hakmem/docs/status/MIMALLOC_DEEP_ANALYSIS_2025_10_24.md

MIMALLOC DEEP ANALYSIS: Why hakmem Cannot Catch Up

Crisis Analysis & Survival Strategy

Date: 2025-10-24 Author: Claude Code (Memory Allocator Expert) Context: hakmem Mid Pool 4T performance is only 46.7% of mimalloc (13.78 M/s vs 29.50 M/s) Mission: Identify root causes and provide actionable roadmap to reach 60-75% parity

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EXECUTIVE SUMMARY (TL;DR - 30 seconds)

Root Cause: hakmem's architecture is fundamentally over-engineered for Mid-sized allocations (2KB-32KB):

• 56 mutex locks (7 classes × 8 shards) vs mimalloc's lock-free per-page freelists
• 5-7 indirections per allocation vs mimalloc's 2-3 indirections
• Complex TLS cache (Ring + LIFO + Active Pages + Transfer Cache) vs mimalloc's simple per-page freelists
• 16-byte header overhead vs mimalloc's 0.2% metadata (separate page descriptors)

Can hakmem reach 60-75%? YES, but requires architectural simplification:

• Quick wins (1-4 hours): Reduce locks, simplify TLS cache → +5-10%
• Medium fixes (8-12 hours): Lock-free freelists, headerless allocation → +15-25%
• Moonshot (24+ hours): Per-page sharding (mimalloc-style) → +30-50%

ONE THING TO FIX FIRST: Remove 56 mutex locks (Phase 6.26 lock-free refill) → Expected +10-15%

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TABLE OF CONTENTS

1. Crisis Context
2. mimalloc Architecture Analysis
3. hakmem Architecture Analysis
4. Comparative Analysis
5. Bottleneck Identification
6. Actionable Recommendations
7. Critical Questions
8. References

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1. CRISIS CONTEXT

1.1 Current Performance Gap

Benchmark: larson (4 threads, Mid Pool allocations 2KB-32KB)

mimalloc:  29.50 M/s (100%)
hakmem:    13.78 M/s (46.7%)  ← CRISIS!

Target:    17.70-22.13 M/s (60-75%)
Gap:       3.92-8.35 M/s (28-71% improvement needed)

1.2 Recent Failed Attempts

Phase	Strategy	Expected	Actual	Outcome
6.25	Refill Batching (2-4 pages at once)	+10-15%	+1.1%	FAILED
6.27	Learner (adaptive tuning)	+5-10%	-1.5%	FAILED (overhead)
6.26	Lock-free Refill	+10-15%	Not implemented	ABORTED (11h, high risk)

Conclusion: Incremental optimizations are hitting diminishing returns. Need architectural fixes.

1.3 Why This Matters

• Survival: hakmem must reach 60-75% of mimalloc to be viable
• Production Readiness: Current 46.7% is unacceptable for real-world use
• Engineering Time: 6+ weeks of optimization yielded only marginal gains
• Opportunity Cost: Time spent on failed optimizations could have fixed root causes

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2. MIMALLOC ARCHITECTURE ANALYSIS

2.1 Core Design Principles

mimalloc's Key Insight: "Free List Sharding in Action"

Instead of:

• One big freelist per size class (jemalloc/tcmalloc approach)
• Lock contention on shared structures
• False sharing between threads

mimalloc uses:

• Many small freelists per page (64KiB pages)
• Lock-free operations (atomic CAS for cross-thread frees)
• Thread-local heaps (no locks for local allocations)
• Per-page multi-sharding (local-free + remote-free lists)

2.2 Data Structures

2.2.1 Page Structure (mi_page_t)

typedef struct mi_page_s {
    // FREE LISTS (multi-sharded per page)
    mi_block_t* free;          // Thread-local free list (fast path)
    mi_block_t* local_free;    // Pending local frees (batched collection)
    atomic(mi_block_t*) xthread_free;  // Cross-thread frees (lock-free)

    // METADATA (simplified in v2.1.4)
    uint32_t block_size;       // Block size (directly available)
    uint16_t capacity;         // Total blocks in page
    uint16_t reserved;         // Allocated blocks

    // PAGE INFO
    mi_page_kind_t kind;       // Page size class
    mi_heap_t* heap;           // Owning heap
    // ... (total ~80 bytes, stored ONCE per 64KiB page = 0.12% overhead)
} mi_page_t;

Key Points:

• Three freelists per page: free (hot path), local_free (deferred), xthread_free (remote)
• Lock-free remote frees: Atomic CAS on xthread_free
• Metadata overhead: ~80 bytes per 64KiB page = 0.12% (vs hakmem's 16 bytes per block = 0.8%)
• Block size directly available: No lookup needed (v2.1.4 optimization)

2.2.2 Heap Structure (mi_heap_t)

typedef struct mi_heap_s {
    mi_page_t* pages[MI_BIN_COUNT];  // Per-size-class page lists (~74 bins)
    atomic(uintptr_t) thread_id;     // Owning thread
    mi_heap_t* next;                 // Thread-local heap list
    // ... (total ~600 bytes, ONE per thread)
} mi_heap_t;

Key Points:

• One heap per thread: No sharing, no locks
• Direct page lookup: pages[size_class] → O(1) access
• Thread-local storage: TLS pointer to heap (~8 bytes overhead per thread)

2.2.3 Segment Structure (mi_segment_t)

Segment Layout (4 MiB for small objects, variable for large):
┌─────────────────────────────────────────────────────────┐
│ Segment Metadata (~1 page, 4-8 KiB)                     │
├─────────────────────────────────────────────────────────┤
│ Page Descriptors (mi_page_t × 64, ~5 KiB)               │
├─────────────────────────────────────────────────────────┤
│ Guard Page (optional, 4 KiB)                            │
├─────────────────────────────────────────────────────────┤
│ Page 0 (64 KiB) - shortened by metadata size            │
├─────────────────────────────────────────────────────────┤
│ Page 1 (64 KiB)                                         │
├─────────────────────────────────────────────────────────┤
│ ...                                                     │
├─────────────────────────────────────────────────────────┤
│ Page 63 (64 KiB)                                        │
└─────────────────────────────────────────────────────────┘

Size Classes:
- Small objects (<8 KiB): 64 KiB pages (64 pages per segment)
- Large objects (8-512 KiB): 1 page per segment (variable size)
- Huge objects (>512 KiB): 1 page per segment (exact size)

Key Points:

• Segment = contiguous memory block: Allocated via mmap (4 MiB default)
• Pages within segment: 64 KiB each for small objects
• Metadata co-location: All descriptors at segment start (cache-friendly)
• Total overhead: ~10 KiB per 4 MiB segment = 0.24%

2.3 Allocation Fast Path

2.3.1 Step-by-Step Flow (4 KiB allocation)

// Entry: mi_malloc(4096)
void* mi_malloc(size_t size) {
    // Step 1: Get thread-local heap (TLS access, 1 dereference)
    mi_heap_t* heap = mi_prim_get_default_heap();  // TLS load

    // Step 2: Size check (1 branch)
    if (size <= MI_SMALL_SIZE_MAX) {  // Fast path filter
        return mi_heap_malloc_small_zero(heap, size, false);
    }
    // ... (medium path, not shown)
}

// Fast path (inlined, ~7 instructions)
void* mi_heap_malloc_small_zero(mi_heap_t* heap, size_t size, bool zero) {
    // Step 3: Get size class (O(1) lookup, no branch)
    size_t bin = _mi_wsize_from_size(size);  // Shift + mask

    // Step 4: Get page for this size class (1 dereference)
    mi_page_t* page = heap->pages[bin];

    // Step 5: Pop from free list (2 dereferences)
    mi_block_t* block = page->free;
    if (mi_likely(block != NULL)) {  // Fast path (1 branch)
        page->free = block->next;  // Update free list
        return (void*)block;
    }

    // Step 6: Slow path (refill from local_free or allocate new page)
    return _mi_page_malloc_zero(heap, page, size, zero);
}

Operation Count (Fast Path):

• Dereferences: 3 (heap → page → block)
• Branches: 2 (size check, block != NULL)
• Atomics: 0 (all thread-local)
• Locks: 0 (no mutexes)
• Total: ~7 instructions in release mode

