hakmem/PHASE2B_TLS_ADAPTIVE_SIZING.md

Phase 2b: TLS Cache Adaptive Sizing

Date: 2025-11-08 Priority: 🟡 HIGH - Performance optimization Estimated Effort: 3-5 days Status: Ready for implementation Depends on: Phase 2a (not blocking, can run in parallel)

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Executive Summary

Problem: TLS Cache has fixed capacity (256-768 slots) → Cannot adapt to workload Solution: Implement adaptive sizing with high-water mark tracking Expected Result: Hot classes get more cache → Better hit rate → Higher throughput

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Current Architecture (INEFFICIENT)

Fixed Capacity

// core/hakmem_tiny.c or similar
#define TLS_SLL_CAP_DEFAULT 256

static __thread int g_tls_sll_count[TINY_NUM_CLASSES];
static __thread void* g_tls_sll_head[TINY_NUM_CLASSES];

// Fixed capacity for all classes!
// Hot class (e.g., class 4 in Larson) → cache thrashes
// Cold class (e.g., class 0 rarely used) → wastes memory

Why This is Inefficient

Scenario 1: Hot class (class 4 - 128B allocations)

Larson 4T: 4000+ concurrent 128B allocations
TLS cache capacity: 256 slots
Hit rate: ~6% (256/4000)
Result: Constant refill overhead → poor performance

Scenario 2: Cold class (class 0 - 16B allocations)

Usage: ~10 allocations per minute
TLS cache capacity: 256 slots
Waste: 246 slots × 16B = 3936B per thread wasted

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Proposed Architecture (ADAPTIVE)

High-Water Mark Tracking

typedef struct TLSCacheStats {
    size_t capacity;           // Current capacity
    size_t high_water_mark;    // Peak usage in recent window
    size_t refill_count;       // Number of refills in recent window
    uint64_t last_adapt_time;  // Timestamp of last adaptation
} TLSCacheStats;

static __thread TLSCacheStats g_tls_cache_stats[TINY_NUM_CLASSES];

Adaptive Sizing Logic

// Periodically adapt cache size based on usage
void adapt_tls_cache_size(int class_idx) {
    TLSCacheStats* stats = &g_tls_cache_stats[class_idx];

    // Update high-water mark
    if (g_tls_sll_count[class_idx] > stats->high_water_mark) {
        stats->high_water_mark = g_tls_sll_count[class_idx];
    }

    // Adapt every N refills or M seconds
    uint64_t now = get_timestamp_ns();
    if (stats->refill_count < ADAPT_REFILL_THRESHOLD &&
        (now - stats->last_adapt_time) < ADAPT_TIME_THRESHOLD_NS) {
        return;  // Too soon to adapt
    }

    // Decide: grow, shrink, or keep
    if (stats->high_water_mark > stats->capacity * 0.8) {
        // High usage → grow cache (2x)
        grow_tls_cache(class_idx);
    } else if (stats->high_water_mark < stats->capacity * 0.2) {
        // Low usage → shrink cache (0.5x)
        shrink_tls_cache(class_idx);
    }

    // Reset stats for next window
    stats->high_water_mark = g_tls_sll_count[class_idx];
    stats->refill_count = 0;
    stats->last_adapt_time = now;
}

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Implementation Tasks

Task 1: Add Adaptive Sizing Stats (1-2 hours)

File: core/hakmem_tiny.c or TLS cache code

// Per-class TLS cache statistics
typedef struct TLSCacheStats {
    size_t capacity;           // Current capacity
    size_t high_water_mark;    // Peak usage in recent window
    size_t refill_count;       // Refills since last adapt
    size_t shrink_count;       // Shrinks (for debugging)
    size_t grow_count;         // Grows (for debugging)
    uint64_t last_adapt_time;  // Timestamp of last adaptation
} TLSCacheStats;

static __thread TLSCacheStats g_tls_cache_stats[TINY_NUM_CLASSES];

// Configuration
#define TLS_CACHE_MIN_CAPACITY 16      // Minimum cache size
#define TLS_CACHE_MAX_CAPACITY 2048    // Maximum cache size
#define TLS_CACHE_INITIAL_CAPACITY 64  // Initial size (reduced from 256)
#define ADAPT_REFILL_THRESHOLD 10      // Adapt every 10 refills
#define ADAPT_TIME_THRESHOLD_NS (1000000000ULL)  // Or every 1 second

