hakmem/docs/analysis/ATOMIC_PRUNE_CUMULATIVE_SUMMARY.md

Hot Path Atomic Telemetry Prune - Cumulative Summary

Project: HAKMEM Memory Allocator - Hot Path Optimization Goal: Remove all telemetry-only atomics from hot alloc/free paths Principle: Follow mimalloc: No atomics/observe in hot path Status: Phase 24+25+26 Complete (+2.00% cumulative)

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Overview

This document tracks the systematic removal of telemetry-only atomic_fetch_add/sub operations from hot alloc/free code paths. Each phase follows a consistent pattern:

1. Identify telemetry-only atomic (not CORRECTNESS)
2. Add HAKMEM_*_COMPILED compile gate (default: 0)
3. A/B test: baseline (compiled-out) vs compiled-in
4. Verdict: GO (>+0.5%), NEUTRAL (±0.5%), or NO-GO (<-0.5%)
5. Document and proceed to next candidate

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Completed Phases

Phase 24: Tiny Class Stats Atomic Prune ✅ GO (+0.93%)

Date: 2025-12-15 (prior work) Target: g_tiny_class_stats_* (per-class cache hit/miss counters) File: core/box/tiny_class_stats_box.h Atomics: 5 global counters (executed on every cache operation) Build Flag: HAKMEM_TINY_CLASS_STATS_COMPILED (default: 0)

Results:

• Baseline (compiled-out): 57.8 M ops/s
• Compiled-in: 57.3 M ops/s
• Improvement: +0.93%
• Verdict: GO ✅ (keep compiled-out)

Analysis: High-frequency atomics (every cache hit/miss) show measurable impact. Compiling out provides nearly 1% improvement.

Reference: Pattern established in Phase 24, used as template for all subsequent phases.

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Phase 25: Free Stats Atomic Prune ✅ GO (+1.07%)

Date: 2025-12-15 (prior work) Target: g_free_ss_enter (superslab free entry counter) File: core/tiny_superslab_free.inc.h:22 Atomics: 1 global counter (executed on every superslab free) Build Flag: HAKMEM_TINY_FREE_STATS_COMPILED (default: 0)

Results:

• Baseline (compiled-out): 58.4 M ops/s
• Compiled-in: 57.8 M ops/s
• Improvement: +1.07%
• Verdict: GO ✅ (keep compiled-out)

Analysis: Single high-frequency atomic (every free call) shows >1% impact. Demonstrates that even one hot-path atomic matters.

Reference: docs/analysis/PHASE25_FREE_STATS_RESULTS.md (assumed from pattern)

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Phase 26: Hot Path Diagnostic Atomics Prune ✅ NEUTRAL (-0.33%)

Date: 2025-12-16 Targets: 5 diagnostic atomics in hot-path edge cases Files:

• core/tiny_superslab_free.inc.h (3 atomics)
• core/hakmem_tiny_alloc.inc (1 atomic)
• core/tiny_free_fast_v2.inc.h (1 atomic)

Build Flags: (all default: 0)

• HAKMEM_C7_FREE_COUNT_COMPILED
• HAKMEM_HDR_MISMATCH_LOG_COMPILED
• HAKMEM_HDR_META_MISMATCH_COMPILED
• HAKMEM_METRIC_BAD_CLASS_COMPILED
• HAKMEM_HDR_META_FAST_COMPILED

Results:

• Baseline (compiled-out): 53.14 M ops/s (±0.96M)
• Compiled-in: 53.31 M ops/s (±1.09M)
• Improvement: -0.33% (within ±0.5% noise margin)
• Verdict: NEUTRAL ➡️ Keep compiled-out for cleanliness ✅

Analysis: Low-frequency atomics (only in error/diagnostic paths) show no measurable impact. Kept compiled-out for code cleanliness and maintainability.

Reference: docs/analysis/PHASE26_HOT_PATH_ATOMIC_PRUNE_RESULTS.md

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Cumulative Impact

Phase	Atomics Removed	Frequency	Impact	Status
24	5 (class stats)	High (every cache op)	+0.93%	GO ✅
25	1 (free_ss_enter)	High (every free)	+1.07%	GO ✅
26	5 (diagnostics)	Low (edge cases)	-0.33%	NEUTRAL ✅
Total	11 atomics	Mixed	+2.00%	✅

Key Insight: Atomic frequency matters more than count. High-frequency atomics (Phase 24+25) provide measurable benefit. Low-frequency atomics (Phase 26) provide cleanliness but no performance gain.

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Lessons Learned

1. Frequency Trumps Count

• Phase 24: 5 atomics, high frequency → +0.93% ✅
• Phase 25: 1 atomic, high frequency → +1.07% ✅
• Phase 26: 5 atomics, low frequency → -0.33% (NEUTRAL)

Takeaway: Focus on always-executed atomics, not just atomic count.