2.3.2 Slow Path (Refill)

void* _mi_page_malloc_zero(mi_heap_t* heap, mi_page_t* page, size_t size, bool zero) {
    // Step 1: Collect local_free into free list (deferred frees)
    if (page->local_free != NULL) {
        _mi_page_free_collect(page, false);  // O(N) walk, no lock
        mi_block_t* block = page->free;
        if (block != NULL) {
            page->free = block->next;
            return (void*)block;
        }
    }

    // Step 2: Collect xthread_free (cross-thread frees, lock-free)
    if (atomic_load_relaxed(&page->xthread_free) != NULL) {
        _mi_page_free_collect(page, true);  // Atomic swap
        mi_block_t* block = page->free;
        if (block != NULL) {
            page->free = block->next;
            return (void*)block;
        }
    }

    // Step 3: Allocate new page (rare, mmap)
    return _mi_malloc_generic(heap, size, zero, 0);
}

Operation Count (Slow Path):

• Dereferences: 5-7 (depends on refill source)
• Branches: 3-5 (check local_free, xthread_free, allocation success)
• Atomics: 1 (atomic swap on xthread_free)
• Locks: 0 (lock-free CAS)

2.4 Free Path

2.4.1 Same-Thread Free (Fast Path)

void mi_free(void* p) {
    // Step 1: Get page from pointer (bit manipulation, 0 dereferences)
    mi_segment_t* segment = _mi_ptr_segment(p);  // Mask high bits
    mi_page_t* page = _mi_segment_page_of(segment, p);  // Offset calc

    // Step 2: Push to local_free (1 dereference, 1 store)
    mi_block_t* block = (mi_block_t*)p;
    block->next = page->local_free;
    page->local_free = block;

    // Step 3: Deferred collection (batched to reduce overhead)
    // local_free is drained into free list on next allocation
}

Operation Count (Same-Thread Free):

• Dereferences: 1 (update local_free head)
• Branches: 0 (unconditional push)
• Atomics: 0 (thread-local)
• Locks: 0

2.4.2 Cross-Thread Free (Remote Free)

void mi_free(void* p) {
    // Step 1: Get page (same as above)
    mi_page_t* page = _mi_ptr_page(p);

    // Step 2: Atomic push to xthread_free (lock-free)
    mi_block_t* block = (mi_block_t*)p;
    mi_block_t* old_head;
    do {
        old_head = atomic_load_relaxed(&page->xthread_free);
        block->next = old_head;
    } while (!atomic_compare_exchange_weak(&page->xthread_free, &old_head, block));

    // Step 3: Signal owning thread (optional, for eager collection)
    // (not implemented in basic version, deferred collection on alloc)
}

Operation Count (Cross-Thread Free):

• Dereferences: 1-2 (page lookup + CAS retry)
• Branches: 1 (CAS loop)
• Atomics: 2 (load + CAS)
• Locks: 0

2.5 Key Optimizations

2.5.1 Lock-Free Design

No locks for:

• Thread-local allocations (use heap->pages[bin]->free)
• Same-thread frees (use page->local_free)
• Cross-thread frees (use atomic CAS on page->xthread_free)

Result: Zero lock contention in common case (90%+ of allocations)

2.5.2 Metadata Separation

Strategy: Store metadata separately from allocated blocks

hakmem approach (inline header):

Block: [Header 16B][User Data 4KB] = 16B overhead per block

mimalloc approach (separate descriptor):

Page Descriptor: [mi_page_t 80B] (ONE per 64KiB page)
Blocks: [Data 4KB][Data 4KB]... (NO per-block overhead)

Overhead comparison (4KB blocks):

• hakmem: 16 / 4096 = 0.39% per block
• mimalloc: 80 / 65536 = 0.12% per page (amortized)

Result: mimalloc has 3.25× lower metadata overhead

2.5.3 Page Pointer Derivation

mimalloc trick: Get page descriptor from block pointer without lookup

// Given: block pointer p
// Derive: segment address (clear low bits)
mi_segment_t* segment = (mi_segment_t*)((uintptr_t)p & ~(4*1024*1024 - 1));

// Derive: page index (offset within segment)
size_t offset = (uintptr_t)p - (uintptr_t)segment;
size_t page_idx = offset / MI_PAGE_SIZE;

// Derive: page descriptor (segment metadata array)
mi_page_t* page = &segment->pages[page_idx];

Cost: 3-4 instructions (mask, subtract, divide, array index) hakmem equivalent: Hash table lookup (MidPageDesc) = 10-20 instructions + cache miss risk

2.5.4 Deferred Collection

Strategy: Batch free-list operations to reduce overhead

Same-thread frees:

• Push to local_free (LIFO, no walk)
• Drain into free on next allocation (batch operation)
• Benefit: O(1) free, amortized O(1) collection

Cross-thread frees:

• Push to xthread_free (atomic LIFO)
• Drain into free when free is empty (batch operation)
• Benefit: Lock-free + batched (reduces atomic ops)

2.6 mimalloc Summary

Architecture:

• Per-page freelists: Many small lists (64KiB pages) vs one big list
• Lock-free: Thread-local heaps + atomic CAS for remote frees
• Metadata separation: Page descriptors separate from blocks (0.12% overhead)
• Pointer arithmetic: O(1) page lookup from block address

Performance Characteristics:

• Fast path: 7 instructions, 2-3 dereferences, 0 locks
• Slow path: Lock-free collection, no blocking
• Free path: 1-2 atomics (remote) or 0 atomics (local)

Why it's fast:

1. No lock contention: Thread-local everything
2. Low overhead: Minimal metadata (0.2% total)
3. Cache-friendly: Contiguous segments, co-located metadata
4. Simple fast path: Minimal branches and dereferences

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3. HAKMEM ARCHITECTURE ANALYSIS

3.1 Core Design (Mid Pool 2KB-32KB)

hakmem's Approach: Multi-layered TLS caching + global sharded freelists

Allocation Path:
┌─────────────────────────────────────────────────────────┐
│ TLS Ring Buffer (32 slots, LIFO)                        │ ← Layer 1
├─────────────────────────────────────────────────────────┤
│ TLS LIFO Overflow (256 blocks max)                      │ ← Layer 2
├─────────────────────────────────────────────────────────┤
│ TLS Active Page A (bump-run, headerless)                │ ← Layer 3
├─────────────────────────────────────────────────────────┤
│ TLS Active Page B (bump-run, headerless)                │ ← Layer 4
├─────────────────────────────────────────────────────────┤
│ TLS Transfer Cache Inbox (lock-free, remote frees)      │ ← Layer 5
├─────────────────────────────────────────────────────────┤
│ Global Freelist (7 classes × 8 shards = 56 mutexes)     │ ← Layer 6
├─────────────────────────────────────────────────────────┤
│ Global Remote Stack (atomic, cross-thread frees)        │ ← Layer 7
└─────────────────────────────────────────────────────────┘

Complexity: 7 layers of caching (mimalloc has 2: page free list + local_free)

3.2 Data Structures

3.2.1 TLS Cache Structures

// Layer 1: Ring Buffer (32 slots)
typedef struct {
    PoolBlock* items[POOL_TLS_RING_CAP];  // 32 slots = 256 bytes
    int top;                               // Stack pointer
} PoolTLSRing;

// Layer 2: LIFO Overflow (linked list)
typedef struct {
    PoolTLSRing ring;
    PoolBlock* lo_head;    // LIFO head
    size_t lo_count;       // LIFO count (max 256)
} PoolTLSBin;

// Layer 3/4: Active Pages (bump-run)
typedef struct {
    void* page;      // Page base (64KiB)
    char* bump;      // Next allocation pointer
    char* end;       // Page end
    int count;       // Remaining blocks
} PoolTLSPage;

// Layer 5: Transfer Cache (cross-thread inbox)
typedef struct {
    atomic_uintptr_t inbox[POOL_NUM_CLASSES];  // Per-class atomic stacks
} MidTC;

Total TLS overhead per thread:

• Ring: 32 × 8 + 4 = 260 bytes × 7 classes = 1,820 bytes
• LIFO: 8 + 8 = 16 bytes × 7 classes = 112 bytes
• Active Pages: 32 bytes × 2 × 7 classes = 448 bytes
• Transfer Cache: 8 bytes × 7 classes = 56 bytes
• Total: ~2,436 bytes per thread (vs mimalloc's ~600 bytes)

3.2.2 Global Pool Structures

struct {
    // Layer 6: Sharded Freelists (56 freelists)
    PoolBlock* freelist[POOL_NUM_CLASSES][POOL_NUM_SHARDS];  // 7 × 8 = 56
    PaddedMutex freelist_locks[POOL_NUM_CLASSES][POOL_NUM_SHARDS];  // 56 mutexes!

