117 lines
2.9 KiB
Markdown
117 lines
2.9 KiB
Markdown
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# Phase 6-3 Fast Path: Quick Fix Summary
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## Root Cause (TL;DR)
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Fast Path implementation creates a **double-layered allocation path** that ALWAYS fails due to SuperSlab OOM:
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```
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Fast Path → tiny_fast_refill() → hak_tiny_alloc_slow() → OOM (NULL)
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↓
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Fallback → Box Refactor path → ALSO OOM → crash
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```
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**Result:** -20% regression (4.19M → 3.35M ops/s) + 45 GB memory leak
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---
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## 3 Fix Options (Ranked)
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### ⭐⭐⭐⭐⭐ Fix #1: Disable Fast Path (IMMEDIATE)
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**Time:** 1 minute
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**Confidence:** 100%
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**Target:** 4.19M ops/s (restore baseline)
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```bash
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make clean
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make BOX_REFACTOR_DEFAULT=1 TINY_FAST_PATH_DEFAULT=0 larson_hakmem
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./larson_hakmem 10 8 128 1024 1 12345 4
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```
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**Why this works:** Reverts to proven Box Refactor path (Phase 6-2.2)
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---
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### ⭐⭐⭐⭐ Fix #2: Integrate Fast Path with Box Refactor (2-4 hours)
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**Confidence:** 80%
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**Target:** 5.0-6.0M ops/s (20-40% improvement)
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**Change 1:** Make `tiny_fast_refill()` use Box Refactor backend
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```c
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// File: core/tiny_fastcache.c:tiny_fast_refill()
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void* tiny_fast_refill(int class_idx) {
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// OLD: void* ptr = hak_tiny_alloc_slow(size, class_idx); // OOM!
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// NEW: Use proven Box Refactor path
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void* ptr = hak_tiny_alloc(size); // ← This works!
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// Rest of refill logic stays the same...
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}
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```
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**Change 2:** Remove Fast Path from `hak_alloc_at()` (avoid double-layer)
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```c
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// File: core/hakmem.c:hak_alloc_at()
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// Comment out lines 682-697 (Fast Path check)
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// Keep ONLY in malloc() wrapper (lines 1294-1309)
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```
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**Why this works:**
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- Box Refactor path is proven (4.19M ops/s)
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- Fast Path gets actual cache refills
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- Subsequent allocations hit 3-4 instruction fast path
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- No OOM because Box Refactor handles allocation correctly
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---
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### ⭐⭐ Fix #3: Fix SuperSlab OOM (1-2 weeks)
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**Confidence:** 60%
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**Effort:** High (deep architectural change)
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Only needed if Fix #2 still has OOM issues. See full analysis for details.
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---
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## Recommended Sequence
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1. **Now:** Run Fix #1 (restore baseline)
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2. **Today:** Implement Fix #2 (integrate with Box Refactor)
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3. **Test:** A/B compare Fix #1 vs Fix #2
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4. **Decision:**
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- If Fix #2 > 4.5M ops/s → Ship it! ✅
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- If Fix #2 still has OOM → Need Fix #3 (long-term)
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---
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## Expected Outcomes
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| Fix | Time | Score | Status |
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|-----|------|-------|--------|
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| #1 (Disable) | 1 min | 4.19M ops/s | ✅ Safe baseline |
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| #2 (Integrate) | 2-4 hrs | 5.0-6.0M ops/s | 🎯 Target |
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| #3 (Root cause) | 1-2 weeks | Unknown | ⚠️ High risk |
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---
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## Why Statistics Don't Show
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`HAKMEM_TINY_FAST_STATS=1` produces no output because:
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1. **No shutdown hook** - `tiny_fast_print_stats()` never called
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2. **Thread-local counters** - Lost when threads exit
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3. **Early crash** - OOM kills benchmark before stats printed
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**Fix:** Add to `hak_flush_tiny_exit()` in `hakmem.c`:
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```c
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// Line ~206
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extern void tiny_fast_print_stats(void);
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tiny_fast_print_stats();
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```
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---
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**Full analysis:** `PHASE6_3_REGRESSION_ULTRATHINK.md`
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