hakmem/docs/specs/CONFIGURATION.md

HAKMEM Configuration Guide

Last Updated: 2025-11-28 (After ENV Cleanup Phase 1-3)

This guide documents all canonical HAKMEM environment variables after Phase 0-2 cleanup and ENV Cleanup Phase 1-3.

Recent Changes:

• 2025-11-28: ENV Cleanup Phase 1-3 completed - 13 debug variables now gated behind !HAKMEM_BUILD_RELEASE
• 2025-11-26: Phase 2.2 - Learning Systems Consolidation (18→6 variables)

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📋 Quick Reference

Use the validation tool to check your configuration:

# Validate current environment
./scripts/validate_config.sh

# Strict mode (treat warnings as errors)
./scripts/validate_config.sh --strict

# Quiet mode (errors only)
./scripts/validate_config.sh --quiet

Deprecated variables? See DEPRECATED.md for migration guide.

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🔧 Debug Variables (Gated in Release Builds)

Important: The following debug-only variables are compiled out when HAKMEM_BUILD_RELEASE=1 (default for production builds). They have zero overhead in release builds.

Phase 1-3 Gated Variables (2025-11-28)

Core Debug Infrastructure:

• HAKMEM_TINY_ALLOC_DEBUG - TLS allocation state dumps (4 call sites)
• HAKMEM_TINY_PROFILE - FastCache profiling
• HAKMEM_WATCH_ADDR - Watch specific address for debugging

Trace & Timing:

• HAKMEM_PTR_TRACE_DUMP - Pointer trace dumps
• HAKMEM_PTR_TRACE_VERBOSE - Verbose pointer tracing
• HAKMEM_TIMING - Timing instrumentation

Freelist Diagnostics:

• HAKMEM_TINY_SLL_DIAG - SLL (singly-linked list) diagnostics (multiple call sites)
• HAKMEM_TINY_FREELIST_MASK - Freelist mask updates
• HAKMEM_SS_FREE_DEBUG - SuperSlab free debug logging

SuperSlab Registry Debug:

• HAKMEM_SUPER_LOOKUP_DEBUG - SuperSlab lookup verbose logging
• HAKMEM_SUPER_REG_DEBUG - Register/unregister debug (2 sites)
• HAKMEM_SS_LRU_DEBUG - LRU cache operation logging (3 sites)
• HAKMEM_SS_PREWARM_DEBUG - Prewarm initialization logging (2 sites)

Production Config (NOT gated): These variables remain available in release builds for operational tuning:

• HAKMEM_SUPERSLAB_MAX_CACHED - LRU cache capacity limit
• HAKMEM_SUPERSLAB_MAX_MEMORY_MB - LRU memory limit
• HAKMEM_SUPERSLAB_TTL_SEC - LRU time-to-live
• HAKMEM_PREWARM_SUPERSLABS - Prewarm count per class

Performance Impact: Gating these 13 debug variables improved Larson benchmark from 30.2M to 30.5M ops/s (+1.0%).

For details, see docs/status/ENV_CLEANUP_TASK.md.

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🎯 Core Configuration

Allocator Path Selection

Variable	Values	Default	Description
HAKMEM_WRAP_TINY	0, 1	1	Enable TINY allocator (1-2048B)
HAKMEM_WRAP_POOL	0, 1	1	Enable POOL allocator (2-8KB)
HAKMEM_WRAP_MID	0, 1	1	Enable MID allocator (8-32KB)
HAKMEM_WRAP_LARGE	0, 1	1	Enable LARGE allocator (>32KB)

Example:

# Disable all HAKMEM allocators (use system malloc)
export HAKMEM_WRAP_TINY=0 HAKMEM_WRAP_POOL=0 HAKMEM_WRAP_MID=0 HAKMEM_WRAP_LARGE=0

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🏗️ SuperSlab Management

Canonical Variables (After P0.1 - SuperSlab Unification):

Variable	Values	Default	Description
HAKMEM_SUPERSLAB_REUSE	0, 1	0	Reuse empty slabs (reduces mmap/munmap syscalls)
HAKMEM_SUPERSLAB_LAZY	0, 1	1	Lazy deallocation (Phase 9, keep slabs cached)
HAKMEM_SUPERSLAB_PREWARM	0-128	0	Preallocate N SuperSlabs at startup
HAKMEM_SUPERSLAB_LRU_CAP	0-1024	256	Max cached SuperSlabs (LRU eviction)
HAKMEM_SUPERSLAB_SOFT_CAP	0-1024	128	Soft cap for SuperSlab pool (before eviction)

Examples:

# High performance (aggressive reuse + large cache)
export HAKMEM_SUPERSLAB_REUSE=1
export HAKMEM_SUPERSLAB_LAZY=1
export HAKMEM_SUPERSLAB_PREWARM=16
export HAKMEM_SUPERSLAB_LRU_CAP=512

# Low memory footprint (minimal caching)
export HAKMEM_SUPERSLAB_REUSE=0
export HAKMEM_SUPERSLAB_LAZY=0
export HAKMEM_SUPERSLAB_LRU_CAP=32
export HAKMEM_SUPERSLAB_SOFT_CAP=16

Note: Phase 12 (Shared SuperSlab Pool) removed per-class registry population, making SUPERSLAB_REUSE less effective. Default is OFF.