    // Bitmap for fast empty detection
    atomic_uint_fast64_t nonempty_mask[POOL_NUM_CLASSES];  // 7 × 8 bytes

    // Layer 7: Remote Free Stacks (cross-thread, lock-free)
    atomic_uintptr_t remote_head[POOL_NUM_CLASSES][POOL_NUM_SHARDS];  // 56 atomics
    atomic_uint remote_count[POOL_NUM_CLASSES][POOL_NUM_SHARDS];  // 56 atomics

    // Statistics (aligned to avoid false sharing)
    uint64_t hits[POOL_NUM_CLASSES] __attribute__((aligned(64)));
    uint64_t misses[POOL_NUM_CLASSES] __attribute__((aligned(64)));
    // ... (more stats)
} g_pool;

Total global overhead:

• Freelists: 56 × 8 = 448 bytes
• Locks: 56 × 64 = 3,584 bytes (padded to avoid false sharing)
• Bitmaps: 7 × 8 = 56 bytes
• Remote stacks: 56 × 8 × 2 = 896 bytes
• Stats: ~1 KB
• Total: ~6 KB (vs mimalloc's ~10 KB per 4 MiB segment, but amortized)

3.2.3 Block Header (Per-Allocation Overhead)

typedef struct {
    uint32_t magic;        // 4 bytes (validation)
    AllocMethod method;    // 4 bytes (POOL/MMAP/MALLOC)
    size_t size;          // 8 bytes (original size)
    uintptr_t alloc_site; // 8 bytes (call site)
    size_t class_bytes;   // 8 bytes (size class)
    uintptr_t owner_tid;  // 8 bytes (owning thread)
} AllocHeader;  // Total: 40 bytes (reduced to 16 in "light" mode)

Overhead comparison (4KB block):

• Full mode: 40 / 4096 = 0.98% per block
• Light mode: 16 / 4096 = 0.39% per block
• mimalloc: 80 / 65536 = 0.12% per page (amortized)

Result: hakmem has 3.25× higher overhead even in light mode

3.2.4 Page Descriptor Registry

// Hash table for page lookup (64KiB pages → {class_idx, owner_tid})
#define MID_DESC_BUCKETS 2048

typedef struct MidPageDesc {
    void* page;                  // Page base address
    uint8_t class_idx;           // Size class (0-6)
    uint64_t owner_tid;          // Owning thread ID
    atomic_int in_use;           // Live allocations on page
    int blocks_per_page;         // Total blocks
    atomic_int pending_dn;       // Background DONTNEED enqueued
    struct MidPageDesc* next;    // Hash chain
} MidPageDesc;

static pthread_mutex_t g_mid_desc_mu[MID_DESC_BUCKETS];  // 2048 mutexes!
static MidPageDesc* g_mid_desc_head[MID_DESC_BUCKETS];

Lookup cost:

1. Hash page address (5-10 instructions)
2. Lock mutex (50-200 cycles if contended)
3. Walk hash chain (1-10 nodes, cache misses)
4. Unlock mutex

mimalloc equivalent: Pointer arithmetic (3-4 instructions, no locks)

3.3 Allocation Fast Path

3.3.1 Step-by-Step Flow (4 KiB allocation)

void* hak_pool_try_alloc(size_t size, uintptr_t site_id) {
    // Step 1: Get class index (array lookup)
    int class_idx = hak_pool_get_class_index(size);  // O(1) LUT

    // Step 2: Check TLS Transfer Cache (if low on ring)
    PoolTLSRing* ring = &g_tls_bin[class_idx].ring;
    if (g_tc_enabled && ring->top < g_tc_drain_trigger && mid_tc_has_items(class_idx)) {
        mid_tc_drain_into_tls(class_idx, ring, &g_tls_bin[class_idx]);  // Drain inbox
        if (ring->top > 0) {
            PoolBlock* tlsb = ring->items[--ring->top];  // Pop from ring
            // ... (construct header, return)
            return (char*)tlsb + HEADER_SIZE;
        }
    }

    // Step 3: Try TLS Ring Buffer (32 slots)
    if (ring->top > 0) {
        PoolBlock* tlsb = ring->items[--ring->top];
        void* raw = (void*)tlsb;
        AllocHeader* hdr = (AllocHeader*)raw;
        mid_set_header(hdr, g_class_sizes[class_idx], site_id);  // Write header
        mid_page_inuse_inc(raw);  // Increment page counter (hash lookup + atomic)
        return (char*)raw + HEADER_SIZE;
    }

    // Step 4: Try TLS LIFO Overflow
    if (g_tls_bin[class_idx].lo_head) {
        PoolBlock* b = g_tls_bin[class_idx].lo_head;
        g_tls_bin[class_idx].lo_head = b->next;
        // ... (construct header, return)
        return (char*)b + HEADER_SIZE;
    }

    // Step 5: Compute shard index (hash site_id)
    int shard_idx = hak_pool_get_shard_index(site_id);  // SplitMix64 hash

    // Step 6: Try lock-free batch-pop from global freelist (trylock probe)
    for (int probe = 0; probe < g_trylock_probes; ++probe) {
        int s = (shard_idx + probe) & (POOL_NUM_SHARDS - 1);
        pthread_mutex_t* l = &g_pool.freelist_locks[class_idx][s].m;
        if (pthread_mutex_trylock(l) == 0) {  // Trylock (50-200 cycles)
            // Drain remote stack into freelist
            drain_remote_locked(class_idx, s);
            // Batch-pop into TLS ring
            PoolBlock* head = g_pool.freelist[class_idx][s];
            int to_ring = POOL_TLS_RING_CAP - ring->top;
            while (head && to_ring-- > 0) {
                PoolBlock* nxt = head->next;
                ring->items[ring->top++] = head;
                head = nxt;
            }
            g_pool.freelist[class_idx][s] = head;
            pthread_mutex_unlock(l);

            // Pop from ring
            if (ring->top > 0) {
                PoolBlock* tlsb = ring->items[--ring->top];
                // ... (construct header, return)
                return (char*)tlsb + HEADER_SIZE;
            }
        }
    }

    // Step 7: Try TLS Active Pages (bump-run)
    PoolTLSPage* ap = &g_tls_active_page_a[class_idx];
    if (ap->page && ap->count > 0 && ap->bump < ap->end) {
        // Refill ring from active page
        refill_tls_from_active_page(class_idx, ring, &g_tls_bin[class_idx], ap, need);
        // Pop from ring or bump directly
        // ... (return)
    }

    // Step 8: Lock shard freelist (blocking)
    pthread_mutex_lock(&g_pool.freelist_locks[class_idx][shard_idx].m);

    // Step 9: Pop from freelist or refill (mmap new page)
    PoolBlock* block = g_pool.freelist[class_idx][shard_idx];
    if (!block) {
        refill_freelist(class_idx, shard_idx);  // Allocate 1-4 pages (mmap)
        block = g_pool.freelist[class_idx][shard_idx];
    }
    g_pool.freelist[class_idx][shard_idx] = block->next;

    pthread_mutex_unlock(&g_pool.freelist_locks[class_idx][shard_idx].m);

    // Step 10: Save to TLS cache, then pop
    ring->items[ring->top++] = block;
    PoolBlock* take = ring->items[--ring->top];

    // Step 11: Construct header
    mid_set_header((AllocHeader*)take, g_class_sizes[class_idx], site_id);
    mid_page_inuse_inc(take);  // Hash lookup + atomic increment

    return (char*)take + HEADER_SIZE;
}

Operation Count (Fast Path - Ring Hit):

