hakmem/docs/archive/OPTIMIZATION_NEXT_STEPS.md

# HAKMEM Tiny Pool - Next Optimization Steps

## Quick Status

**Current Performance**: 120-164 M ops/sec (15-57% faster than glibc)
**Target Performance**: 180-250 M ops/sec (70-140% faster than glibc)
**Gap**: 40-86 M ops/sec achievable with next optimization

---

## Bottleneck Identified: hak_tiny_alloc (22.75% CPU)

### Root Causes (from perf annotate)

1. **Heavy stack usage** (10.5% CPU):
   - 88 bytes (0x58) of stack allocated
   - Multiple stack reads/writes per call
   - Register spilling due to pressure

2. **Repeated global reads** (7.2% CPU):
   - `g_tiny_initialized` read every call (3.52%)
   - `g_wrap_tiny_enabled` read every call (0.28%)
   - Should cache in TLS or check once

3. **Complex control flow** (5.0% CPU):
   - Multiple branches for size validation
   - Magazine refill logic in main path
   - Should separate fast/slow paths

### Optimization Strategy (Priority Order)

#### 1. Inline Fast Path (HIGH PRIORITY)
**Impact**: -5 to -7% CPU
**Effort**: 2-3 hours
**Complexity**: Medium

Create `hak_tiny_alloc_fast()` inline helper:
```c
static inline void* hak_tiny_alloc_fast(size_t size) {
    // Quick validation
    if (UNLIKELY(size > MAX_TINY_SIZE)) 
        return hak_tiny_alloc(size);
    
    // Inline bin calculation
    unsigned bin = size_to_bin_fast(size);
    
    // Fast path: TLS magazine hit
    mag_t* mag = &g_tls_mags[bin];
    if (LIKELY(mag->count > 0))
        return mag->objects[--mag->count];
    
    // Slow path: refill magazine
    return hak_tiny_alloc_slow(size, bin);
}
```

**Key optimizations**:
- Move size validation inline (avoid function call for common case)
- Direct TLS magazine access (no indirection)
- Separate slow path (refill) to different function (better branch prediction)

#### 2. Reduce Stack Usage (MEDIUM PRIORITY)
**Impact**: -3 to -4% CPU
**Effort**: 1-2 hours
**Complexity**: Low

Current: 88 bytes stack
Target: <32 bytes stack

**Actions**:
- Audit local variables in `hak_tiny_alloc()`
- Use fewer temporaries
- Pass calculated values as function params (use registers)
- Move rarely-used locals to slow path

#### 3. Cache Globals in TLS (LOW PRIORITY)
**Impact**: -2 to -3% CPU
**Effort**: 1 hour
**Complexity**: Low

Instead of reading `g_tiny_initialized` on every call:
```c
// In TLS init
tls->tiny_available = g_tiny_initialized && g_wrap_tiny_enabled;

// In allocation fast path
if (UNLIKELY(!tls->tiny_available))
    return fallback_alloc(size);
```

**Benefit**: Removes 2 global reads per allocation (3.8% CPU saved)

#### 4. Optimize Size-to-Bin (OPTIONAL)
**Impact**: -1 to -2% CPU
**Effort**: 1 hour
**Complexity**: Medium

Current: Uses `lzcnt` (3.06% CPU in that instruction alone)

**Option A**: Lookup table for sizes ≤128 bytes
```c
static const uint8_t size_to_bin_table[128] = {
    0, 0, 0, 0, 0, 0, 0, 0,  // 0-7   → bin 0
    1, 1, 1, 1, 1, 1, 1, 1,  // 8-15  → bin 1
    2, 2, 2, 2, 2, 2, 2, 2,  // 16-23 → bin 2
    // ...
};

static inline unsigned size_to_bin_fast(size_t sz) {
    if (sz <= 128)
        return size_to_bin_table[sz];
    return size_to_bin_lzcnt(sz);  // Fallback
}
```

**Option B**: Use multiply-shift instead of lzcnt
- May be faster on older CPUs
- Profile to confirm

---

## Implementation Plan

### Phase 1: Quick Wins (2-3 hours) ← START HERE
- [ ] Inline fast path (`hak_tiny_alloc_fast()`)
- [ ] Reduce stack usage (88 → 32 bytes)
- Expected: 120-164 M → 160-220 M ops/sec

