead: mmap/munmap syscalls → 55% CPU in kernel

**Why No Performance Difference**:
- SuperSlab ON: Uses legacy backend (same as SuperSlab OFF)
- SuperSlab OFF: Uses legacy backend (expected)
- Both configurations → same code path → same performance

---

## 3. Missing Implementation: EMPTY→Freelist Recycling

### 3.1 What Needs to Be Implemented

**Phase 9-2 Option A** (from investigation report):

#### Step 1: Add EMPTY Detection to Remote Drain
**File**: `core/superslab_slab.c` (after line 109)
```c
void _ss_remote_drain_to_freelist_unsafe(SuperSlab* ss, int slab_idx, TinySlabMeta* meta) {
    // ... existing drain logic ...

    meta->freelist = prev;
    atomic_store(&ss->remote_counts[slab_idx], 0);

    // ✅ NEW: Check if slab is now EMPTY
    if (meta->used == 0 && meta->capacity > 0) {
        ss_mark_slab_empty(ss, slab_idx);  // Set empty_mask bit

        // Notify shared pool: push to per-class freelist
        int class_idx = (int)meta->class_idx;
        if (class_idx >= 0 && class_idx < TINY_NUM_CLASSES_SS) {
            shared_pool_release_slab(ss, slab_idx);
        }
    }

    // ... update masks ...
}
```

#### Step 2: Add EMPTY Detection to TLS SLL Drain
**File**: `core/box/tls_sll_drain_box.c`
```c
uint32_t tiny_tls_sll_drain(int class_idx, uint32_t batch_size) {
    // ... existing drain logic ...

    // After draining N blocks from TLS SLL to freelist:
    if (meta->used == 0 && meta->capacity > 0) {
        ss_mark_slab_empty(ss, slab_idx);
        shared_pool_release_slab(ss, slab_idx);
    }
}
```

### 3.2 Expected Impact (After Implementation)

**Performance Prediction** (from Phase 9-2 investigation, Section 9.2):

| Configuration | Throughput | Kernel Overhead | Stage 1 Hit Rate |
|--------------|------------|-----------------|------------------|
| Current (no recycling) | 16.5 M ops/s | 55% | 0% |
| **Option A (EMPTY recycling)** | **25-28 M ops/s** | 15% | 80% |
| Option A+B (+ 2MB SS) | 30-35 M ops/s | 12% | 85% |

**Why +50-70% Improvement**:
- EMPTY slabs recycle instantly via lock-free Stage 1
- Soft cap never hit (slots reused, not created)
- Eliminates mmap/munmap overhead from legacy fallback
- SuperSlab backend becomes **fully functional**

---

## 4. Comparison with Phase 9-1

### 4.1 Phase 9-1 Status

From PHASE9_1_PROGRESS.md:

**Phase 9-1 Goal**: Optimize SuperSlab lookup (50-80 cycles → 8-12 cycles)
**Status**: Infrastructure complete (4/6 steps), **migration not started**
- ✅ Step 1-4: Hash table + TLS hints implementation
- ⏸️ Step 5: Migration (IN PROGRESS)
- ⏸️ Step 6: Benchmark (PENDING)

**Key Point**: Phase 9-1 optimizations are **not yet integrated** into hot path.

### 4.2 Phase 9-2 Status

**Phase 9-2 Goal**: Fix SuperSlab backend (eliminate legacy fallbacks)
**Status**: Investigation complete, **implementation not started**
- ✅ Root cause identified (EMPTY recycling missing)
- ✅ 4 fix options proposed (Option A recommended)
- ⏸️ Implementation: NOT STARTED
- ⏸️ Benchmark: NOT STARTED

**Key Point**: Phase 9-2 is still in **planning phase**.

---

## 5. Performance Budget Analysis

### 5.1 Current Bottlenecks (WS8192)

```
Total: 212 cycles/op (16.5 M ops/s @ 2.8 GHz)
  - SuperSlab Lookup:   50-80 cycles  ← Phase 9-1 target
  - Legacy Fallback:    30-50 cycles  ← Phase 9-2 target
  - Fragmentation:      30-50 cycles
  - TLS Drain:          10-15 cycles
  - Actual Work:        30-40 cycles
```

**Kernel Overhead**: 55% (mmap/munmap from legacy fallback)

### 5.2 Expected After Phase 9-1 + 9-2

**After Phase 9-1** (lookup optimization):
```
Total: 152 cycles/op (18.4 M ops/s baseline)
  - SuperSlab Lookup:   8-12 cycles   ✅ Fixed (hash + TLS hints)
  - Legacy Fallback:    30-50 cycles  ← Still broken
  - Fragmentation:      30-50 cycles
  - TLS Drain:          10-15 cycles
  - Actual Work:        30-40 cycles
```
**Expected**: 16.5M → 23-25M ops/s (+39-52%)