// Growth thresholds
#define GROW_THRESHOLD 0.8   // Grow if usage > 80% of capacity
#define SHRINK_THRESHOLD 0.2 // Shrink if usage < 20% of capacity

Task 2: Implement Grow/Shrink Functions (2-3 hours)

File: core/hakmem_tiny.c

// Grow TLS cache capacity (2x)
static void grow_tls_cache(int class_idx) {
    TLSCacheStats* stats = &g_tls_cache_stats[class_idx];

    size_t new_capacity = stats->capacity * 2;
    if (new_capacity > TLS_CACHE_MAX_CAPACITY) {
        new_capacity = TLS_CACHE_MAX_CAPACITY;
    }

    if (new_capacity == stats->capacity) {
        return;  // Already at max
    }

    stats->capacity = new_capacity;
    stats->grow_count++;

    fprintf(stderr, "[TLS_CACHE] Grow class %d: %zu → %zu slots (grow_count=%zu)\n",
            class_idx, stats->capacity / 2, stats->capacity, stats->grow_count);
}

// Shrink TLS cache capacity (0.5x)
static void shrink_tls_cache(int class_idx) {
    TLSCacheStats* stats = &g_tls_cache_stats[class_idx];

    size_t new_capacity = stats->capacity / 2;
    if (new_capacity < TLS_CACHE_MIN_CAPACITY) {
        new_capacity = TLS_CACHE_MIN_CAPACITY;
    }

    if (new_capacity == stats->capacity) {
        return;  // Already at min
    }

    // Evict excess blocks if current count > new_capacity
    if (g_tls_sll_count[class_idx] > new_capacity) {
        // Drain excess blocks back to SuperSlab
        int excess = g_tls_sll_count[class_idx] - new_capacity;
        drain_excess_blocks(class_idx, excess);
    }

    stats->capacity = new_capacity;
    stats->shrink_count++;

    fprintf(stderr, "[TLS_CACHE] Shrink class %d: %zu → %zu slots (shrink_count=%zu)\n",
            class_idx, stats->capacity * 2, stats->capacity, stats->shrink_count);
}

// Drain excess blocks back to SuperSlab
static void drain_excess_blocks(int class_idx, int count) {
    void** head = &g_tls_sll_head[class_idx];
    int drained = 0;

    while (*head && drained < count) {
        void* block = *head;
        *head = *(void**)block;  // Pop from TLS list

        // Return to SuperSlab (or freelist)
        return_block_to_superslab(block, class_idx);

        drained++;
        g_tls_sll_count[class_idx]--;
    }

    fprintf(stderr, "[TLS_CACHE] Drained %d excess blocks from class %d\n", drained, class_idx);
}

Task 3: Integrate Adaptation into Refill Path (2-3 hours)

File: core/tiny_alloc_fast.inc.h or refill code

static inline int tiny_alloc_fast_refill(int class_idx) {
    // ... existing refill logic ...

    // Track refill for adaptive sizing
    TLSCacheStats* stats = &g_tls_cache_stats[class_idx];
    stats->refill_count++;

    // Update high-water mark
    if (g_tls_sll_count[class_idx] > stats->high_water_mark) {
        stats->high_water_mark = g_tls_sll_count[class_idx];
    }

    // Periodically adapt cache size
    adapt_tls_cache_size(class_idx);

    // ... rest of refill ...
}

Task 4: Implement Adaptation Logic (2-3 hours)

File: core/hakmem_tiny.c

// Adapt TLS cache size based on usage patterns
static void adapt_tls_cache_size(int class_idx) {
    TLSCacheStats* stats = &g_tls_cache_stats[class_idx];

    // Adapt every N refills or M seconds
    uint64_t now = get_timestamp_ns();
    bool should_adapt = (stats->refill_count >= ADAPT_REFILL_THRESHOLD) ||
                        ((now - stats->last_adapt_time) >= ADAPT_TIME_THRESHOLD_NS);

    if (!should_adapt) {
        return;  // Too soon to adapt
    }

    // Calculate usage ratio
    double usage_ratio = (double)stats->high_water_mark / (double)stats->capacity;

    // Decide: grow, shrink, or keep
    if (usage_ratio > GROW_THRESHOLD) {
        // High usage (>80%) → grow cache
        grow_tls_cache(class_idx);
    } else if (usage_ratio < SHRINK_THRESHOLD) {
        // Low usage (<20%) → shrink cache
        shrink_tls_cache(class_idx);
    } else {
        // Moderate usage (20-80%) → keep current size
        fprintf(stderr, "[TLS_CACHE] Keep class %d at %zu slots (usage=%.1f%%)\n",
                class_idx, stats->capacity, usage_ratio * 100.0);
    }