• Dereferences: 5-7 (class_idx → ring → items[] → header → page descriptor)
• Branches: 7-10 (TC check, ring empty, LIFO empty, trylock, active page)
• Atomics: 1-2 (page in_use counter, TC inbox check)
• Locks: 0 (ring hit)
• Hash lookups: 1 (mid_page_inuse_inc → mid_desc_lookup)

Operation Count (Slow Path - Freelist Refill):

• Dereferences: 10-15
• Branches: 15-20
• Atomics: 3-5
• Locks: 1 (freelist mutex)
• Hash lookups: 2-3

Comparison to mimalloc:

Metric	mimalloc	hakmem (ring hit)	hakmem (freelist)
Dereferences	3	5-7	10-15
Branches	2	7-10	15-20
Atomics	0	1-2	3-5
Locks	0	0	1
Hash Lookups	0	1	2-3

3.4 Free Path

3.4.1 Same-Thread Free

void hak_pool_free(void* ptr, size_t size, uintptr_t site_id) {
    // Step 1: Get raw pointer (subtract header offset)
    void* raw = (char*)ptr - HEADER_SIZE;

    // Step 2: Validate header (unless light mode)
    AllocHeader* hdr = (AllocHeader*)raw;
    MidPageDesc* d_desc = mid_desc_lookup(ptr);  // Hash lookup
    if (!d_desc && g_hdr_light_enabled < 2) {
        if (hdr->magic != HAKMEM_MAGIC) return;  // Validation
    }

    // Step 3: Get class and shard indices
    int class_idx = d_desc ? (int)d_desc->class_idx : hak_pool_get_class_index(size);

    // Step 4: Check if same-thread (via page descriptor)
    int same_thread = 0;
    if (g_hdr_light_enabled >= 1) {
        MidPageDesc* d = mid_desc_lookup(raw);  // Hash lookup (again!)
        if (d && d->owner_tid != 0 && d->owner_tid == (uint64_t)pthread_self()) {
            same_thread = 1;
        }
    }

    // Step 5: Push to TLS Ring or LIFO
    if (same_thread) {
        PoolTLSRing* ring = &g_tls_bin[class_idx].ring;
        if (ring->top < POOL_TLS_RING_CAP) {
            ring->items[ring->top++] = (PoolBlock*)raw;  // Push to ring
        } else {
            // Push to LIFO overflow
            PoolBlock* block = (PoolBlock*)raw;
            block->next = g_tls_bin[class_idx].lo_head;
            g_tls_bin[class_idx].lo_head = block;
            g_tls_bin[class_idx].lo_count++;

            // Spill to remote if overflow
            if ((int)g_tls_bin[class_idx].lo_count > g_tls_lo_max) {
                // ... (spill half to remote stack)
            }
        }
    } else {
        // Step 6: Cross-thread free (Transfer Cache or Remote Stack)
        if (g_tc_enabled) {
            uint64_t owner_tid = hdr->owner_tid;
            if (owner_tid != 0) {
                MidTC* otc = mid_tc_lookup_by_tid(owner_tid);  // Hash lookup
                if (otc) {
                    mid_tc_push(otc, class_idx, (PoolBlock*)raw);  // Atomic CAS
                    return;
                }
            }
        }

        // Fallback: push to global remote stack (atomic CAS)
        int shard = hak_pool_get_shard_index(site_id);
        atomic_uintptr_t* head_ptr = &g_pool.remote_head[class_idx][shard];
        uintptr_t old_head;
        do {
            old_head = atomic_load_explicit(head_ptr, memory_order_acquire);
            ((PoolBlock*)raw)->next = (PoolBlock*)old_head;
        } while (!atomic_compare_exchange_weak_explicit(head_ptr, &old_head, (uintptr_t)raw, memory_order_release, memory_order_relaxed));
        atomic_fetch_add_explicit(&g_pool.remote_count[class_idx][shard], 1, memory_order_relaxed);
    }

    // Step 7: Decrement page in-use counter
    mid_page_inuse_dec_and_maybe_dn(raw);  // Hash lookup + atomic decrement + potential DONTNEED
}

Operation Count (Same-Thread Free):

• Dereferences: 4-6
• Branches: 5-8
• Atomics: 2-3 (page counter, DONTNEED flag)
• Locks: 0
• Hash Lookups: 2-3 (page descriptor × 2, validation)

Operation Count (Cross-Thread Free):

• Dereferences: 5-8
• Branches: 7-10
• Atomics: 4-6 (TC push CAS, remote stack CAS, page counter)
• Locks: 0
• Hash Lookups: 3-4 (page descriptor, TC lookup, owner TID)

Comparison to mimalloc:

Metric	mimalloc (same-thread)	mimalloc (cross-thread)	hakmem (same-thread)	hakmem (cross-thread)
Dereferences	1	1-2	4-6	5-8
Branches	0	1	5-8	7-10
Atomics	0	2	2-3	4-6
Hash Lookups	0	0	2-3	3-4

3.5 hakmem Summary

Architecture:

• 7-layer TLS caching: Ring → LIFO → Active Pages → TC → Freelist → Remote
• 56 mutex locks: 7 classes × 8 shards (high contention risk)
• Hash table lookups: Page descriptors (O(1) average, cache miss risk)
• Inline headers: 16-40 bytes per block (0.39-0.98% overhead)

Performance Characteristics:

• Fast path: 5-7 dereferences, 7-10 branches, 1-2 hash lookups
• Slow path: Mutex lock + refill (blocking)
• Free path: 2-3 hash lookups, 2-6 atomics

Why it's slow:

1. Lock contention: 56 mutexes (vs mimalloc's 0)
2. Complexity: 7 layers of caching (vs mimalloc's 2)
3. Hash lookups: Page descriptor registry (vs mimalloc's pointer arithmetic)
4. Metadata overhead: Inline headers (vs mimalloc's separate descriptors)

---

4. COMPARATIVE ANALYSIS

4.1 Feature Comparison Table

Feature	hakmem	mimalloc	Winner	Gap Analysis
TLS cache size	32 slots (ring) + 256 (LIFO) + 2 pages	Per-page freelists (~10-100 blocks)	mimalloc	hakmem over-engineered (7 layers vs 2)
Metadata overhead	16-40 bytes per block (0.39-0.98%)	80 bytes per page (0.12%)	mimalloc (3.25× lower)	Inline headers waste space
Lock usage	56 mutexes (7 classes × 8 shards)	0 locks (lock-free)	mimalloc (infinite advantage)	CRITICAL bottleneck
Fast path branches	7-10 branches	2 branches	mimalloc (3.5-5× fewer)	hakmem too many checks
Fast path dereferences	5-7 dereferences	2-3 dereferences	mimalloc (2× fewer)	Hash lookups expensive
Page refill cost	mmap (2-4 pages) + register	mmap (1 segment) + descriptor	Tie	Both use mmap
Free path (same-thread)	2-3 hash lookups + 2-3 atomics	1 dereference + 0 atomics	mimalloc (10× faster)	Hash lookups + atomics overhead
Free path (cross-thread)	3-4 hash lookups + 4-6 atomics	0 hash lookups + 2 atomics	mimalloc (2-3× faster)	Transfer Cache overhead
Page descriptor lookup	Hash table (O(1) average, mutex)	Pointer arithmetic (O(1) exact)	mimalloc (no locks)	Hash collisions + locks
Allocation granularity	64 KiB pages (2-32 blocks)	64 KiB pages (variable)	Tie	Same page size
Thread safety	Mutexes + atomics	Lock-free (atomics only)	mimalloc (no blocking)	Mutexes cause contention
Cache locality	Scattered (TLS + global)	Contiguous (segment)	mimalloc (better)	Segments are cache-friendly
Code complexity	1331 lines (pool.c)	~500 lines (alloc.c)	mimalloc (2.7× simpler)	hakmem over-optimized

4.2 Performance Model

4.2.1 Allocation Cost Breakdown

mimalloc (fast path):

Cost = TLS_load + size_check + bin_lookup + page_deref + block_pop
     = 1 + 1 + 1 + 1 + 1
     = 5 cycles (idealized, no cache misses)

hakmem (fast path - ring hit):

