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)

---

## 📋 Quick Reference

Use the validation tool to check your configuration:

```bash
# 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](DEPRECATED.md) for migration guide.

---

## 🔧 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`.

---

## 🎯 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**:
```bash
# Disable all HAKMEM allocators (use system malloc)
export HAKMEM_WRAP_TINY=0 HAKMEM_WRAP_POOL=0 HAKMEM_WRAP_MID=0 HAKMEM_WRAP_LARGE=0
```

---

## 🏗️ 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**:
```bash
# 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.

---

## 🧠 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**:
```bash
# 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**:
```bash
# 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
```

---

## 📊 Statistics & Profiling

| Variable | Values | Default | Description |
|----------|--------|---------|-------------|

**Example**:
```bash
# Enable stats for performance analysis
```

---

## 🧪 Experimental Features

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

| Variable | Values | Default | Description |
|----------|--------|---------|-------------|

---

## 🚀 Quick Start Examples

### 1. Production (Default Recommended)
```bash
# 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
```bash
# Verbose logging, tracing, integrity checks
export HAKMEM_TRACE_ALLOCATIONS=1
export HAKMEM_TINY_P0_LOG=1
```

### 3. Low-Latency Workload
```bash
# 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
```bash
# 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)
```bash
# 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
```

---

## ✅ Validation & Testing

### Validate Configuration
```bash
# 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
```bash
# 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
```

---

## ❓ 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](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.

---

## 📚 See Also

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

---

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