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🧠 Learning Systems

Canonical Variables (After P2.2 - Learning Consolidation, 18→6 variables):

Allocation Learning

Controls adaptive sizing for allocator caches (TLS, SFC, capacity tuning).

Variable	Values	Default	Description

Memory Learning

Controls THP (Transparent Huge Pages), RSS optimization, and max-size learning.

Variable	Values	Default	Description

Advanced Overrides

For troubleshooting only - enables legacy advanced knobs that are auto-tuned by default.

Variable	Values	Default	Description

Examples:

# Production (learning disabled, use static tuning)
## 🎯 TINY Allocator (1-2048B)

### TLS Cache Configuration

| Variable | Values | Default | Description |
|----------|--------|---------|-------------|
| `HAKMEM_TINY_TLS_CAP` | 16-1024 | 64 | Per-class TLS cache capacity |
| `HAKMEM_TINY_TLS_REFILL` | 4-256 | 16 | Batch refill size |
| `HAKMEM_TINY_DRAIN_THRESH` | 0-1024 | 128 | Remote free drain threshold |

### Super Front Cache (SFC)
**Note**: SFC is **ACTIVE** and provides 95%+ hit rate for hot allocations.

| Variable | Values | Default | Description |
|----------|--------|---------|-------------|
| `HAKMEM_TINY_SFC_ENABLE` | 0, 1 | 1 | Enable Super Front Cache (ultra-fast TLS cache) |
| `HAKMEM_TINY_SFC_CAPACITY` | 32-512 | 128 | SFC slot count |
| `HAKMEM_TINY_SFC_HOT_CLASSES` | 1-16 | 8 | Number of hot classes to cache |

### P0 Batch Optimization

| Variable | Values | Default | Description |
|----------|--------|---------|-------------|
| `HAKMEM_TINY_P0_ENABLE` | 0, 1 | 1 | Enable P0 batch refill (O(1) freelist pop) |
| `HAKMEM_TINY_P0_BATCH` | 4-128 | 16 | P0 batch size |
| `HAKMEM_TINY_P0_NO_DRAIN` | 0, 1 | 0 | Disable remote drain (debug only) |
| `HAKMEM_TINY_P0_LOG` | 0, 1 | 0 | Enable P0 counter validation logging |

### Header Configuration

| Variable | Values | Default | Description |
|----------|--------|---------|-------------|
| `HAKMEM_TINY_HEADER_CLASSIDX` | 0, 1 | 1 | Store class_idx in header (Phase 7, enables fast free) |