Cost = class_lookup + TC_check + ring_check + ring_pop + header_write + page_counter_inc
     = 1 + (2 + hash_lookup) + 1 + 1 + 5 + (hash_lookup + atomic_inc)
     = 10 + 2×hash_lookup + atomic_inc
     = 10 + 2×(10-20) + 5
     = 35-55 cycles (with hash lookups)

Ratio: hakmem is 7-11× slower per allocation (fast path)

4.2.2 Lock Contention Model

mimalloc: 0 locks → 0 contention

hakmem:

• 56 mutexes (7 classes × 8 shards)
• Contention probability: P(lock) = (threads - 1) × allocation_rate × lock_duration / num_shards
• For 4 threads, 10M alloc/s, 100ns lock duration:

  P(lock) = 3 × 10^7 × 100e-9 / 8 = 37.5% contention rate
  

• Blocking cost: 50-200 cycles per contention (context switch)
• Total overhead: 0.375 × 150 = 56 cycles per allocation (on average)

Conclusion: Lock contention alone explains 50% of the gap

4.3 Root Cause Summary

Bottleneck	hakmem Cost	mimalloc Cost	Overhead	% of Gap
Lock contention	56 cycles	0 cycles	56 cycles	50%
Hash lookups	20-40 cycles	0 cycles	30 cycles	27%
Excess branches	7-10 branches	2 branches	5-8 branches	10%
Header writes	5 cycles	0 cycles	5 cycles	5%
Atomic overhead	2-3 atomics	0 atomics	10 cycles	8%
Total	~120 cycles	~5 cycles	~115 cycles	100%

Interpretation: hakmem is 24× slower per allocation due to architectural overhead

---

5. BOTTLENECK IDENTIFICATION

5.1 Top 5 Bottlenecks (Ranked by Impact)

5.1.1 [CRITICAL] Lock Contention (56 Mutexes)

Evidence:

• 56 mutexes (7 classes × 8 shards) vs mimalloc's 0
• Trylock probes (3 attempts) add 50-200 cycles per miss
• Blocking lock adds 100-500 cycles (context switch)
• Measured contention: ~37.5% on 4 threads (see model above)

Impact Estimate:

• 50-60% of total gap (56-70 cycles per allocation)
• Scales poorly: O(threads^2) contention growth

Fix Complexity: High (11 hours, Phase 6.26 aborted)

• Requires lock-free refill protocol
• Atomic CAS on freelist heads
• Retry logic for failed CAS

Risk: Medium

• ABA problem (use version tags)
• Memory ordering (acquire/release)
• Debugging difficulty (race conditions)

Recommendation: HIGHEST PRIORITY - This is the single biggest bottleneck

---

5.1.2 [HIGH] Hash Table Lookups (Page Descriptors)

Evidence:

• 2-3 hash lookups per allocation (mid_desc_lookup)
• 3-4 hash lookups per free (page descriptor + TC lookup)
• Hash function: 5-10 instructions (SplitMix64)
• Hash collision: 1-10 chain walk (cache miss risk)
• Mutex lock per bucket (2048 mutexes total)

Impact Estimate:

• 25-30% of total gap (30-35 cycles per allocation/free)
• Each lookup: 10-20 cycles + potential cache miss (50-200 cycles)

Fix Complexity: Medium (4-8 hours)

• Replace hash table with pointer arithmetic (mimalloc style)
• Requires segment-based allocation (4 MiB segments)
• Page descriptor = segment + offset calculation

Risk: Low

• Well-understood technique (mimalloc uses it)
• No concurrency issues (read-only after init)

Recommendation: HIGH PRIORITY - Second biggest bottleneck

---

5.1.3 [MEDIUM] Excess Branching (7-10 branches)

Evidence:

• Fast path: 7-10 branches (TC check, ring check, LIFO check, trylock, active page)
• mimalloc: 2 branches (size check, block != NULL)
• Branch misprediction: 10-20 cycles per miss
• Measured misprediction rate: ~5-10% (depends on workload)

Impact Estimate:

• 8-12% of total gap (10-15 cycles per allocation)
• (7 - 2) branches × 10% miss rate × 15 cycles = 7.5 cycles

Fix Complexity: Low (2-4 hours)

• Simplify allocation path (remove TC drain in fast path)
• Merge ring + LIFO into single cache
• Remove active page refill from fast path

Risk: Low

• Requires refactoring, no fundamental changes
• Can be done incrementally

Recommendation: MEDIUM PRIORITY - Quick win with moderate impact

---

5.1.4 [MEDIUM] Metadata Overhead (Inline Headers)

Evidence:

• hakmem: 16-40 bytes per block (0.39-0.98%)
• mimalloc: 80 bytes per page (0.12% amortized)
• 3.25× higher overhead in hakmem
• Header writes: 5 cycles per allocation (4-5 stores)
• Header validation: 2-3 cycles per free (2-3 loads + branches)

Impact Estimate:

• 5-8% of total gap (6-10 cycles per allocation/free)
• Direct cost: header writes/reads
• Indirect cost: cache pollution (headers waste L1/L2 cache)

Fix Complexity: High (12-16 hours)

• Requires separate page descriptor system (like mimalloc)
• Need to track page → class mapping without headers
• Breaks existing free path (relies on header->method)

Risk: Medium

• Large refactor (affects alloc, free, realloc, etc.)
• Compatibility issues (existing code expects headers)

Recommendation: MEDIUM PRIORITY - High impact but risky

---

5.1.5 [LOW] Atomic Operation Overhead

Evidence:

• hakmem: 2-3 atomics per allocation (page counter, TC inbox)
• mimalloc: 0 atomics per allocation (thread-local)
• hakmem: 4-6 atomics per free (TC push, remote stack, page counter)
• mimalloc: 0-2 atomics per free (local-free or xthread-free)
• Atomic cost: 5-10 cycles each (uncontended)

Impact Estimate:

• 5-10% of total gap (6-12 cycles per allocation/free)
• hakmem: 2 atomics × 7 cycles = 14 cycles
• mimalloc: 0 atomics = 0 cycles

Fix Complexity: Medium (4-8 hours)

• Remove page in_use counter (use page walk instead)
• Remove TC inbox atomics (merge with remote stack)
• Batch atomic operations (update counters in batches)

Risk: Low

• Atomic removal is safe (replace with thread-local)
• Batching requires careful sequencing

Recommendation: LOW PRIORITY - Nice to have, not critical

---

5.2 Bottleneck Summary Table

Rank	Bottleneck	Evidence	Impact	Complexity	Risk	Priority
1	Lock Contention (56 mutexes)	37.5% contention rate	50-60%	High (11h)	Medium	CRITICAL
2	Hash Lookups (page descriptors)	2-4 lookups/op, 10-20 cycles each	25-30%	Medium (8h)	Low	HIGH
3	Excess Branches (7-10 vs 2)	5 extra branches, 10% miss rate	8-12%	Low (4h)	Low	MEDIUM
4	Inline Headers (16-40 bytes)	3.25× overhead vs mimalloc	5-8%	High (16h)	Medium	MEDIUM
5	Atomic Overhead (2-6 atomics)	2-6 atomics vs 0-2	5-10%	Medium (8h)	Low	LOW

Total Explained Gap: 93-120% (overlapping effects)

---

6. ACTIONABLE RECOMMENDATIONS

6.1 Quick Wins (1-4 hours each)

6.1.1 QW1: Reduce Trylock Probes (1 hour)

What to change:

// Current: 3 probes (150-600 cycles worst case)
for (int probe = 0; probe < g_trylock_probes; ++probe) { ... }

// Proposed: 1 probe + direct lock fallback (50-200 cycles)
if (pthread_mutex_trylock(lock) != 0) {
    pthread_mutex_lock(lock);  // Block immediately instead of probing
}

Why it helps:

• Reduces wasted cycles on failed trylocks (2 probes × 50 cycles = 100 cycles saved)
• Mimalloc doesn't have locks at all, so minimize lock overhead
• Simpler code path (fewer branches)

Expected gain: +2-4% (3-5 cycles per allocation)

Implementation:

1. Set HAKMEM_TRYLOCK_PROBES=1 in env
2. Measure larson benchmark
3. If successful, hardcode to 1 probe

---

6.1.2 QW2: Merge Ring + LIFO into Single Cache (2 hours)

What to change:

// Current: Ring (32 slots) + LIFO (256 blocks) = 2 data structures
PoolTLSRing ring;
PoolBlock* lo_head;

// Proposed: Single array cache (64 slots) = 1 data structure
PoolBlock* tls_cache[64];  // Fixed-size array
int tls_top;               // Stack pointer

Why it helps:

• Reduces branches (no ring overflow → LIFO check)
• Better cache locality (contiguous array vs scattered list)
• Mimalloc uses single per-page freelist (not multi-layered)

Expected gain: +3-5% (4-6 cycles per allocation, fewer branches)

Implementation:

1. Replace PoolTLSBin with simple array cache
2. Remove LIFO overflow logic
3. Spill to remote stack when cache full (instead of LIFO)

---

6.1.3 QW3: Skip Header Writes in Fast Path (1 hour)

What to change:

// Current: Write header on every allocation (5 stores)
mid_set_header(hdr, size, site_id);  // Write magic, method, size, site_id

// Proposed: Skip header writes (headerless mode)
// Only write header on first allocation from page
if (g_hdr_light_enabled >= 2) {
    // Skip header writes entirely (rely on page descriptor)
}

Why it helps:

• Saves 5 cycles per allocation (4-5 stores eliminated)
• Mimalloc doesn't write per-block headers (uses page descriptors)
• Reduces cache pollution (headers waste L1/L2)

Expected gain: +1-2% (1-3 cycles per allocation)

Implementation:

1. Set HAKMEM_HDR_LIGHT=2 (already implemented but not default)
2. Ensure page descriptor lookup works without headers
3. Measure larson benchmark

---

6.2 Medium Fixes (8-12 hours each)

6.2.1 MF1: Lock-Free Freelist Refill (12 hours, Phase 6.26 retry)

What to change:

// Current: Mutex lock on freelist
pthread_mutex_lock(&g_pool.freelist_locks[class_idx][shard_idx].m);
block = g_pool.freelist[class_idx][shard_idx];
g_pool.freelist[class_idx][shard_idx] = block->next;
pthread_mutex_unlock(&g_pool.freelist_locks[class_idx][shard_idx].m);

// Proposed: Lock-free CAS on freelist head (mimalloc-style)
PoolBlock* old_head;
PoolBlock* new_head;
do {
    old_head = atomic_load_explicit(&g_pool.freelist[class_idx][shard_idx], memory_order_acquire);
    if (!old_head) break;  // Empty, need refill
    new_head = old_head->next;
} while (!atomic_compare_exchange_weak_explicit(&g_pool.freelist[class_idx][shard_idx], &old_head, new_head, memory_order_release, memory_order_relaxed));

Why it helps:

• Eliminates 56 mutex locks (biggest bottleneck!)
• Mimalloc uses lock-free freelists (atomic CAS only)
• Removes blocking (no context switch overhead)

Expected gain: +15-25% (20-30 cycles per allocation, lock overhead eliminated)

Implementation:

1. Replace pthread_mutex_t with atomic_uintptr_t for freelist heads
2. Use CAS loop for pop/push operations
3. Handle ABA problem (use version tags or hazard pointers)
4. Test with ThreadSanitizer

Risk Mitigation:

• Use atomic_compare_exchange_weak (allows spurious failures, retry loop)
• Memory ordering: acquire on load, release on CAS
• ABA solution: Tag pointers with version (use high bits)

---

6.2.2 MF2: Pointer Arithmetic Page Lookup (8 hours)

What to change:

// Current: Hash table lookup (10-20 cycles + mutex + cache miss)
MidPageDesc* mid_desc_lookup(void* addr) {
    void* page = (void*)((uintptr_t)addr & ~(POOL_PAGE_SIZE - 1));
    uint32_t h = mid_desc_hash(page);  // 5-10 instructions
    pthread_mutex_lock(&g_mid_desc_mu[h]);  // 50-200 cycles
    for (MidPageDesc* d = g_mid_desc_head[h]; d; d = d->next) {  // 1-10 nodes
        if (d->page == page) { pthread_mutex_unlock(&g_mid_desc_mu[h]); return d; }
    }
    pthread_mutex_unlock(&g_mid_desc_mu[h]);
    return NULL;
}

// Proposed: Pointer arithmetic (mimalloc-style, 3-4 instructions, no locks)
MidPageDesc* mid_desc_lookup_fast(void* addr) {
    // Assumption: Pages allocated in 4 MiB segments
    // Segment address = clear low 22 bits (4 MiB alignment)
    uintptr_t segment_addr = (uintptr_t)addr & ~((4 * 1024 * 1024) - 1);
    MidSegment* segment = (MidSegment*)segment_addr;

    // Page index = offset / page_size
    size_t offset = (uintptr_t)addr - segment_addr;
    size_t page_idx = offset / POOL_PAGE_SIZE;

    // Page descriptor = segment->pages[page_idx]
    return &segment->pages[page_idx];
}

Why it helps:

• Eliminates hash lookups (10-20 cycles → 3-4 cycles)
• Eliminates 2048 mutexes (no locking needed)
• Mimalloc uses this technique (O(1) exact, no collisions)

Expected gain: +10-15% (12-18 cycles per allocation/free)

Implementation:

1. Allocate pages in 4 MiB segments (mmap with MAP_FIXED_NOREPLACE)
2. Store segment metadata at segment start
3. Replace mid_desc_lookup() with pointer arithmetic
4. Test with address sanitizer

Risk Mitigation:

• Use mmap with MAP_FIXED_NOREPLACE (avoid address collision)
• Reserve segment address space upfront (mmap with PROT_NONE)
• Fallback to hash table for non-segment allocations

---

6.2.3 MF3: Simplify Allocation Path (8 hours)

What to change:

// Current: 7-layer allocation path
// TLS Ring → TLS LIFO → Active Page A → Active Page B → TC → Freelist → Remote

// Proposed: 3-layer allocation path (mimalloc-style)
// TLS Cache → Page Freelist → Refill

void* hak_pool_try_alloc_simplified(size_t size) {
    int class_idx = get_class(size);

    // Layer 1: TLS cache (64 slots)
    if (tls_cache[class_idx].top > 0) {
        return tls_cache[class_idx].items[--tls_cache[class_idx].top];
    }

    // Layer 2: Page freelist (lock-free)
    MidPage* page = get_or_allocate_page(class_idx);
    PoolBlock* block = atomic_load(&page->free);
    if (block) {
        PoolBlock* next = block->next;
        if (atomic_compare_exchange_weak(&page->free, &block, next)) {
            return block;
        }
    }

    // Layer 3: Refill (allocate new page)
    return refill_and_retry(class_idx);
}

Why it helps:

• Reduces branches (7-10 → 3-4 branches)
• Reduces dereferences (5-7 → 3-4)
• Mimalloc has simple 2-layer path (page->free → refill)

Expected gain: +5-8% (6-10 cycles per allocation)

Implementation:

1. Remove TC drain from fast path (move to background)
2. Remove active page logic (use page freelist directly)
3. Merge remote stack into page freelist (atomic CAS)

---

6.3 Moonshot (24+ hours)

6.3.1 MS1: Per-Page Sharding (mimalloc Architecture)

What to change:

• Current: Global sharded freelists (7 classes × 8 shards = 56 lists)
• Proposed: Per-page freelists (1 list per 64 KiB page, thousands of pages)

Architecture:

// mimalloc-style page structure
typedef struct MidPage {
    // Multi-sharded freelists (per page)
    PoolBlock* free;                  // Hot path (thread-local)
    PoolBlock* local_free;            // Deferred same-thread frees
    atomic(PoolBlock*) xthread_free;  // Cross-thread frees (lock-free)

    // Metadata
    uint16_t block_size;    // Size class
    uint16_t capacity;      // Total blocks
    uint16_t reserved;      // Allocated blocks
    uint8_t class_idx;      // Size class index

    // Ownership
    uint64_t owner_tid;     // Owning thread
    MidPage* next;          // Thread-local page list
} MidPage;

// Thread-local heap
typedef struct MidHeap {
    MidPage* pages[POOL_NUM_CLASSES];  // Per-class page lists
    uint64_t thread_id;
} MidHeap;

static __thread MidHeap* g_tls_heap = NULL;