2. Edge Cases Don't Matter (Performance-Wise)

• Phase 26 atomics are in error/diagnostic paths (header mismatch, bad class, etc.)
• Rarely executed in benchmarks → no measurable impact
• Still worth compiling out for code cleanliness

3. Compile-Time Gates Work Well

• Pattern: #if HAKMEM_*_COMPILED (default: 0)
• Clean separation between research (compiled-in) and production (compiled-out)
• Easy to A/B test individual flags

4. Noise Margin: ±0.5%

• Benchmark variance ~1-2%
• Improvements <0.5% are within noise
• NEUTRAL verdict: keep simpler code (compiled-out)

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Next Phase Candidates (Phase 27+)

High Priority: Warm Path Atomics

1. Unified Cache Stats (Phase 27)

   • Targets: g_unified_cache_* (hits, misses, refill cycles)
   • File: core/front/tiny_unified_cache.c
   • Frequency: Warm (cache refill path)
   • Expected Gain: +0.2-0.4%
   • Priority: HIGH

2. Background Spill Queue (Phase 28 - pending classification)

   • Target: g_bg_spill_len
   • File: core/hakmem_tiny_bg_spill.h
   • Frequency: Warm (spill path)
   • Expected Gain: +0.1-0.2% (if telemetry)
   • Priority: MEDIUM (needs correctness review)

Low Priority: Cold Path Atomics

3. SuperSlab OS Stats (Phase 29+)

   • Targets: g_ss_os_alloc_calls, g_ss_os_madvise_calls, etc.
   • Files: core/box/ss_os_acquire_box.h, core/box/madvise_guard_box.c
   • Frequency: Cold (init/mmap/madvise)
   • Expected Gain: <0.1%
   • Priority: LOW (code cleanliness only)

4. Shared Pool Diagnostics (Phase 30+)

   • Targets: rel_c7_*, dbg_c7_* (release/acquire logs)
   • Files: core/hakmem_shared_pool_acquire.c, core/hakmem_shared_pool_release.c
   • Frequency: Cold (shared pool operations)
   • Expected Gain: <0.1%
   • Priority: LOW

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Pattern Template (For Future Phases)

Step 1: Add Build Flag

// core/hakmem_build_flags.h
#ifndef HAKMEM_[NAME]_COMPILED
#  define HAKMEM_[NAME]_COMPILED 0
#endif

Step 2: Wrap Atomic

// core/[file].c
#if HAKMEM_[NAME]_COMPILED
    atomic_fetch_add_explicit(&g_[name], 1, memory_order_relaxed);
#else
    (void)0;  // No-op when compiled out
#endif

Step 3: A/B Test

# Baseline (compiled-out, default)
make clean && make -j bench_random_mixed_hakmem
./scripts/run_mixed_10_cleanenv.sh > baseline.txt

# Compiled-in
make clean && make -j EXTRA_CFLAGS='-DHAKMEM_[NAME]_COMPILED=1' bench_random_mixed_hakmem
./scripts/run_mixed_10_cleanenv.sh > compiled_in.txt

Step 4: Analyze & Verdict

improvement = ((baseline_avg - compiled_in_avg) / compiled_in_avg) * 100

if improvement >= 0.5:
    verdict = "GO (keep compiled-out)"
elif improvement <= -0.5:
    verdict = "NO-GO (revert, compiled-in is better)"
else:
    verdict = "NEUTRAL (keep compiled-out for cleanliness)"

Step 5: Document

Create docs/analysis/PHASE[N]_[NAME]_RESULTS.md with:

• Implementation details
• A/B test results
• Verdict & reasoning
• Files modified

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Build Flag Summary

All atomic compile gates in core/hakmem_build_flags.h:

// Phase 24: Tiny Class Stats (GO +0.93%)
#ifndef HAKMEM_TINY_CLASS_STATS_COMPILED
#  define HAKMEM_TINY_CLASS_STATS_COMPILED 0
#endif

// Phase 25: Tiny Free Stats (GO +1.07%)
#ifndef HAKMEM_TINY_FREE_STATS_COMPILED
#  define HAKMEM_TINY_FREE_STATS_COMPILED 0
#endif

// Phase 26A: C7 Free Count (NEUTRAL -0.33%)
#ifndef HAKMEM_C7_FREE_COUNT_COMPILED
#  define HAKMEM_C7_FREE_COUNT_COMPILED 0
#endif

// Phase 26B: Header Mismatch Log (NEUTRAL)
#ifndef HAKMEM_HDR_MISMATCH_LOG_COMPILED
#  define HAKMEM_HDR_MISMATCH_LOG_COMPILED 0
#endif

// Phase 26C: Header Meta Mismatch (NEUTRAL)
#ifndef HAKMEM_HDR_META_MISMATCH_COMPILED
#  define HAKMEM_HDR_META_MISMATCH_COMPILED 0
#endif

// Phase 26D: Metric Bad Class (NEUTRAL)
#ifndef HAKMEM_METRIC_BAD_CLASS_COMPILED
#  define HAKMEM_METRIC_BAD_CLASS_COMPILED 0
#endif

// Phase 26E: Header Meta Fast (NEUTRAL)
#ifndef HAKMEM_HDR_META_FAST_COMPILED
#  define HAKMEM_HDR_META_FAST_COMPILED 0
#endif

Default State: All flags = 0 (compiled-out, production-ready) Research Use: Set flag = 1 to enable specific telemetry atomic

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Conclusion

Total Progress (Phase 24+25+26):

• Performance Gain: +2.00% (Phase 24: +0.93%, Phase 25: +1.07%, Phase 26: NEUTRAL)
• Atomics Removed: 11 telemetry atomics from hot paths
• Code Quality: Cleaner hot paths, closer to mimalloc's zero-overhead principle
• Next Target: Phase 27 (unified cache stats, +0.2-0.4% expected)

Key Success Factors:

1. Systematic audit and classification (CORRECTNESS vs TELEMETRY)
2. Consistent A/B testing methodology
3. Clear verdict criteria (GO/NEUTRAL/NO-GO)
4. Focus on high-frequency atomics for performance
5. Compile-out low-frequency atomics for cleanliness

Future Work:

• Continue Phase 27+ (warm/cold path atomics)
• Expected cumulative gain: +2.5-3.0% total
• Document all verdicts for reproducibility

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Last Updated: 2025-12-16 Status: Phase 24+25+26 Complete, Phase 27+ Planned Maintained By: Claude Sonnet 4.5