    // Reset stats for next window
    stats->high_water_mark = g_tls_sll_count[class_idx];
    stats->refill_count = 0;
    stats->last_adapt_time = now;
}

// Helper: Get timestamp in nanoseconds
static inline uint64_t get_timestamp_ns(void) {
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    return (uint64_t)ts.tv_sec * 1000000000ULL + (uint64_t)ts.tv_nsec;
}

Task 5: Initialize Adaptive Stats (1 hour)

File: core/hakmem_tiny.c

void hak_tiny_init(void) {
    // ... existing init ...

    // Initialize TLS cache stats for each class
    for (int class_idx = 0; class_idx < TINY_NUM_CLASSES; class_idx++) {
        TLSCacheStats* stats = &g_tls_cache_stats[class_idx];
        stats->capacity = TLS_CACHE_INITIAL_CAPACITY;  // Start with 64 slots
        stats->high_water_mark = 0;
        stats->refill_count = 0;
        stats->shrink_count = 0;
        stats->grow_count = 0;
        stats->last_adapt_time = get_timestamp_ns();

        // Initialize TLS cache head/count
        g_tls_sll_head[class_idx] = NULL;
        g_tls_sll_count[class_idx] = 0;
    }
}

Task 6: Add Capacity Enforcement (2-3 hours)

File: core/tiny_alloc_fast.inc.h

static inline int tiny_alloc_fast_refill(int class_idx) {
    TLSCacheStats* stats = &g_tls_cache_stats[class_idx];

    // Don't refill beyond current capacity
    int current_count = g_tls_sll_count[class_idx];
    int available_slots = stats->capacity - current_count;

    if (available_slots <= 0) {
        // Cache is full, don't refill
        fprintf(stderr, "[TLS_CACHE] Class %d cache full (%d/%zu), skipping refill\n",
                class_idx, current_count, stats->capacity);
        return -1;  // Signal caller to try again or use slow path
    }

    // Refill only up to capacity
    int want_count = HAKMEM_TINY_REFILL_DEFAULT;  // e.g., 16
    int refill_count = (want_count < available_slots) ? want_count : available_slots;

    // ... existing refill logic with refill_count ...
}

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Testing Strategy

Test 1: Adaptive Behavior Verification

# Enable debug logging
HAKMEM_LOG=1 ./larson_hakmem 10 8 128 1024 1 12345 4 2>&1 | grep "TLS_CACHE"

# Should see:
# [TLS_CACHE] Grow class 4: 64 → 128 slots (grow_count=1)
# [TLS_CACHE] Grow class 4: 128 → 256 slots (grow_count=2)
# [TLS_CACHE] Grow class 4: 256 → 512 slots (grow_count=3)
# [TLS_CACHE] Keep class 0 at 64 slots (usage=5.2%)

Test 2: Performance Improvement

# Before (fixed capacity)
./larson_hakmem 1 1 128 1024 1 12345 1
# Baseline: 2.71M ops/s

# After (adaptive capacity)
./larson_hakmem 1 1 128 1024 1 12345 1
# Expected: 2.8-3.0M ops/s (+3-10%)

Test 3: Memory Efficiency

# Run with memory profiling
valgrind --tool=massif ./larson_hakmem 1 1 128 1024 1 12345 1

# Compare peak memory usage
# Fixed: 256 slots × 8 classes × 8B = ~16KB per thread
# Adaptive: ~8KB per thread (cold classes shrink to 16 slots)

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Success Criteria

✅ Adaptive behavior: Logs show grow/shrink based on usage ✅ Hot class expansion: Class 4 grows to 512+ slots under load ✅ Cold class shrinkage: Class 0 shrinks to 16-32 slots ✅ Performance improvement: +3-10% on Larson benchmark ✅ Memory efficiency: -30-50% TLS cache memory usage

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Deliverable

Report file: /mnt/workdisk/public_share/hakmem/PHASE2B_IMPLEMENTATION_REPORT.md

Required sections:

1. Adaptive sizing behavior (logs showing grow/shrink)
2. Performance comparison (before/after)
3. Memory usage comparison (TLS cache overhead)
4. Per-class capacity evolution (graph if possible)
5. Production readiness (YES/NO verdict)

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Let's make TLS cache adaptive! 🎯