**After Phase 9-1 + 9-2** (lookup + backend):
```
Total: 95 cycles/op (29.5 M ops/s baseline)
  - SuperSlab Lookup:   8-12 cycles   ✅ Fixed (Phase 9-1)
  - Legacy Fallback:    0 cycles      ✅ Fixed (Phase 9-2)
  - SuperSlab Backend:  15-20 cycles  ✅ Stage 1 reuse
  - Fragmentation:      20-30 cycles
  - TLS Drain:          10-15 cycles
  - Actual Work:        30-40 cycles
```
**Expected**: 16.5M → **30-35M ops/s** (+80-110%)
**Kernel Overhead**: 55% → 12-15%

---

## 6. Diagnostic Output Analysis

### 6.1 Repeated Warnings

**TLS_SLL_POP_POST_INVALID**:
```
[TLS_SLL_POP_POST_INVALID] cls=6 next=0x7 last_writer=pop
[TLS_SLL_HDR_RESET] cls=6 base=0x... got=0x00 expect=0xa6 count=0
[TLS_SLL_POP_POST_INVALID] cls=6 next=0x5b last_writer=pop
```

**Analysis** (from Phase 9-2 investigation, Section 2):
- **cls=6**: Class 6 (512-byte blocks)
- **got=0x00**: Header corrupted/zeroed
- **count=0**: One-time event (not recurring)
- **Hypothesis**: Use-after-free or slab reuse race
- **Mitigation**: Existing guards (`tiny_tls_slab_reuse_guard()`) should prevent
- **Verdict**: **Not critical** (one-time event, guards in place)
- **Action**: Monitor with `HAKMEM_SUPER_REG_DEBUG=1` for recurrence

### 6.2 Shared Fail Events

```
[SS_BACKEND] shared_fail→legacy cls=7
```

**Count**: 4 events per benchmark run
**Class**: Class 7 (2048-byte allocations, 1024-1040B range in benchmark)
**Reason**: Soft cap reached (Stage 3 blocked)
**Impact**: Falls back to system malloc → kernel overhead

**This is the PRIMARY bottleneck** that Phase 9-2 Option A will fix.

---

## 7. Verification of Test Configuration

### 7.1 Benchmark Parameters

**Command Used**:
```bash
./bench_random_mixed_hakmem 10000000 8192
```

**Breakdown**:
- `10000000`: 10M cycles (steady-state measurement)
- `8192`: Working set size (WS8192)

**From bench_random_mixed.c (line 45-46)**:
```c
int cycles = (argc>1)? atoi(argv[1]) : 10000000; // total ops
int ws     = (argc>2)? atoi(argv[2]) : 8192;    // working-set slots
```

**Allocation Pattern** (line 116):
```c
size_t sz = 16u + (r & 0x3FFu); // 16..1040 bytes (approx 16..1024)
```

**Class Distribution** (estimated):
```
16-64B   → Classes 0-3 (~40%)
64-256B  → Classes 4-5 (~30%)
256-512B → Class 6      (~20%)
512-1040B → Class 7     (~10% = ~820 live allocations)
```

**Why Class 7 Exhausts**:
- 820 live allocations ÷ 511 blocks/SuperSlab = 1.6 SuperSlabs (rounded to 2)
- Soft cap = 2 → any additional allocation fails → legacy fallback

### 7.2 Comparison with Phase 9-1 Baseline

**From PHASE9_1_PROGRESS.md (line 142)**:
```bash
./bench_random_mixed_hakmem 10000000 8192  # ~16.5 M ops/s
```

**Current Measurement**:
- SuperSlab OFF: 16.23 M ops/s
- SuperSlab ON:  16.15 M ops/s

**Match**: ✅ Values align with Phase 9-1 baseline (16.5M vs 16.2M, within variance)

---

## 8. Next Steps

### 8.1 Immediate Actions

1. **Implement Phase 9-2 Option A** (EMPTY→Freelist recycling)
   - Modify `core/superslab_slab.c` (remote drain)
   - Modify `core/box/tls_sll_drain_box.c` (TLS SLL drain)
   - Add EMPTY detection: `if (meta->used == 0) { shared_pool_release_slab(...) }`

2. **Run Debug Build** to verify EMPTY recycling
   ```bash
   make clean
   make CFLAGS="-O2 -g -DHAKMEM_BUILD_RELEASE=0" bench_random_mixed_hakmem

   HAKMEM_TINY_USE_SUPERSLAB=1 \
   HAKMEM_SS_ACQUIRE_DEBUG=1 \
   HAKMEM_SHARED_POOL_STAGE_STATS=1 \
     ./bench_random_mixed_hakmem 100000 256 42
   ```

