hakmem/docs/archive/PHASE_6.6_SUMMARY.md

Phase 6.6 Complete Summary

Date: 2025-10-21 Status: ✅ COMPLETE

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🎯 Goal & Achievement

Goal: Fix ELO control flow bug that prevented batch madvise activation Result: ✅ Successfully fixed and verified - Batch madvise now working correctly

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🐛 Problem

After Phase 6.5 (Learning Lifecycle) integration:

• 2MB allocations were using MALLOC instead of MMAP
• BigCache eviction called free() instead of hak_batch_add()
• Batch madvise statistics showed 0 blocks batched (completely inactive)

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🔍 Root Cause (Diagnosed by Gemini Pro)

Control flow ordering bug in hakmem.c:hak_alloc_at():

1. OLD policy decision (infer_policy()) executed FIRST → returned POLICY_DEFAULT
2. Allocation happened using old policy → alloc_malloc() called
3. ELO strategy selection executed TOO LATE → results completely ignored
4. ELO results only used for BigCache eligibility, not allocation method

Key insight: "The right answer computed at the wrong time is the wrong answer"

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✅ Fix Applied

Modified: hakmem.c (lines 645-720)

Before (WRONG):

void* hak_alloc_at(size_t size, ...) {
    // 1. Old policy (WRONG!)
    policy = POLICY_DEFAULT;

    // 2. Allocate (TOO EARLY!)
    ptr = allocate_with_policy(size, policy);  // Uses malloc

    // 3. ELO selection (TOO LATE!)
    strategy_id = hak_elo_select_strategy();   // Result not used!
    threshold = hak_elo_get_threshold(strategy_id);
}

After (CORRECT):

void* hak_alloc_at(size_t size, ...) {
    // 1. ELO selection FIRST!
    strategy_id = hak_elo_select_strategy();
    threshold = hak_elo_get_threshold(strategy_id);

    // 2. BigCache check
    if (hak_bigcache_try_get(...)) return cached_ptr;

    // 3. Use ELO threshold to decide malloc vs mmap
    ptr = (size >= threshold) ? alloc_mmap(size) : alloc_malloc(size);
}

Result: 2MB allocations now correctly use mmap, enabling batch madvise.

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📊 Benchmark Results

Configuration: bench_runner.sh --warmup 2 --runs 10 (200 total runs)

VM Scenario (2MB allocations)

Allocator	Median (ns)	vs Phase 6.4	vs mimalloc
mimalloc	19,964	+12.6%	baseline
jemalloc	26,241	-3.0%	+31.4%
hakmem-evolving	37,602	+2.6%	+88.3%
hakmem-baseline	40,282	+9.1%	+101.7%
system	59,995	-4.4%	+200.4%

Analysis

1. ✅ No regression: +2.6% difference vs Phase 6.4 is within measurement variance
2. ✅ ELO working: hakmem-evolving beats hakmem-baseline
3. ✅ Batch madvise active: Verified with debug logging
4. ⚠️ Overhead gap: Still 2× slower than mimalloc → Phase 6.7 investigation

Note: README.md claimed "16,125 ns" for Phase 6.4, but FINAL_RESULTS.md shows 36,647 ns (the correct baseline for comparison).

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🧪 Verification

Batch Madvise Activation Confirmed

[DEBUG] BigCache eviction: method=1 (MMAP), size=2097152  ✅
[DEBUG] Calling hak_batch_add(raw=0x..., size=2097152)    ✅

Batch Statistics:
  Total blocks added:       1                              ✅
  Flush operations:         1                              ✅
  Total bytes flushed:      2097152                        ✅

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🎓 Lessons Learned

Design Mistakes

1. Control flow ordering: Strategy selection must happen BEFORE usage
2. Dead code accumulation: Old infer_policy() logic left behind
3. Silent failures: ELO results computed but not used

Detection Challenges

1. High-level symptoms: "Batch not activating" didn't point to control flow
2. Required detailed tracing: Had to add debug logging to discover MALLOC usage
3. Multi-layer architecture: Problem spanned ELO, allocation, BigCache, batch

AI Collaboration Success

• Gemini Pro: Root cause diagnosis from logs + code analysis
• Claude: Applied fix, tested, documented
• Synergy: Gemini saw the forest (control flow), Claude fixed the trees (code)

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📝 Bonus Findings

BigCache Size Check Bug (Already Fixed)

Gemini Task 5cfad9 diagnosed a heap-buffer-overflow bug:

• Problem: BigCache returning undersized blocks without actual_bytes >= requested_bytes check
• Impact: cold-churn benchmark (varying sizes) triggers buffer overflow
• Status: ✅ Already fixed in previous session
• Code: hakmem_bigcache.c:151 has size check with "Segfault fix!" comment

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🚀 Next Steps (Phase 6.7)

1. Overhead Analysis

Goal: Identify why hakmem is 2× slower than mimalloc

Candidates (from OVERHEAD_ANALYSIS_PLAN.md):

• P0: BigCache lookup (~50-100 ns)
• P0: ELO strategy selection (~100-200 ns)
• P1: mmap/munmap syscalls (~1,000-5,000 ns) ← Main suspect
• P1: Page faults (~100-500 ns per page)

Strategy:

1. Feature isolation testing (environment variables)
2. perf profiling (hotspot identification)
3. strace syscall counting

2. Optimization Ideas

1. FROZEN mode by default (after learning) → -5% overhead
2. BigCache direct indexing (instead of linear search) → -5% overhead
3. Pre-allocated arena (Phase 7+) → -50% overhead target

Realistic goal: Reduce gap from +88% to +40% (Phase 7), then +20% (Phase 8)

Limit: Cannot beat mimalloc without slab allocator (industry standard, 10+ years optimization)

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📁 Documentation Created

1. PHASE_6.6_ELO_CONTROL_FLOW_FIX.md (updated with benchmark results)
2. OVERHEAD_ANALYSIS_PLAN.md (Phase 6.7 preparation)
3. PHASE_6.6_SUMMARY.md (this file)
4. GEMINI_BIGCACHE_ANALYSIS.md (confirmed existing fix)

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🏆 Final Status

Phase 6.6: ✅ COMPLETE

Achievements:

• ✅ ELO control flow bug fixed
• ✅ Batch madvise activation verified
• ✅ Performance parity with Phase 6.4 maintained (+2.6% variance)
• ✅ Comprehensive documentation created
• ✅ Phase 6.7 roadmap prepared

Code quality:

• Modified files: 1 (hakmem.c)
• Lines changed: ~75 lines (reordering + cleanup)
• Test coverage: VM scenario verified (200 runs)

Time investment: ~6 hours (diagnosis + fix + benchmarking + documentation)

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Ready for Phase 6.7: Overhead Analysis & Optimization 🚀