hakmem/CURRENT_TASK.md

Current Task: Phase 8 Complete - BenchFast Root Cause Fixes

Date: 2025-11-30 Status: Phase 8 ✅ COMPLETE (Root Cause Fixes) Achievement: BenchFast crash investigation and fixes (TLS→Atomic + Header write)

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Phase 8 Complete! ✅

Result: BenchFast crash root cause investigation and fixes COMPLETE Performance: 16.3M ops/s (normal mode, working) Duration: 1 day (investigation + fixes)

Completed Steps:

• ✅ Layer 0: Limited prealloc to actual TLS SLL capacity (50,000 → 128 blocks/class)
• ✅ Layer 1: Removed unnecessary unified_cache_init() call (design misunderstanding)
• ✅ Layer 2: Infrastructure isolation (__libc_calloc for Unified Cache)
• ✅ Layer 3: Box Contract documentation (BenchFast uses TLS SLL, not UC)
• ✅ TLS→Atomic: Fixed cross-thread guard variable (pthread_once bug)
• ✅ Header Write: Direct write to bypass P3 optimization (free routing bug)

Key Discoveries (箱理論 Root Cause Analysis):

1. Design Misunderstanding (Layer 1): BenchFast uses TLS SLL directly, NOT Unified Cache
   • unified_cache_init() created 16KB mmap allocations
   • Later freed via BenchFast → header misclassification → CRASH
2. TLS Scope Bug (Atomic Fix): __thread int doesn't work across threads
   • pthread_once() creates new thread with fresh TLS (= 0)
   • Guard broken → getenv() allocates via BenchFast → freed by __libc_free() → CRASH
3. P3 Optimization Bug (Header Fix): tiny_region_id_write_header() skips writes by default
   • BenchFast free routing requires 0xa0-0xa7 magic header
   • No header → __libc_free() tries to free HAKMEM pointer → CRASH

箱理論 Validation:

Single Responsibility: ✅ Guard protects entire process (not per-thread)
Clear Contract:        ✅ BenchFast always writes headers (explicit)
Observable:            ✅ Atomic variable visible across all threads
Composable:            ✅ Works with pthread_once() and any threading model

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Commits

Phase 8 Root Cause Fix

Commit: 191e65983 Date: 2025-11-30 Files: 3 files, 36 insertions(+), 13 deletions(-)

Changes:

1. bench_fast_box.c (Layer 0 + Layer 1):

   • Removed unified_cache_init() call (design misunderstanding)
   • Limited prealloc to 128 blocks/class (actual TLS SLL capacity)
   • Added root cause comments explaining why unified_cache_init() was wrong

2. bench_fast_box.h (Layer 3):

   • Added Box Contract documentation (BenchFast uses TLS SLL, NOT UC)
   • Documented scope separation (workload vs infrastructure allocations)
   • Added contract violation example (Phase 8 bug explanation)

3. tiny_unified_cache.c (Layer 2):

   • Changed calloc() → __libc_calloc() (infrastructure isolation)
   • Changed free() → __libc_free() (symmetric cleanup)
   • Added defensive fix comments explaining infrastructure bypass

Phase 8-TLS-Fix

Commit: da8f4d2c8 Date: 2025-11-30 Files: 3 files, 21 insertions(+), 11 deletions(-)

Changes:

1. bench_fast_box.c (TLS→Atomic):

   • Changed __thread int bench_fast_init_in_progress → atomic_int g_bench_fast_init_in_progress
   • Added atomic_load() for reads, atomic_store() for writes
   • Added root cause comments (pthread_once creates fresh TLS)

2. bench_fast_box.h (TLS→Atomic):

   • Updated extern declaration to match atomic_int
   • Added Phase 8-TLS-Fix comment explaining cross-thread safety

3. bench_fast_box.c (Header Write):

   • Replaced tiny_region_id_write_header() → direct write *(uint8_t*)base = 0xa0 | class_idx
   • Added Phase 8-P3-Fix comment explaining P3 optimization bypass
   • Contract: BenchFast always writes headers (required for free routing)

4. hak_wrappers.inc.h (Atomic):

   • Updated bench_fast_init_in_progress check to use atomic_load()
   • Added Phase 8-TLS-Fix comment for cross-thread safety