### Phase 2: Polish (1-2 hours)
- [ ] Cache globals in TLS
- [ ] Optimize size-to-bin with lookup table
- Expected: Additional 10-20% gain

### Phase 3: Validate (30 min)
- [ ] Run comprehensive benchmark
- [ ] Collect new perf data
- [ ] Verify hak_tiny_alloc reduced from 22.75% → ~10%

**Total time**: 3-6 hours
**Total impact**: +50-70% throughput improvement

---

## Success Criteria

After optimization, verify:
- [ ] hak_tiny_alloc CPU: 22.75% → <12% (primary goal)
- [ ] Total throughput: 120-164 M → 180-250 M ops/sec
- [ ] Faster than glibc: +70% to +140% (vs current +15-57%)
- [ ] No correctness regressions (all tests pass)
- [ ] No new bottleneck >10% (except benchmark overhead like rand())

---

## Files to Modify

**Primary**:
- `/home/tomoaki/git/hakmem/hakmem_pool.c` - Main allocation logic
  - `hak_tiny_alloc()` - Current slow implementation
  - Add `hak_tiny_alloc_fast()` - New inline fast path
  - Add `hak_tiny_alloc_slow()` - Refill logic

**Supporting**:
- `/home/tomoaki/git/hakmem/hakmem_pool.h` - Add inline function
- Check if size-to-bin logic is in separate function

---

## Validation Commands

After implementing optimizations:

```bash
# Rebuild
make clean && make bench_comprehensive_hakmem

# Benchmark
HAKMEM_WRAP_TINY=1 ./bench_comprehensive_hakmem

# Perf analysis
HAKMEM_WRAP_TINY=1 perf record -g --call-graph dwarf \
  -o perf_post_tiny_opt.data ./bench_comprehensive_hakmem

perf report --stdio -i perf_post_tiny_opt.data --sort=symbol \
  | grep -E "^\s+[0-9]+\.[0-9]+%"
```

Check that:
1. hak_tiny_alloc < 12%
2. No new bottleneck appeared >15%
3. Throughput improved by 40-80 M ops/sec

---

## Risk Assessment

**Low Risk**:
- Inline fast path: Only affects common path, easy to test
- Stack reduction: Mechanical refactoring
- TLS caching: Simple optimization

**Medium Risk**:
- Size-to-bin lookup table: Need to ensure correctness of mapping
- Separation of fast/slow paths: Need to ensure all cases covered

**Mitigation**:
- Test all size classes (8, 16, 32, 64, 128, 256, 512, 1024 bytes)
- Run comprehensive benchmark suite
- Verify with perf that CPU moved to expected places

---

## Reference Data

### Current Hottest Instructions in hak_tiny_alloc

```asm
3.71%:  push %r14                       ← Register spill
3.52%:  mov g_tiny_initialized,%r14d    ← Global read
3.53%:  mov 0x1c(%rsp),%ebp            ← Stack read
3.33%:  cmpq $0x80,0x10(%rsp)          ← Size check
3.06%:  mov %rbp,0x38(%rsp)            ← Stack write
```

After optimization, expect:
- Push/pop overhead: reduced (fewer registers needed in fast path)
- Global reads: eliminated (cached in TLS)
- Stack access: reduced (smaller frame)
- Size checks: inlined (better prediction)

### Expected New Bottlenecks After Fix

If hak_tiny_alloc drops to ~10%, next hottest will be:
1. mid_desc_lookup: 12.55% (may become #1)
2. hak_tiny_owner_slab: 9.09% (close to threshold)

But both <15%, so diminishing returns starting to apply.

---

## Questions Before Starting?

1. Do we want to target 10% or 12% for hak_tiny_alloc?
   - 10%: More aggressive, may need all optimizations
   - 12%: Conservative, Phase 1 may be enough

2. Should we also tackle mid_desc_lookup (12.55%)?
   - Could do both in one session
   - Hash table optimization is independent

3. When to stop optimizing?
   - When top bottleneck <10%? 
   - When beating glibc by 2x (target achieved)?
   - When effort exceeds returns?

**Recommendation**: Do Phase 1 (fast path inline), measure, then decide on Phase 2.

---

## Ready to Start?

Next command to run:
```bash
# Read current implementation
cat /home/tomoaki/git/hakmem/hakmem_pool.c | grep -A 200 "hak_tiny_alloc"
```

Then start implementing inline fast path!