hakmem/CURRENT_TASK.md

# Current Task: Phase 4 - Tiny Front Optimization

**Date**: 2025-11-29
**Goal**: Tiny allocation throughput 2x improvement (56.8M → 110M+ ops/s)
**Strategy**: Box化 + PGO + Hot/Cold separation

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## Phase 4 Overview: 3-Step Approach

### Step 1: PGO Workflow Box ✅ COMPLETE (+6.25%)
- **Duration**: ~~1-2 days~~ **Completed: 2025-11-29**
- **Risk**: Low
- **Target**: 56.8M → 60-62M ops/s
- **Actual**: **57.0M → 60.6M ops/s (+6.25%)** ✓

**Deliverables**:
1. ✅ `scripts/box/pgo_tiny_profile_box.sh` - Profile collection automation
2. ✅ `scripts/box/pgo_tiny_profile_config.sh` - Workload configuration
3. ✅ Makefile targets: `pgo-tiny-profile`, `pgo-tiny-collect`, `pgo-tiny-build`, `pgo-tiny-full`
4. ✅ Makefile help target updated with PGO instructions
5. ✅ Benchmark comparison (before/after PGO)
6. ✅ Completion report: `PHASE4_STEP1_COMPLETE.md`

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### Step 2: Hot/Cold Path Box (Expected: +10-15%)
- **Duration**: 3-5 days
- **Risk**: Medium
- **Target**: 60-62M → 68-75M ops/s (cumulative +15-25%)

**Deliverables**:
1. `core/box/tiny_front_hot_box.h` - Ultra-fast path (5-7 branches max)
2. `core/box/tiny_front_cold_box.h` - Slow path (noinline, cold)
3. Refactor `tiny_alloc_fast()` to use Hot/Cold boxes
4. PGO re-optimization with new structure

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### Step 3: Front Config Box (Expected: +5-8%)
- **Duration**: 2-3 days
- **Risk**: Low
- **Target**: 68-75M → 73-83M ops/s (cumulative +20-33%)

**Deliverables**:
1. `core/box/tiny_front_config_box.h` - Compile-time config management
2. Replace runtime checks with `TINY_FRONT_*_ENABLED` macros
3. Build flag: `HAKMEM_TINY_FRONT_PGO=1`
4. Final PGO optimization + full benchmark suite

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## Success Criteria

**bench_random_mixed (ws=256)**:
- Phase 3 baseline: 56.8M ops/s
- Phase 4.1 (PGO): 60-62M ops/s
- Phase 4.2 (Hot/Cold): 68-75M ops/s
- Phase 4.3 (Config): **73-83M ops/s** ✓ (vs mimalloc 107M = 68-77%)

**bench_tiny_hot (64B)**:
- Phase 3 baseline: 81.0M ops/s
- Phase 4.3 target: **100-115M ops/s** ✓ (vs system 156M = 64-74%)

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## Current Status: Step 1 Complete ✅ → Ready for Step 2

**Completed**:
1. ✅ PGO Profile Collection Box implemented (+6.25% improvement)
2. ✅ Makefile workflow automation (`make pgo-tiny-full`)
3. ✅ Help target updated for discoverability
4. ✅ Completion report written

**Next Actions (Step 2)**:
1. Implement Tiny Front Hot Path Box (5-7 branches max)
2. Implement Tiny Front Cold Path Box (noinline, cold)
3. Refactor `tiny_alloc_fast()` to use Hot/Cold separation
4. Re-run PGO optimization with new structure
5. Benchmark: Target 68-75M ops/s (+10-15% over Step 1)

**Design Reference**: `docs/design/PHASE4_TINY_FRONT_BOX_DESIGN.md` (already complete)

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## Notes from ChatGPT Analysis

**Real bottleneck**:
- NOT front_gate_v2 alone
- BUT `tiny_alloc_fast()` overall complexity (15-20 branches)

**Branch explosion sources**:
1. ultra_slim_mode_enabled() gate
2. hak_tiny_size_to_class range check
3. tiny_sizeclass_hist_hit (profile)
4. HeapV2 enabled/disabled
5. FastCache enabled/disabled
6. SFC enabled/disabled + hit/miss
7. TLS SLL enabled/disabled + per-class branches
8. Multiple env gates in refill path

**Pool/Tiny boundary**: Negligible overhead (0.1-0.2% in bench)

**memset/page fault**: Already optimized (TRUST_MMAP_ZERO=1)

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Updated: 2025-11-29
Phase: 4 (Tiny Front Optimization)
Previous: Phase 3 (mincore removal, +10.7%)