84 lines
2.9 KiB
Markdown
84 lines
2.9 KiB
Markdown
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# Phase 15 v1: UnifiedCache FIFO→LIFO (Stack) A/B Test Results
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**Date:** 2025-12-15
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**Benchmark:** Mixed (16–1024B) + C7-only (1025–2048B) 10-run cleanenv
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**Target:** Transform existing UnifiedCache from FIFO ring to LIFO stack
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**Expected ROI:** +5-10% (design estimate, cache locality improvement)
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**GO Threshold:** +1.0% mean improvement
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---
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## 1. Implementation Summary
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Phase 15 v1 transforms the existing array-based UnifiedCache from FIFO (ring buffer) to LIFO (stack) layout.
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**Key Changes:**
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- **Patch 1**: L0 ENV gate box (`tiny_unified_lifo_env_box.{h,c}`)
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- **Patch 2**: L1 LIFO operations (`tiny_unified_lifo_box.h`)
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- **Patch 3**: Hot path integration (`tiny_front_hot_box.h` - alloc/free both)
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- **Patch 4**: Makefile updates (added `.o` files)
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- **Patch 5**: bench_profile.h refresh sync
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**Design:**
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- Reuses existing `TinyUnifiedCache.slots[]` array (no intrusive pointers)
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- `tail` treated as stack top (depth), `head` unused (always 0)
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- Mode check at function entry (once per call)
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- No wrap-around (`mask` unused in LIFO mode)
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**ENV Control:**
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```bash
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export HAKMEM_TINY_UNIFIED_LIFO=0 # Baseline (FIFO)
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export HAKMEM_TINY_UNIFIED_LIFO=1 # Optimized (LIFO)
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```
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**Bonus Fix:**
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- Discovered and fixed pre-existing LTO linkage bug for `tiny_c7_preserve_header_enabled()` (Phase 13/14 latent issue)
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- Converted static inline to extern declaration + non-inline implementation
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---
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## 2. A/B Test Results
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### Mixed (16–1024B):
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- **Baseline (LIFO=0):** 52,965,966 ops/s
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- **Optimized (LIFO=1):** 52,593,948 ops/s
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- **Delta:** **-0.70%** (regression)
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### C7-only (1025–2048B):
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- **Baseline (LIFO=0):** 78,010,783 ops/s
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- **Optimized (LIFO=1):** 78,335,509 ops/s
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- **Delta:** **+0.42%** (slight improvement)
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---
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## 3. Verdict: NEUTRAL
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**Result:** Mixed -0.70%, C7-only +0.42% (both below GO threshold)
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**Comparison to Phase 14:**
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- Phase 14 v1 (tcache free-side only): Mixed +0.20% (NEUTRAL)
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- Phase 14 v2 (tcache alloc+free): Mixed +0.08%, C7-only -0.39% (NEUTRAL)
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- Phase 15 v1 (FIFO→LIFO): Mixed -0.70%, C7-only +0.42% (NEUTRAL)
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**Root Cause:**
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1. **Mode check overhead**: Entry-point `tiny_unified_lifo_enabled()` call adds branch
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2. **Minimal locality delta**: LIFO vs FIFO temporal locality difference is small in practice
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3. **Existing optimization**: FIFO ring implementation already well-optimized
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4. **Cache warming**: TLS cache pre-warming reduces locality sensitivity
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---
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## 4. Recommendation: Freeze as Research Box
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**Decision:** Freeze Phase 15 v1 as research box (HAKMEM_TINY_UNIFIED_LIFO=0 default, OFF)
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**Rationale:**
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- Neither LIFO nor FIFO shows significant advantage
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- Mode switching overhead outweighs potential locality gains
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- Existing FIFO ring is simple and already fast
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**Next:** Explore alternative approaches:
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- Hybrid strategies (per-class mode selection)
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- Batch operations (reduce per-call overhead)
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- Hardware prefetch hints (explicit locality control)
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