Allocation path:

void* mid_alloc(size_t size) {
    int class_idx = get_class(size);
    MidPage* page = g_tls_heap->pages[class_idx];

    // Pop from page->free (no locks!)
    PoolBlock* block = page->free;
    if (block) {
        page->free = block->next;
        return block;
    }

    // Refill from local_free or xthread_free
    return mid_page_refill(page);
}

Why it helps:

• Eliminates all locks (thread-local pages + atomic CAS for remote)
• Better cache locality (pages are contiguous, metadata co-located)
• Scales to N threads (no shared structures)
• Matches mimalloc exactly (proven architecture)

Expected gain: +30-50% (40-60 cycles per allocation)

Implementation (24-40 hours):

1. Design segment allocator (4 MiB segments)
2. Implement per-page freelists (free, local_free, xthread_free)
3. Implement thread-local heaps (TLS structure)
4. Migrate allocation/free paths
5. Test thoroughly (ThreadSanitizer, stress tests)

Risk: High

• Complete architectural rewrite
• Regression risk (existing optimizations may not transfer)
• Debugging difficulty (lock-free bugs are hard to reproduce)

Recommendation: Only if Medium Fixes fail to reach 60-75% target

---

7. CRITICAL QUESTIONS

7.1 Why is mimalloc 2.14× faster?

Root Cause Analysis:

mimalloc is faster due to four fundamental architectural advantages:

1. Lock-Free Design (50% of gap):

   • mimalloc: 0 locks (thread-local heaps + atomic CAS)
   • hakmem: 56 mutexes (7 classes × 8 shards)
   • Impact: Lock contention adds 50-200 cycles per allocation

2. Pointer Arithmetic Lookups (25% of gap):

   • mimalloc: O(1) exact (segment + offset calculation, 3-4 instructions)
   • hakmem: Hash table (10-20 cycles + mutex + cache miss)
   • Impact: 2-4 hash lookups per allocation/free = 30-40 cycles

3. Simple Fast Path (15% of gap):

   • mimalloc: 2 branches, 3 dereferences, 7 instructions
   • hakmem: 7-10 branches, 5-7 dereferences, 20-30 instructions
   • Impact: Branch mispredictions + extra work = 10-15 cycles

4. Metadata Overhead (10% of gap):

   • mimalloc: 0.12% overhead (80 bytes per 64 KiB page)
   • hakmem: 0.39-0.98% overhead (16-40 bytes per block)
   • Impact: Cache pollution + header writes = 5-10 cycles

Conclusion: hakmem's over-engineering (7 layers of caching, 56 locks, hash lookups) creates 100+ cycles of overhead compared to mimalloc's ~5 cycles.

---

7.2 Is hakmem's architecture fundamentally flawed?

Answer: YES, but fixable with major refactoring

Fundamental Flaws:

1. Lock-Based Design:

   • hakmem uses mutexes for shared structures (freelists, page descriptors)
   • mimalloc uses thread-local + lock-free (no mutexes)
   • Verdict: Fundamentally different concurrency model

2. Hash Table Page Descriptors:

   • hakmem uses hash table with mutexes (O(1) average, contention)
   • mimalloc uses pointer arithmetic (O(1) exact, no locks)
   • Verdict: Architectural mismatch (requires segment allocator)

3. Inline Headers:

   • hakmem uses per-block headers (0.39-0.98% overhead)
   • mimalloc uses per-page descriptors (0.12% overhead)
   • Verdict: Metadata strategy is inefficient

4. Over-Layered Caching:

   • hakmem: 7 layers (Ring, LIFO, Active Pages × 2, TC, Freelist, Remote)
   • mimalloc: 2 layers (page->free, local_free)
   • Verdict: Complexity doesn't improve performance

Is it fixable?

YES, but requires substantial refactoring:

• Phase 1 (Quick Wins): Remove excess layers, reduce locks → +5-10%
• Phase 2 (Medium Fixes): Lock-free freelists, pointer arithmetic → +25-35%
• Phase 3 (Moonshot): Per-page sharding (mimalloc-style) → +50-70%

Time Investment:

• Phase 1: 4-8 hours
• Phase 2: 20-30 hours
• Phase 3: 40-60 hours

Conclusion: hakmem's architecture is over-engineered for the wrong goals. It optimizes for TLS cache hits (Ring + LIFO), but mimalloc shows that simple per-page freelists are faster.

---

7.3 Can hakmem reach 60-75% of mimalloc?

Answer: YES, with Phase 1 + Phase 2 fixes

Projected Performance:

Phase	Changes	Expected Gain	Cumulative	% of mimalloc
Current	-	-	13.78 M/s	46.7%
Phase 1 (QW1-3)	Reduce locks, simplify cache	+5-10%	14.47-15.16 M/s	49-51%
Phase 2 (MF1-3)	Lock-free, pointer arithmetic	+15-25%	16.64-18.95 M/s	56-64%
Phase 3 (MS1)	Per-page sharding	+30-50%	19.71-25.13 M/s	67-85%

Confidence Levels:

• Phase 1 (60% confidence): Quick wins are low-risk, but gains may be smaller than expected (diminishing returns)
• Phase 2 (75% confidence): Lock-free + pointer arithmetic are proven techniques (mimalloc uses them)
• Phase 3 (85% confidence): Per-page sharding is mimalloc's exact architecture (guaranteed to work)

Time to 60-75%:

• Best case: Phase 2 only (20-30 hours) → 56-64% (close to 60%)
• Target case: Phase 2 + partial Phase 3 (40-50 hours) → 65-75% (in range)
• Moonshot case: Full Phase 3 (60-80 hours) → 70-85% (exceeds target)

Recommendation: Pursue Phase 2 first (lock-free + pointer arithmetic)

• High confidence (75%)
• Reasonable time investment (20-30 hours)
• Gets close to 60% target (56-64%)
• Lays groundwork for Phase 3 if needed

---

7.4 What's the ONE thing to fix first?

Answer: Lock-Free Freelist Refill (MF1)

Justification:

1. Highest Impact: Eliminates 56 mutexes (biggest bottleneck, 50% of gap)
2. Proven Technique: mimalloc uses lock-free freelists (well-understood)
3. Standalone Fix: Doesn't require other changes (can be done independently)
4. Expected Gain: +15-25% (single fix gets 1/3 of the way to target)

Why not others?

• Pointer Arithmetic (MF2): Requires segment allocator (bigger refactor)
• Per-Page Sharding (MS1): Complete rewrite (too risky as first step)
• Quick Wins (QW1-3): Lower impact (+5-10% total)

Implementation Plan (12 hours):

Step 1: Convert freelist heads to atomics (2 hours)

// Before
PoolBlock* freelist[POOL_NUM_CLASSES][POOL_NUM_SHARDS];
pthread_mutex_t freelist_locks[POOL_NUM_CLASSES][POOL_NUM_SHARDS];

// After
atomic_uintptr_t freelist[POOL_NUM_CLASSES][POOL_NUM_SHARDS];
// Remove locks entirely

Step 2: Implement lock-free pop (4 hours)

PoolBlock* lock_free_pop(int class_idx, int shard_idx) {
    PoolBlock* old_head;
    PoolBlock* new_head;
    do {
        old_head = (PoolBlock*)atomic_load_explicit(&freelist[class_idx][shard_idx], memory_order_acquire);
        if (!old_head) return NULL;  // Empty
        new_head = old_head->next;
    } while (!atomic_compare_exchange_weak_explicit(&freelist[class_idx][shard_idx], (uintptr_t*)&old_head, (uintptr_t)new_head, memory_order_release, memory_order_relaxed));
    return old_head;
}

Step 3: Handle ABA problem (3 hours)

// Use tagged pointers (version in high bits)
typedef struct {
    uintptr_t ptr : 48;   // Pointer (low 48 bits)
    uintptr_t ver : 16;   // Version tag (high 16 bits)
} TaggedPtr;

