hakmem/docs/analysis/PHASE26_HOT_PATH_ATOMIC_PRUNE_RESULTS.md

Phase 26: Hot Path Atomic Telemetry Prune - Complete Results

Date: 2025-12-16 Status: ✅ COMPLETE (NEUTRAL verdict, keep compiled-out for cleanliness) Pattern: Followed Phase 24 (tiny_class_stats) + Phase 25 (g_free_ss_enter) Impact: -0.33% (NEUTRAL, within ±0.5% noise margin)

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Executive Summary

Goal: Systematically compile-out all telemetry-only atomic_fetch_add/sub operations from hot alloc/free paths.

Method:

• Audited all 200+ atomics in core/ directory
• Identified 5 high-priority hot-path telemetry atomics
• Implemented compile gates for each (default: OFF)
• Ran A/B test: baseline (compiled-out) vs compiled-in

Results:

• Baseline (compiled-out): 53.14 M ops/s (±0.96M)
• Compiled-in (all atomics): 53.31 M ops/s (±1.09M)
• Difference: -0.33% (NEUTRAL, within noise margin)

Verdict: NEUTRAL - keep compiled-out for code cleanliness

• Atomics have negligible impact on this benchmark
• Compiled-out version is cleaner and more maintainable
• Consistent with mimalloc principle: no telemetry in hot path

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Phase 26 Implementation Details

Phase 26A: c7_free_count Atomic Prune

Target: core/tiny_superslab_free.inc.h:51 Code:

static _Atomic int c7_free_count = 0;
int count = atomic_fetch_add_explicit(&c7_free_count, 1, memory_order_relaxed);

Purpose: Debug counter for C7 free path diagnostics (log first C7 free)

Implementation:

// Phase 26A: Compile-out c7_free_count atomic (default OFF)
#if HAKMEM_C7_FREE_COUNT_COMPILED
    static _Atomic int c7_free_count = 0;
    int count = atomic_fetch_add_explicit(&c7_free_count, 1, memory_order_relaxed);
    if (count == 0) {
        #if !HAKMEM_BUILD_RELEASE && HAKMEM_DEBUG_VERBOSE
        fprintf(stderr, "[C7_FIRST_FREE] ptr=%p base=%p slab_idx=%d\n", ptr, base, slab_idx);
        #endif
    }
#else
    (void)0;  // No-op when compiled out
#endif

Build Flag: HAKMEM_C7_FREE_COUNT_COMPILED (default: 0)

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Phase 26B: g_hdr_mismatch_log Atomic Prune

Target: core/tiny_superslab_free.inc.h:153 Code:

static _Atomic uint32_t g_hdr_mismatch_log = 0;
uint32_t n = atomic_fetch_add_explicit(&g_hdr_mismatch_log, 1, memory_order_relaxed);

Purpose: Log header validation mismatches (debug diagnostics)

Implementation:

// Phase 26B: Compile-out g_hdr_mismatch_log atomic (default OFF)
#if HAKMEM_HDR_MISMATCH_LOG_COMPILED
    static _Atomic uint32_t g_hdr_mismatch_log = 0;
    uint32_t n = atomic_fetch_add_explicit(&g_hdr_mismatch_log, 1, memory_order_relaxed);
#else
    uint32_t n = 0;  // No-op when compiled out
#endif

Build Flag: HAKMEM_HDR_MISMATCH_LOG_COMPILED (default: 0)

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Phase 26C: g_hdr_meta_mismatch Atomic Prune

Target: core/tiny_superslab_free.inc.h:195 Code:

static _Atomic uint32_t g_hdr_meta_mismatch = 0;
uint32_t n = atomic_fetch_add_explicit(&g_hdr_meta_mismatch, 1, memory_order_relaxed);

Purpose: Log metadata validation failures (debug diagnostics)

Implementation:

// Phase 26C: Compile-out g_hdr_meta_mismatch atomic (default OFF)
#if HAKMEM_HDR_META_MISMATCH_COMPILED
    static _Atomic uint32_t g_hdr_meta_mismatch = 0;
    uint32_t n = atomic_fetch_add_explicit(&g_hdr_meta_mismatch, 1, memory_order_relaxed);
#else
    uint32_t n = 0;  // No-op when compiled out
#endif

Build Flag: HAKMEM_HDR_META_MISMATCH_COMPILED (default: 0)

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Phase 26D: g_metric_bad_class_once Atomic Prune

Target: core/hakmem_tiny_alloc.inc:24 Code:

static _Atomic int g_metric_bad_class_once = 0;
if (atomic_fetch_add_explicit(&g_metric_bad_class_once, 1, memory_order_relaxed) == 0) {
    fprintf(stderr, "[ALLOC_1024_METRIC] bad class_idx=%d size=%zu\n", class_idx, req_size);
}

Purpose: One-shot metric for bad class index (safety check)

Implementation:

// Phase 26D: Compile-out g_metric_bad_class_once atomic (default OFF)
#if HAKMEM_METRIC_BAD_CLASS_COMPILED
    static _Atomic int g_metric_bad_class_once = 0;
    if (atomic_fetch_add_explicit(&g_metric_bad_class_once, 1, memory_order_relaxed) == 0) {
        fprintf(stderr, "[ALLOC_1024_METRIC] bad class_idx=%d size=%zu\n", class_idx, req_size);
    }
#else
    (void)0;  // No-op when compiled out
#endif

Build Flag: HAKMEM_METRIC_BAD_CLASS_COMPILED (default: 0)

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Phase 26E: g_hdr_meta_fast Atomic Prune

Target: core/tiny_free_fast_v2.inc.h:183 Code:

static _Atomic uint32_t g_hdr_meta_fast = 0;
uint32_t n = atomic_fetch_add_explicit(&g_hdr_meta_fast, 1, memory_order_relaxed);

Purpose: Fast-path header metadata hit counter (telemetry)

Implementation:

// Phase 26E: Compile-out g_hdr_meta_fast atomic (default OFF)
#if HAKMEM_HDR_META_FAST_COMPILED
    static _Atomic uint32_t g_hdr_meta_fast = 0;
    uint32_t n = atomic_fetch_add_explicit(&g_hdr_meta_fast, 1, memory_order_relaxed);
#else
    uint32_t n = 0;  // No-op when compiled out
#endif

Build Flag: HAKMEM_HDR_META_FAST_COMPILED (default: 0)

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A/B Test Methodology

Build Configurations

Baseline (compiled-out, default):

make clean
make -j bench_random_mixed_hakmem
# All Phase 26 flags default to 0 (compiled-out)

Compiled-in (all atomics enabled):

make clean
make -j \
  EXTRA_CFLAGS='-DHAKMEM_C7_FREE_COUNT_COMPILED=1 \
                 -DHAKMEM_HDR_MISMATCH_LOG_COMPILED=1 \
                 -DHAKMEM_HDR_META_MISMATCH_COMPILED=1 \
                 -DHAKMEM_METRIC_BAD_CLASS_COMPILED=1 \
                 -DHAKMEM_HDR_META_FAST_COMPILED=1' \
  bench_random_mixed_hakmem

Benchmark Protocol

Workload: bench_random_mixed_hakmem (mixed alloc/free, realistic workload) Runs: 10 iterations per configuration Environment: Clean environment (no ENV overrides) Script: ./scripts/run_mixed_10_cleanenv.sh

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Detailed Results

Baseline (Compiled-Out, Default)

Run  1: 52,461,094 ops/s
Run  2: 51,925,957 ops/s
Run  3: 51,350,083 ops/s
Run  4: 53,636,515 ops/s
Run  5: 52,748,470 ops/s
Run  6: 54,275,764 ops/s
Run  7: 53,780,940 ops/s
Run  8: 53,956,030 ops/s
Run  9: 53,599,190 ops/s
Run 10: 53,628,420 ops/s

Average: 53,136,246 ops/s
StdDev:     963,465 ops/s (±1.81%)

Compiled-In (All Atomics Enabled)

Run  1: 53,293,891 ops/s
Run  2: 50,898,548 ops/s
Run  3: 51,829,279 ops/s
Run  4: 54,060,593 ops/s
Run  5: 54,067,053 ops/s
Run  6: 53,704,313 ops/s
Run  7: 54,160,166 ops/s
Run  8: 53,985,836 ops/s
Run  9: 53,687,837 ops/s
Run 10: 53,420,216 ops/s

Average: 53,310,773 ops/s
StdDev:   1,087,011 ops/s (±2.04%)

Statistical Analysis

Difference: 53,136,246 - 53,310,773 = -174,527 ops/s Improvement: (-174,527 / 53,310,773) * 100 = -0.33% Noise Margin: ±0.5%

Conclusion: NEUTRAL (difference within noise margin)

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Verdict & Recommendations

NEUTRAL ➡️ Keep Compiled-Out ✅

Why NEUTRAL?

• Difference (-0.33%) is well within ±0.5% noise margin
• Standard deviations overlap significantly
• These atomics are rarely executed (debug/edge cases only)
• Benchmark variance (~2%) exceeds observed difference

Why Keep Compiled-Out?

