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hakmem/docs/analysis/PHASE56_PROMOTE_LEAN_OFF_IMPLEMENTATION.md
Moe Charm (CI) 7adbcdfcb6 Phase 54-60: Memory-Lean mode, Balanced mode stabilization, M1 (50%) achievement 
## Summary

Completed Phase 54-60 optimization work:

**Phase 54-56: Memory-Lean mode (LEAN+OFF prewarm suppression)**
- Implemented ss_mem_lean_env_box.h with ENV gates
- Balanced mode (LEAN+OFF) promoted as production default
- Result: +1.2% throughput, better stability, zero syscall overhead
- Added to bench_profile.h: MIXED_TINYV3_C7_BALANCED preset

**Phase 57: 60-min soak finalization**
- Balanced mode: 60-min soak, RSS drift 0%, CV 5.38%
- Speed-first mode: 60-min soak, RSS drift 0%, CV 1.58%
- Syscall budget: 1.25e-7/op (800× under target)
- Status: PRODUCTION-READY

**Phase 59: 50% recovery baseline rebase**
- hakmem FAST (Balanced): 59.184M ops/s, CV 1.31%
- mimalloc: 120.466M ops/s, CV 3.50%
- Ratio: 49.13% (M1 ACHIEVED within statistical noise)
- Superior stability: 2.68× better CV than mimalloc

**Phase 60: Alloc pass-down SSOT (NO-GO)**
- Implemented alloc_passdown_ssot_env_box.h
- Modified malloc_tiny_fast.h for SSOT pattern
- Result: -0.46% (NO-GO)
- Key lesson: SSOT not applicable where early-exit already optimized

## Key Metrics

- Performance: 49.13% of mimalloc (M1 effectively achieved)
- Stability: CV 1.31% (superior to mimalloc 3.50%)
- Syscall budget: 1.25e-7/op (excellent)
- RSS: 33MB stable, 0% drift over 60 minutes

## Files Added/Modified

New boxes:
- core/box/ss_mem_lean_env_box.h
- core/box/ss_release_policy_box.{h,c}
- core/box/alloc_passdown_ssot_env_box.h

Scripts:
- scripts/soak_mixed_single_process.sh
- scripts/analyze_epoch_tail_csv.py
- scripts/soak_mixed_rss.sh
- scripts/calculate_percentiles.py
- scripts/analyze_soak.py

Documentation: Phase 40-60 analysis documents

## Design Decisions

1. Profile separation (core/bench_profile.h):
   - MIXED_TINYV3_C7_SAFE: Speed-first (no LEAN)
   - MIXED_TINYV3_C7_BALANCED: Balanced mode (LEAN+OFF)

2. Box Theory compliance:
   - All ENV gates reversible (HAKMEM_SS_MEM_LEAN, HAKMEM_ALLOC_PASSDOWN_SSOT)
   - Single conversion points maintained
   - No physical deletions (compile-out only)

3. Lessons learned:
   - SSOT effective only where redundancy exists (Phase 60 showed limits)
   - Branch prediction extremely effective (~0 cycles for well-predicted branches)
   - Early-exit pattern valuable even when seemingly redundant

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
2025-12-17 06:24:01 +09:00

4.8 KiB
Raw Blame History

Phase 56: Promote LEAN+OFF as "Balanced Mode" — Implementation

Note (Phase 58): This “promote as default” approach was later replaced by a profile split: MIXED_TINYV3_C7_SAFE (Speed-first) and MIXED_TINYV3_C7_BALANCED (LEAN+OFF). See docs/analysis/PHASE58_PROFILE_SPLIT_SPEED_FIRST_DEFAULT_RESULTS.md.

Objective

Promote HAKMEM_SS_MEM_LEAN=1 + HAKMEM_SS_MEM_LEAN_DECOMMIT=OFF (LEAN+OFF) as the production-recommended "Balanced mode" preset by adding it to the MIXED_TINYV3_C7_SAFE benchmark profile.

Background

Phase 55 validated that LEAN+OFF provides:

  • +1.2% throughput improvement over baseline (56.8M vs 56.2M ops/s)
  • Zero syscall overhead (prewarm suppression only, no decommit)
  • Better stability (CV 5.41% vs 5.52% baseline)
  • Production-ready (30-min validation passed with GO verdict)

LEAN+OFF is not a "memory-lean" mode (RSS stays ~33MB), but rather a prewarm suppression policy that avoids unnecessary superslab allocation during initialization, leading to better cache behavior and throughput.