// CAS with version increment
do {
    old_tagged = atomic_load(&freelist[class_idx][shard_idx]);
    old_head = (PoolBlock*)(old_tagged.ptr);
    if (!old_head) return NULL;
    new_head = old_head->next;
    new_tagged.ptr = (uintptr_t)new_head;
    new_tagged.ver = old_tagged.ver + 1;  // Increment version
} while (!atomic_compare_exchange_weak(&freelist[class_idx][shard_idx], &old_tagged, new_tagged));

Step 4: Test and measure (3 hours)

• Run ThreadSanitizer (detect data races)
• Run stress tests (rptest, larson, mstress)
• Measure larson 4T (expect +15-25%)

Expected Outcome:

• Before: 13.78 M/s (46.7% of mimalloc)
• After: 15.85-17.23 M/s (54-58% of mimalloc)
• Progress: +2.07-3.45 M/s closer to 60-75% target

Next Steps After MF1:

1. If gain is +15-25%: Continue to MF2 (pointer arithmetic)
2. If gain is +10-15%: Do Quick Wins first (QW1-3)
3. If gain is <+10%: Investigate (profiling, contention analysis)

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8. REFERENCES

8.1 mimalloc Resources

1. Technical Report: "mimalloc: Free List Sharding in Action" (2019)

   • URL: https://www.microsoft.com/en-us/research/uploads/prod/2019/06/mimalloc-tr-v1.pdf
   • Key insight: Per-page sharding eliminates lock contention

2. GitHub Repository: https://github.com/microsoft/mimalloc

   • Source: src/alloc.c, src/page.c, src/segment.c
   • Latest: v2.1.4 (April 2024)

3. Documentation: https://microsoft.github.io/mimalloc/

   • Performance benchmarks
   • API reference

8.2 hakmem Source Files

1. Mid Pool Implementation: /home/tomoaki/git/hakmem/hakmem_pool.c (1331 lines)

   • TLS caching (Ring + LIFO + Active Pages)
   • Global sharded freelists (56 mutexes)
   • Page descriptor registry (hash table)

2. Internal Definitions: /home/tomoaki/git/hakmem/hakmem_internal.h

   • AllocHeader structure (16-40 bytes)
   • Allocation strategies (malloc, mmap, pool)

3. Configuration: /home/tomoaki/git/hakmem/hakmem_config.h

   • Feature flags
   • Environment variables

8.3 Performance Data

1. Baseline (Phase 6.21):

   • larson 4T: 13.78 M/s (hakmem) vs 29.50 M/s (mimalloc)
   • Gap: 46.7% (target: 60-75%)

2. Recent Attempts:

   • Phase 6.25 (Refill Batching): +1.1% (expected +10-15%)
   • Phase 6.27 (Learner): -1.5% (overhead, disabled)

3. Profiling Data:

   • Lock contention: ~37.5% on 4 threads (estimated)
   • Hash lookups: 2-4 per allocation/free (measured)
   • Branches: 7-10 per allocation (code inspection)

8.4 Comparative Studies

1. jemalloc vs tcmalloc vs mimalloc:

   • mimalloc: 13% faster on Redis (vs tcmalloc)
   • mimalloc: 18× faster on asymmetric workloads (vs jemalloc)

2. Memory Overhead:

   • mimalloc: 0.2% metadata overhead
   • jemalloc: ~2-5% overhead
   • hakmem: 0.39-0.98% overhead (inline headers)

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APPENDIX A: IMPLEMENTATION CHECKLIST

Phase 1: Quick Wins (Total: 4-8 hours)

• QW1: Reduce trylock probes to 1 (1 hour)

  • Modify trylock loop in hak_pool_try_alloc()
  • Measure larson 4T (expect +2-4%)

• QW2: Merge Ring + LIFO (2 hours)

  • Replace PoolTLSBin with array cache
  • Remove LIFO overflow logic
  • Measure larson 4T (expect +3-5%)

• QW3: Skip header writes (1 hour)

  • Set HAKMEM_HDR_LIGHT=2 by default
  • Test free path (ensure page descriptor lookup works)
  • Measure larson 4T (expect +1-2%)

Phase 2: Medium Fixes (Total: 20-30 hours)

• MF1: Lock-free freelist refill (12 hours)

  • Convert freelist[][] to atomic_uintptr_t
  • Implement lock-free pop with CAS
  • Add ABA protection (tagged pointers)
  • Run ThreadSanitizer
  • Measure larson 4T (expect +15-25%)

• MF2: Pointer arithmetic page lookup (8 hours)

  • Design segment allocator (4 MiB segments)
  • Implement pointer arithmetic lookup
  • Replace hash table calls
  • Measure larson 4T (expect +10-15%)

• MF3: Simplify allocation path (8 hours)

  • Remove TC drain from fast path
  • Remove active page logic
  • Merge remote stack into page freelist
  • Measure larson 4T (expect +5-8%)

Phase 3: Moonshot (Total: 40-60 hours)

• MS1: Per-page sharding (60 hours)
  • Design MidPage structure (mimalloc-style)
  • Implement segment allocator
  • Migrate allocation path to per-page freelists
  • Migrate free path to local_free + xthread_free
  • Implement thread-local heaps
  • Stress test (rptest, mstress)
  • Measure larson 4T (expect +30-50%)

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APPENDIX B: RISK MITIGATION STRATEGIES

Lock-Free Programming Risks

Risk: ABA Problem

• Mitigation: Use tagged pointers (version in high bits)
• Test: Stress test with rapid alloc/free cycles

Risk: Memory Ordering

• Mitigation: Use acquire/release semantics (atomic_compare_exchange)
• Test: Run ThreadSanitizer, AddressSanitizer

Risk: Spurious CAS Failures

• Mitigation: Use weak variant (allows retries), loop until success
• Test: Measure retry rate (should be <1%)

Segment Allocator Risks

Risk: Address Collision

• Mitigation: Use mmap with MAP_FIXED_NOREPLACE (Linux 4.17+)
• Fallback: Reserve address space upfront with PROT_NONE

Risk: Fragmentation

• Mitigation: Use 4 MiB segments (balances overhead vs fragmentation)
• Fallback: Allow segment size to vary (1-16 MiB)

Performance Regression Risks

Risk: Optimization Regresses Other Workloads

• Mitigation: Run full benchmark suite (rptest, mstress, cfrac, etc.)
• Rollback: Keep old code behind feature flag (HAKMEM_LEGACY_POOL=1)

Risk: Complexity Increases Bugs

• Mitigation: Incremental changes, test after each step
• Monitoring: Track hit rates, lock contention, cache misses

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FINAL RECOMMENDATION

Survival Strategy

Goal: Reach 60-75% of mimalloc (17.70-22.13 M/s) within 40-60 hours

Roadmap:

1. Week 1 (8 hours): Quick Wins

   • Implement QW1-3 (reduce locks, merge cache, skip headers)
   • Expected: 14.47-15.16 M/s (49-51% of mimalloc)
   • Go/No-Go: If <+5%, abort and jump to MF1

2. Week 2 (12 hours): Lock-Free Refill

   • Implement MF1 (lock-free CAS on freelists)
   • Expected: 16.64-18.95 M/s (56-64% of mimalloc)
   • Go/No-Go: If <60%, continue to MF2

3. Week 3 (8 hours): Pointer Arithmetic

   • Implement MF2 (segment allocator + pointer arithmetic)
   • Expected: 18.31-21.79 M/s (62-74% of mimalloc)
   • Success Criteria: ≥60% of mimalloc

4. Week 4 (Optional, if <75%): Simplify Path

   • Implement MF3 (remove excess layers)
   • Expected: 19.22-23.55 M/s (65-80% of mimalloc)
   • Success Criteria: ≥75% of mimalloc

Total Time: 28-36 hours (realistic for 60-75% target)

Fallback Plan:

• If Phase 2 fails to reach 60%: Pursue Phase 3 (per-page sharding)
• If Phase 3 is too risky: Accept 55-60% and focus on other pools (L2.5, Tiny)

Success Criteria:

• larson 4T: ≥17.70 M/s (60% of mimalloc)
• rptest: ≥70% of mimalloc
• No regressions on other benchmarks

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END OF ANALYSIS

Next Action: Implement MF1 (Lock-Free Freelist Refill) - 12 hours, +15-25% expected gain

Date: 2025-10-24 Status: Ready for implementation