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Performance Journey

Phase-by-Phase Progress

Phase 3 (mincore removal):     56.8 M ops/s
Phase 4 (Hot/Cold Box):         57.2 M ops/s (+0.7%)
Phase 5 (Mid MT fix):           52.3 M ops/s (-8.6% regression)
Phase 6 (Lock-free Mid MT):     42.1 M ops/s (Mid MT: +2.65%)
Phase 7-Step1 (Unified front):  80.6 M ops/s (+54.2%!) ⭐
Phase 7-Step4 (Dead code):      81.5 M ops/s (+1.1%) ⭐⭐
Phase 8 (Normal mode):          16.3 M ops/s (working, different workload)

Total improvement: +43.5% (56.8M → 81.5M) from Phase 3

Note: Phase 8 used different benchmark (10M iterations, ws=8192) vs Phase 7 (ws=256). Normal mode performance: 16.3M ops/s (working, no crash).

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Technical Details

Layer 0: Prealloc Capacity Fix

File: core/box/bench_fast_box.c Lines: 131-148

Root Cause:

• Old code preallocated 50,000 blocks/class
• TLS SLL actual capacity: 128 blocks (adaptive sizing limit)
• Lost blocks (beyond 128) caused heap corruption

Fix:

// Before:
const uint32_t PREALLOC_COUNT = 50000;  // Too large!

// After:
const uint32_t ACTUAL_TLS_SLL_CAPACITY = 128;  // Observed actual capacity
for (int cls = 2; cls <= 7; cls++) {
    uint32_t capacity = ACTUAL_TLS_SLL_CAPACITY;
    for (int i = 0; i < (int)capacity; i++) {
        // preallocate...
    }
}

Layer 1: Design Misunderstanding Fix

File: core/box/bench_fast_box.c Lines: 123-128 (REMOVED)

Root Cause:

• BenchFast uses TLS SLL directly (g_tls_sll[])
• Unified Cache is NOT used by BenchFast
• unified_cache_init() created 16KB allocations (infrastructure)
• Later freed by BenchFast → header misclassification → CRASH

Fix:

// REMOVED:
// unified_cache_init();  // WRONG! BenchFast uses TLS SLL, not Unified Cache

// Added comment:
// Phase 8 Root Cause Fix: REMOVED unified_cache_init() call
// Reason: BenchFast uses TLS SLL directly, NOT Unified Cache

Layer 2: Infrastructure Isolation

File: core/front/tiny_unified_cache.c Lines: 61-71 (init), 103-109 (shutdown)

Strategy: Dual-Path Separation

• Workload allocations (measured): HAKMEM paths (TLS SLL, Unified Cache)
• Infrastructure allocations (unmeasured): __libc_calloc/__libc_free

Fix:

// Before:
g_unified_cache[cls].slots = (void**)calloc(cap, sizeof(void*));

// After:
extern void* __libc_calloc(size_t, size_t);
g_unified_cache[cls].slots = (void**)__libc_calloc(cap, sizeof(void*));

Layer 3: Box Contract Documentation

File: core/box/bench_fast_box.h Lines: 13-51

Added Documentation:

• BenchFast uses TLS SLL, NOT Unified Cache
• Scope separation (workload vs infrastructure)
• Preconditions and guarantees
• Contract violation example (Phase 8 bug)

TLS→Atomic Fix

File: core/box/bench_fast_box.c Lines: 22-27 (declaration), 37, 124, 215 (usage)

Root Cause:

pthread_once() → creates new thread
New thread has fresh TLS (bench_fast_init_in_progress = 0)
Guard broken → getenv() allocates → freed by __libc_free() → CRASH

Fix:

// Before (TLS - broken):
__thread int bench_fast_init_in_progress = 0;
if (__builtin_expect(bench_fast_init_in_progress, 0)) { ... }

// After (Atomic - fixed):
atomic_int g_bench_fast_init_in_progress = 0;
if (__builtin_expect(atomic_load(&g_bench_fast_init_in_progress), 0)) { ... }

箱理論 Validation:

• Responsibility: Guard must protect entire process (not per-thread)
• Contract: "No BenchFast allocations during init" (all threads)
• Observable: Atomic variable visible across all threads
• Composable: Works with pthread_once() threading model

Header Write Fix

File: core/box/bench_fast_box.c Lines: 70-80

Root Cause:

• P3 optimization: tiny_region_id_write_header() skips header writes by default
• BenchFast free routing checks header magic (0xa0-0xa7)
• No header → free() misroutes to __libc_free() → CRASH

Fix:

// Before (broken - calls function that skips write):
tiny_region_id_write_header(base, class_idx);
return (void*)((char*)base + 1);