**Examples**:
```bash
# High-throughput (large caches, aggressive batching)
export HAKMEM_TINY_TLS_CAP=256
export HAKMEM_TINY_TLS_REFILL=32
export HAKMEM_TINY_SFC_CAPACITY=256
export HAKMEM_TINY_P0_ENABLE=1
export HAKMEM_TINY_P0_BATCH=32

# Low-latency (small caches, fine-grained refill)
export HAKMEM_TINY_TLS_CAP=32
export HAKMEM_TINY_TLS_REFILL=4
export HAKMEM_TINY_SFC_CAPACITY=64
export HAKMEM_TINY_P0_BATCH=8

# Debug P0 issues
export HAKMEM_TINY_P0_LOG=1
export HAKMEM_TINY_P0_NO_DRAIN=1  # Isolate batch refill from remote free

---

🏊 Pool TLS Allocator (2-8KB)

Arena Management

Variable	Values	Default	Description
HAKMEM_POOL_TLS_ARENA_MB_INIT	1-64	1	Initial arena size (MB)
HAKMEM_POOL_TLS_ARENA_MB_MAX	1-64	8	Maximum arena size (MB)
HAKMEM_POOL_TLS_ARENA_GROWTH_LEVELS	1-8	3	Growth levels (1MB→2MB→4MB→8MB)

Example:

# Large arena for high-throughput 8KB allocations
export HAKMEM_POOL_TLS_ARENA_MB_INIT=4
export HAKMEM_POOL_TLS_ARENA_MB_MAX=32
export HAKMEM_POOL_TLS_ARENA_GROWTH_LEVELS=5  # 4MB→8MB→16MB→32MB

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📊 Statistics & Profiling

Variable	Values	Default	Description

Example:

# Enable stats for performance analysis

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🧪 Experimental Features

Warning: These features are experimental and may change or be removed.

Variable	Values	Default	Description

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🚀 Quick Start Examples

1. Production (Default Recommended)

# High performance, stable, integrity checks enabled
export HAKMEM_SUPERSLAB_LAZY=1
export HAKMEM_SUPERSLAB_LRU_CAP=256
export HAKMEM_TINY_P0_ENABLE=1

2. Debug Session

# Verbose logging, tracing, integrity checks
export HAKMEM_TRACE_ALLOCATIONS=1
export HAKMEM_TINY_P0_LOG=1

3. Low-Latency Workload

# Small caches, fine-grained batching, minimal syscalls
export HAKMEM_TINY_TLS_CAP=32
export HAKMEM_TINY_TLS_REFILL=4
export HAKMEM_TINY_SFC_CAPACITY=64
export HAKMEM_SUPERSLAB_LAZY=1
export HAKMEM_SUPERSLAB_LRU_CAP=128

4. High-Throughput Workload

# Large caches, aggressive batching, prewarm
export HAKMEM_TINY_TLS_CAP=256
export HAKMEM_TINY_TLS_REFILL=32
export HAKMEM_TINY_SFC_CAPACITY=256
export HAKMEM_TINY_P0_BATCH=32
export HAKMEM_SUPERSLAB_PREWARM=16
export HAKMEM_SUPERSLAB_LRU_CAP=512

5. Memory-Efficient (Low RSS)

# Minimal caching, eager deallocation
export HAKMEM_SUPERSLAB_LAZY=0
export HAKMEM_SUPERSLAB_LRU_CAP=32
export HAKMEM_SUPERSLAB_SOFT_CAP=16
export HAKMEM_TINY_TLS_CAP=32
export HAKMEM_TINY_SFC_CAPACITY=64
export HAKMEM_POOL_TLS_ARENA_MB_MAX=2

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✅ Validation & Testing

Validate Configuration

# Check for deprecated/invalid variables
./scripts/validate_config.sh

# Example output:
#   Sunset date: 2026-05-26 (6 months from 2025-11-26)
#   See DEPRECATED.md for migration guide
#
# [WARN] HAKMEM_TINY_TLS_CAP=2048 is outside typical range (16-1024)
#
# [OK] HAKMEM_SUPERSLAB_LAZY=1

Test Performance

# Baseline (10M iterations, 10 runs recommended)
./out/release/bench_random_mixed_hakmem

# Custom workload
./out/release/bench_random_mixed_hakmem 10000000 256 42

# Multi-threaded (Larson benchmark)
./out/release/larson_hakmem 8  # 8 threads

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❓ FAQ

Q: What's the difference between ALLOC_LEARN and MEM_LEARN?

A:

Q: Should I enable learning in production?

A: Generally NO. Learning adds overhead (~5-10%) and is best for:

• Adaptive workloads with unpredictable patterns
• Benchmarking different configurations
• Initial tuning phase (then bake learned values into static config)

For production, use static tuning based on profiling.

Q: Why is SUPERSLAB_REUSE default OFF?

A: Phase 12 (Shared SuperSlab Pool) removed per-class registry population. Reuse is now less effective and can cause fragmentation. Use SUPERSLAB_LAZY=1 (default) instead for syscall reduction.

Q: What's the performance impact of INTEGRITY_CHECKS?

A: ~2-5% overhead. Recommended for production (default ON) to catch memory corruption early. Disable only for performance testing.

Q: How do I migrate from deprecated learning variables?

A: See DEPRECATED.md Section "Learning Systems (P2.2 Consolidation)" for complete mapping of 18→6 variables. The 6-month deprecation period provides backward compatibility.

Q: What's SFC and why is it still active?

A: SFC (Super Front Cache) is an ultra-fast TLS cache (95%+ hit rate, 3-4 instructions). Unified Cache was tested in Phase 3d-B but found slower than SFC, so SFC remained as the active implementation.

Q: What are "gated" debug variables?

A: Debug variables gated behind !HAKMEM_BUILD_RELEASE (13 variables as of Phase 1-3) are compiled out entirely in production builds. This means:

• Zero runtime overhead - no getenv() calls, no branch checks
• Smaller binary size - debug code removed
• Still available in debug builds - set HAKMEM_BUILD_RELEASE=0 to enable

This differs from production config variables (like HAKMEM_SUPERSLAB_MAX_CACHED) which remain accessible for operational tuning.

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📚 See Also

• ENV_CLEANUP_TASK.md - ENV Cleanup Phase 1-3 completion report
• DEPRECATED.md - Deprecated variables and migration guide
• BUILDING_QUICKSTART.md - Build instructions
• CLAUDE.md - Development history and performance benchmarks
• hakmem_cleanup_proposal.txt - Cleanup roadmap

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Generated: 2025-11-28 (Phase 1-3 ENV Cleanup Complete)