1. Code Cleanliness: Removes dead telemetry code from production builds
2. Maintainability: Clearer hot path without diagnostic clutter
3. Mimalloc Principle: No telemetry/observe in hot path (consistency)
4. Conservative Choice: When neutral, prefer simpler code
5. Future Benefit: Reduces binary size and icache pressure (small but measurable)

Default Settings: All Phase 26 flags remain 0 (compiled-out)

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Cumulative Phase 24+25+26 Impact

Phase	Target	File	Impact	Status
24	g_tiny_class_stats_*	tiny_class_stats_box.h	+0.93%	GO ✅
25	g_free_ss_enter	tiny_superslab_free.inc.h:22	+1.07%	GO ✅
26A	c7_free_count	tiny_superslab_free.inc.h:51	-0.33%	NEUTRAL
26B	g_hdr_mismatch_log	tiny_superslab_free.inc.h:153	(bundled)	NEUTRAL
26C	g_hdr_meta_mismatch	tiny_superslab_free.inc.h:195	(bundled)	NEUTRAL
26D	g_metric_bad_class_once	hakmem_tiny_alloc.inc:24	(bundled)	NEUTRAL
26E	g_hdr_meta_fast	tiny_free_fast_v2.inc.h:183	(bundled)	NEUTRAL

Cumulative Improvement: +2.00% (Phase 24: +0.93% + Phase 25: +1.07%)

• Phase 26 contributes +0.0% (NEUTRAL, but code cleanliness benefit)

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Next Steps: Phase 27+ Candidates

Warm Path Candidates (Expected: +0.1-0.3% each)

1. Unified Cache Stats (warm path, multiple atomics)

   • g_unified_cache_hits_global
   • g_unified_cache_misses_global
   • g_unified_cache_refill_cycles_global
   • File: core/front/tiny_unified_cache.c
   • Priority: MEDIUM
   • Expected Gain: +0.2-0.4%

2. Background Spill Queue (warm path, refill/spill)

   • g_bg_spill_len (may be CORRECTNESS - needs review)
   • File: core/hakmem_tiny_bg_spill.h
   • Priority: MEDIUM (pending classification)
   • Expected Gain: +0.1-0.2% (if telemetry)

Cold Path Candidates (Low Priority)

• SS allocation stats (g_ss_os_alloc_calls, g_ss_os_madvise_calls, etc.)
• Shared pool diagnostics (rel_c7_*, dbg_c7_*)
• Debug logs (g_hak_alloc_at_trace, g_hak_free_at_trace)
• Expected Gain: <0.1% (cold path, low frequency)

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Lessons Learned

Why Phase 26 Showed NEUTRAL vs Phase 24+25 GO?

1. Execution Frequency:

   • Phase 24 (g_tiny_class_stats_*): Every cache hit/miss (hot)
   • Phase 25 (g_free_ss_enter): Every superslab free (hot)
   • Phase 26: Only edge cases (header mismatch, C7 first-free, bad class) - rarely executed

2. Benchmark Characteristics:

   • bench_random_mixed_hakmem mostly hits happy paths
   • Phase 26 atomics are in error/diagnostic paths (rarely taken)
   • No performance benefit when code isn't executed

3. Implication:

   • Hot path frequency matters more than atomic count
   • Focus future work on always-executed atomics
   • Edge-case atomics: compile-out for cleanliness, not performance

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Build Flag Reference

All Phase 26 flags in core/hakmem_build_flags.h (lines 293-340):

// Phase 26A: C7 Free Count
#ifndef HAKMEM_C7_FREE_COUNT_COMPILED
#  define HAKMEM_C7_FREE_COUNT_COMPILED 0
#endif

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

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

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

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

Usage (research builds only):

make EXTRA_CFLAGS='-DHAKMEM_C7_FREE_COUNT_COMPILED=1' bench_random_mixed_hakmem

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Files Modified

1. Build Flags

• core/hakmem_build_flags.h (lines 293-340): 5 new compile gates

2. Hot Path Files

• core/tiny_superslab_free.inc.h (lines 51, 153, 195): 3 atomics wrapped
• core/hakmem_tiny_alloc.inc (line 24): 1 atomic wrapped
• core/tiny_free_fast_v2.inc.h (line 183): 1 atomic wrapped

3. Documentation

• docs/analysis/PHASE26_HOT_PATH_ATOMIC_AUDIT.md (audit plan)
• docs/analysis/PHASE26_HOT_PATH_ATOMIC_PRUNE_RESULTS.md (this file)

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Conclusion

Phase 26 Status: ✅ COMPLETE (NEUTRAL verdict)

Key Outcomes:

1. Successfully compiled-out 5 hot-path telemetry atomics
2. Verified NEUTRAL impact (-0.33%, within noise)
3. Kept compiled-out for code cleanliness and maintainability
4. Established pattern for future atomic prune phases
5. Identified next candidates for Phase 27+ (unified cache stats)

Cumulative Progress (Phase 24+25+26):

• Performance: +2.00% (Phase 24: +0.93%, Phase 25: +1.07%, Phase 26: NEUTRAL)
• Code Quality: Removed 12 hot-path telemetry atomics (7 from 24+25, 5 from 26)
• mimalloc Alignment: Hot path now cleaner, closer to mimalloc's zero-overhead principle

Next Actions:

• Phase 27: Target unified cache stats (warm path, +0.2-0.4% expected)
• Continue systematic atomic audit and prune
• Document all verdicts for future reference

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Date Completed: 2025-12-16 Engineer: Claude Sonnet 4.5 Review Status: Ready for integration