Add SuperSlab Prefault Box with 4MB MAP_POPULATE bug fix
New Feature: ss_prefault_box.h
- Box for controlling SuperSlab page prefaulting policy
- ENV: HAKMEM_SS_PREFAULT (0=OFF, 1=POPULATE, 2=TOUCH)
- Default: OFF (safe mode until further optimization)
Bug Fix: 4MB MAP_POPULATE regression
- Problem: Fallback path allocated 4MB (2x size for alignment) with MAP_POPULATE
causing 52x slower mmap (0.585ms → 30.6ms) and 35% throughput regression
- Solution: Remove MAP_POPULATE from 4MB allocation, apply madvise(MADV_WILLNEED)
only to the aligned 2MB region after trimming prefix/suffix
Changes:
- core/box/ss_prefault_box.h: New prefault policy box (header-only)
- core/box/ss_allocation_box.c: Integrate prefault box, call ss_prefault_region()
- core/superslab_cache.c: Fix fallback path - no MAP_POPULATE on 4MB,
always munmap prefix/suffix, use MADV_WILLNEED for 2MB only
- docs/specs/ENV_VARS*.md: Document HAKMEM_SS_PREFAULT
Performance:
- bench_random_mixed: 4.32M ops/s (regression fixed, slight improvement)
- bench_tiny_hot: 157M ops/s with prefault=1 (no crash)
Box Theory:
- OS layer (ss_os_acquire): "how to mmap"
- Prefault Box: "when to page-in"
- Allocation Box: "when to call prefault"
🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>
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