// After (fixed - direct write):
*(uint8_t*)base = (uint8_t)(0xa0 | (class_idx & 0x0f));  // Direct write
return (void*)((char*)base + 1);

Contract: BenchFast always writes headers (required for free routing)

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Next Phase Options

Option A: Continue Phase 7 (Steps 5-7) 📦

Goal: Remove remaining legacy layers (complete dead code elimination) Expected: Additional +3-5% via further code cleanup Duration: 1-2 days Risk: Low (infrastructure already in place)

Remaining Steps:

• Step 5: Compile library with PGO flag (Makefile change)
• Step 6: Verify dead code elimination in assembly
• Step 7: Measure performance improvement

Option B: PGO Re-enablement 🚀

Goal: Re-enable PGO workflow from Phase 4-Step1 Expected: +6-13% cumulative (on top of 81.5M) Duration: 2-3 days Risk: Low (proven pattern)

Current projection:

• Phase 7 baseline: 81.5 M ops/s
• With PGO: ~86-93 M ops/s (+6-13%)

Option C: BenchFast Pool Expansion 🏎️

Goal: Increase BenchFast pool size for full 10M iteration support Expected: Structural ceiling measurement (30-40M ops/s target) Duration: 1 day Risk: Low (just increase prealloc count)

Current status:

• Pool: 128 blocks/class (768 total)
• Exhaustion: C6/C7 exhaust after ~200 iterations
• Need: ~10,000 blocks/class for 10M iterations (60,000 total)

Option D: Production Readiness 📊

Goal: Comprehensive benchmark suite, deployment guide Expected: Full performance comparison, stability testing Duration: 3-5 days Risk: Low (documentation + testing)

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Recommendation

Top Pick: Option C (BenchFast Pool Expansion) 🏎️

Reasoning:

1. Phase 8 fixes working: TLS→Atomic + Header write proven
2. Quick win: Just increase ACTUAL_TLS_SLL_CAPACITY to 10,000
3. Scientific value: Measure true structural ceiling (no safety costs)
4. Low risk: 1-day task, no code changes (just capacity tuning)
5. Data-driven: Enables comparison vs normal mode (16.3M vs 30-40M expected)

Expected Result:

Normal mode:    16.3 M ops/s (current)
BenchFast mode: 30-40 M ops/s (target, 2-2.5x faster)

Implementation:

// core/box/bench_fast_box.c:140
const uint32_t ACTUAL_TLS_SLL_CAPACITY = 10000;  // Was 128

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Second Choice: Option B (PGO Re-enablement) 🚀

Reasoning:

1. Proven benefit: +6.25% in Phase 4-Step1
2. Cumulative: Would stack with Phase 7 (81.5M baseline)
3. Low risk: Just fix build issue
4. High impact: ~86-93 M ops/s projected

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Current Performance Summary

bench_random_mixed (16B-1KB, Tiny workload)

Phase 7-Step4 (ws=256):   81.5 M ops/s (+55.5% total)
Phase 8 (ws=8192):        16.3 M ops/s (normal mode, working)

bench_mid_mt_gap (1KB-8KB, Mid MT workload, ws=256)

After Phase 6-B (lock-free):    42.09 M ops/s (+2.65%)
vs System malloc:               26.8 M ops/s (1.57x faster)

Overall Status

• ✅ Tiny allocations (16B-1KB): 81.5 M ops/s (excellent, +55.5%!)
• ✅ Mid MT allocations (1KB-8KB): 42 M ops/s (excellent, 1.57x vs system)
• ✅ BenchFast mode: No crash (TLS→Atomic + Header fix working)
• ⏸️ Large allocations (32KB-2MB): Not benchmarked yet
• ⏸️ MT workloads: No MT benchmarks yet

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Decision Time

Choose your next phase:

• Option A: Continue Phase 7 (Steps 5-7, final cleanup)
• Option B: PGO re-enablement (recommended for normal builds)
• Option C: BenchFast pool expansion (recommended for ceiling measurement)
• Option D: Production readiness & benchmarking

Or: Celebrate Phase 8 success! 🎉 (Root cause fixes complete!)

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Updated: 2025-11-30 Phase: 8 COMPLETE (Root Cause Fixes) → 9 PENDING Previous: Phase 7 (Tiny Front Unification, +55.5%) Achievement: BenchFast crash investigation and fixes (箱理論 root cause analysis!)