3. **Verify Stage 1 Hits** in debug output
   - Look for `[SP_ACQUIRE_STAGE1_LOCKFREE]` logs
   - Confirm freelist population: `[SP_SLOT_FREELIST_LOCKFREE]`
   - Verify zero `shared_fail→legacy` events

### 8.2 Performance Validation

4. **Re-run WS8192 Benchmark** (after Option A implementation)
   ```bash
   # Baseline (should be same as before)
   HAKMEM_TINY_USE_SUPERSLAB=0 ./bench_random_mixed_hakmem 10000000 8192

   # Optimized (should show +50-70% improvement)
   HAKMEM_TINY_USE_SUPERSLAB=1 ./bench_random_mixed_hakmem 10000000 8192
   ```

5. **Success Criteria** (from Phase 9-2 Section 11.2):
   - ✅ Throughput: 16.5M → 25-30M ops/s (+50-80%)
   - ✅ Zero `shared_fail→legacy` events
   - ✅ Stage 1 hit rate: 70-80% (after warmup)
   - ✅ Kernel overhead: 55% → <15%

### 8.3 Optional Enhancements

6. **Implement Option B** (revert to 2MB SuperSlab)
   - Change `SUPERSLAB_LG_DEFAULT` from 19 → 21
   - Expected additional gain: +10-15% (30-35M ops/s total)

7. **Implement Option D** (expand EMPTY scan limit)
   - Change `HAKMEM_SS_EMPTY_SCAN_LIMIT` default from 16 → 64
   - Expected additional gain: +3-8% (marginal)

---

## 9. Risk Assessment

### 9.1 Implementation Risks (Option A)

| Risk | Likelihood | Impact | Mitigation |
|------|------------|--------|------------|
| **Double-free in EMPTY detection** | Low | Critical | Add `meta->used > 0` assertion before `shared_pool_release_slab()` |
| **Race: EMPTY→ACTIVE→EMPTY** | Medium | Medium | Use atomic `meta->used` reads; Stage 1 CAS prevents double-activation |
| **Deadlock in release_slab** | Low | Medium | Use lock-free push (already implemented) |

**Overall**: Low risk (Box boundaries well-defined, guards in place)

### 9.2 Performance Risks

| Risk | Likelihood | Impact | Mitigation |
|------|------------|--------|------------|
| **Improvement less than expected** | Medium | Medium | Profile with perf, check Stage 1 hit rate, consider Option B |
| **Regression in other workloads** | Low | Medium | Run full benchmark suite (WS256, cache_thrash, larson) |
| **Memory leak from freelist** | Low | High | Monitor RSS growth, verify EMPTY detection logic |

**Overall**: Medium risk (new feature, but small code change)

---

## 10. Lessons Learned

### 10.1 Benchmark Parameter Confusion

**Issue**: Initial request mentioned "デフォルトパラメータで測定してしまい、ワークロードが軽すぎました"
**Reality**: Default parameters ARE WS8192 (line 46 in bench_random_mixed.c)
```c
int ws = (argc>2)? atoi(argv[2]) : 8192;  // default: 8192
```

**Takeaway**: Always check source code to verify default behavior (documentation may be outdated).

### 10.2 SuperSlab Enabled ≠ SuperSlab Functional

**Issue**: `HAKMEM_TINY_USE_SUPERSLAB=1` enables SuperSlab code, but doesn't guarantee it's used.
**Reality**: Legacy fallback is triggered when SuperSlab backend fails (soft cap, OOM, etc.)

**Takeaway**: Check for `shared_fail→legacy` warnings in output to verify SuperSlab is actually being used.

### 10.3 Phase Dependencies

**Issue**: Assumed Phase 9-2 was complete (based on PHASE9_2_*.md files)
**Reality**: Phase 9-2 investigation is complete, but **implementation is not started**

**Takeaway**: Check document status header (e.g., "Status: Root Cause Analysis Complete" vs "Status: Implementation Complete")

---

## 11. Conclusion

**Current State**: WS8192 benchmark correctly measured at 16.2-16.3 M ops/s, consistent across SuperSlab ON/OFF.

**Root Cause**: SuperSlab backend falls back to legacy system malloc due to missing EMPTY→Freelist recycling (Phase 9-2 Option A).

**Expected Improvement**: After implementing Option A, expect 25-30 M ops/s (+50-80%) by eliminating legacy fallbacks and enabling lock-free Stage 1 EMPTY reuse.

**Next Action**: Implement Phase 9-2 Option A (2-3 hour task), then re-benchmark WS8192 to verify +50-70% improvement.

---

**Report Prepared By**: Claude (Sonnet 4.5)
**Benchmark Date**: 2025-11-30
**Total Test Time**: ~6 seconds (6 runs × 0.6s average)
**Status**: Baseline established, awaiting Phase 9-2 implementation