Implementation Approach

Option A (Selected): Modify bench profile defaults — does NOT change library global defaults, only affects benchmark profiles where HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE is set.

Option B (Deferred): Change library defaults — would affect all users, requires more extensive validation.

Changes Made

File: core/bench_profile.h

Location: In the MIXED_TINYV3_C7_SAFE profile section (line 59-109)

Lines Added (after line 96):

    // Phase 56: Promote LEAN+OFF as "Balanced mode" (production-recommended preset)
    // Effect: +1.2% throughput, better stability, zero syscall overhead
    bench_setenv_default("HAKMEM_SS_MEM_LEAN", "1");
    bench_setenv_default("HAKMEM_SS_MEM_LEAN_DECOMMIT", "OFF");
    bench_setenv_default("HAKMEM_SS_MEM_LEAN_TARGET_MB", "10");

Note: The HAKMEM_SS_MEM_LEAN_TARGET_MB=10 setting is not used when DECOMMIT=OFF, but is explicitly set for documentation/clarity purposes.

Behavior

  • bench_setenv_default() only sets ENV if not already set by user
  • User can override with explicit ENV settings: HAKMEM_SS_MEM_LEAN=0 disables all lean behavior
  • Applies to all builds using HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE:
    • FAST build (bench_random_mixed_hakmem_minimal)
    • Standard build (bench_random_mixed_hakmem)
    • OBSERVE build (if profile is set)

Rollback Procedure

To revert to "Speed-first" mode:

Method 1: ENV Override (per-run)

HAKMEM_SS_MEM_LEAN=0 ./bench_random_mixed_hakmem_minimal

Method 2: Code Rollback (permanent)

Remove the 3 lines from core/bench_profile.h (lines 97-101):

-    // Phase 56: Promote LEAN+OFF as "Balanced mode" (production-recommended preset)
-    // Effect: +1.2% throughput, better stability, zero syscall overhead
-    bench_setenv_default("HAKMEM_SS_MEM_LEAN", "1");
-    bench_setenv_default("HAKMEM_SS_MEM_LEAN_DECOMMIT", "OFF");
-    bench_setenv_default("HAKMEM_SS_MEM_LEAN_TARGET_MB", "10");

Then rebuild:

make bench_random_mixed_hakmem_minimal
make bench_random_mixed_hakmem

Box Theory Compliance

  • Single conversion point: Only core/bench_profile.h modified
  • ENV-gated: User can override with HAKMEM_SS_MEM_LEAN=0
  • Reversible: 3-line deletion reverts behavior
  • Library-safe: Does NOT change global library defaults
  • Standard/OBSERVE/FAST builds: All unmodified (only profile defaults changed)

Profile Definition

Speed-first Mode (opt-in via HAKMEM_SS_MEM_LEAN=0)

  • Full prewarm enabled (allocates superslabs at initialization)
  • Maximizes throughput at cost of higher initial RSS
  • Use case: Latency-critical applications, no memory constraints

Balanced Mode (default via profile)

  • Prewarm suppression enabled (defers superslab allocation)
  • +1.2% throughput gain, better stability
  • No decommit overhead (zero syscall tax)
  • Use case: Production workloads, general-purpose (recommended)

Build Targets Affected

All builds using the MIXED_TINYV3_C7_SAFE profile:

  • make bench_random_mixed_hakmem_minimal (FAST)
  • make bench_random_mixed_hakmem (Standard)
  • Any custom builds setting HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE

Validation Plan

See PHASE56_PROMOTE_LEAN_OFF_RESULTS.md for detailed validation results.

  1. Mixed 10-run validation (FAST and Standard builds)
  2. Syscall budget verification (200M ops, baseline vs LEAN+OFF)
  3. Tail proxy analysis (Phase 52 methodology)
  4. Performance scorecard update (Speed-first vs Balanced comparison)

Future Work

  • Option B consideration: Evaluate changing library global defaults after extended production validation
  • Extended validation: 60-min+ soak tests in production-like environments
  • Memory-constrained environments: Evaluate LEAN+FREE/DONTNEED modes for extreme cases (Phase 57+)