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+# sll_refill_small_from_ss() Bottleneck Analysis
+
+**Date**: 2025-11-05
+**Context**: Refill takes 19,624 cycles (89.6% of execution time), limiting throughput to 1.59M ops/s vs 1.68M baseline
+
+---
+
+## Executive Summary
+
+**Root Cause**: `superslab_refill()` is a **298-line monster** consuming **28.56% CPU time** with:
+- 5 expensive paths (adopt/freelist/virgin/registry/mmap)
+- 4 `getenv()` calls in hot path
+- Multiple nested loops with atomic operations
+- O(n) linear searches despite P0 optimization
+
+**Impact**:
+- Refill: 19,624 cycles (89.6% of execution time)
+- Fast path: 143 cycles (10.4% of execution time)
+- Refill frequency: 6.3% but dominates performance
+
+**Optimization Potential**: **+50-100% throughput** (1.59M → 2.4-3.2M ops/s)
+
+---
+
+## Call Chain Analysis
+
+### Current Flow
+
+```
+tiny_alloc_fast_pop()  [143 cycles, 10.4%]
+  ↓ Miss (6.3% of calls)
+tiny_alloc_fast_refill()
+  ↓
+sll_refill_small_from_ss()  ← Aliased to sll_refill_batch_from_ss()
+  ↓
+sll_refill_batch_from_ss()  [19,624 cycles, 89.6%]
+  │
+  ├─ trc_pop_from_freelist()       [~50 cycles]
+  ├─ trc_linear_carve()            [~100 cycles]
+  ├─ trc_splice_to_sll()           [~30 cycles]
+  └─ superslab_refill() ───────────► [19,400+ cycles] 💥 BOTTLENECK
+       │
+       ├─ getenv() × 4              [~400 cycles each = 1,600 total]
+       ├─ Adopt path                [~5,000 cycles]
+       │   ├─ ss_partial_adopt()    [~1,000 cycles]
+       │   ├─ Scoring loop (32×)    [~2,000 cycles]
+       │   ├─ slab_try_acquire()    [~500 cycles - atomic CAS]
+       │   └─ slab_drain_remote()   [~1,500 cycles]
+       │
+       ├─ Freelist scan             [~3,000 cycles]
+       │   ├─ nonempty_mask build   [~500 cycles]
+       │   ├─ ctz loop (32×)        [~800 cycles]
+       │   ├─ slab_try_acquire()    [~500 cycles - atomic CAS]
+       │   └─ slab_drain_remote()   [~1,500 cycles]
+       │
+       ├─ Virgin slab search        [~800 cycles]
+       │   └─ superslab_find_free() [~500 cycles]
+       │
+       ├─ Registry scan             [~4,000 cycles]
+       │   ├─ Loop (256 entries)    [~2,000 cycles]
+       │   ├─ Atomic loads × 512    [~1,500 cycles]
+       │   └─ freelist scan         [~500 cycles]
+       │
+       ├─ Must-adopt gate           [~2,000 cycles]
+       └─ superslab_allocate()      [~4,000 cycles]
+           └─ mmap() syscall        [~3,500 cycles]
+```
+
+---
+
+## Detailed Breakdown: superslab_refill()
+
+### File Location
+- **Path**: `/home/user/hakmem_private/core/hakmem_tiny_free.inc`
+- **Lines**: 686-984 (298 lines)
+- **Complexity**:
+  - 15+ branches
+  - 4 nested loops
+  - 50+ atomic operations (worst case)
+  - 4 getenv() calls
+
+### Cost Breakdown by Path
+
+| Path | Lines | Cycles | % of superslab_refill | Frequency |
+|------|-------|--------|----------------------|-----------|
+| **getenv × 4** | 693, 704, 835 | ~1,600 | 8% | 100% |
+| **Adopt path** | 759-825 | ~5,000 | 26% | ~40% |
+| **Freelist scan** | 828-886 | ~3,000 | 15% | ~80% |
+| **Virgin slab** | 888-903 | ~800 | 4% | ~60% |
+| **Registry scan** | 906-939 | ~4,000 | 21% | ~20% |
+| **Must-adopt gate** | 943-944 | ~2,000 | 10% | ~10% |
+| **mmap** | 948-983 | ~4,000 | 21% | ~5% |
+| **Total** | - | **~19,400** | **100%** | - |
+
+---
+
+## Critical Bottlenecks
+
+### 1. getenv() Calls in Hot Path (Priority 1) 🔥🔥🔥
+
+**Problem:**
+```c
+// Line 693: Called on EVERY refill!
+if (g_ss_adopt_en == -1) {
+    char* e = getenv("HAKMEM_TINY_SS_ADOPT");  // ~400 cycles!
+    g_ss_adopt_en = (*e != '0') ? 1 : 0;
+}
+
+// Line 704: Another getenv()
+if (g_adopt_cool_period == -1) {
+    char* cd = getenv("HAKMEM_TINY_SS_ADOPT_COOLDOWN");  // ~400 cycles!
+    // ...
+}
+
+// Line 835: INSIDE freelist scan loop!
+if (__builtin_expect(g_mask_en == -1, 0)) {
+    const char* e = getenv("HAKMEM_TINY_FREELIST_MASK");  // ~400 cycles!
+    // ...
+}
+```
+
+**Cost**:
+- Each `getenv()`: ~400 cycles (syscall-like overhead)
+- Total: **1,600 cycles** (8% of superslab_refill)
+
+**Why it's slow**:
+- `getenv()` scans entire `environ` array linearly
+- Involves string comparisons
+- Not cached by libc (must scan every time)
+
+**Fix**: Cache at init time
+```c
+// In hakmem_tiny_init.c (ONCE at startup)
+static int g_ss_adopt_en = 0;
+static int g_adopt_cool_period = 0;
+static int g_mask_en = 0;
+
+void tiny_init_env_cache(void) {
+    const char* e = getenv("HAKMEM_TINY_SS_ADOPT");
+    g_ss_adopt_en = (e && *e != '0') ? 1 : 0;
+
+    e = getenv("HAKMEM_TINY_SS_ADOPT_COOLDOWN");
+    g_adopt_cool_period = e ? atoi(e) : 0;
+
+    e = getenv("HAKMEM_TINY_FREELIST_MASK");
+    g_mask_en = (e && *e != '0') ? 1 : 0;
+}
+```
+
+**Expected gain**: **+8-10%** (1,600 cycles saved)
+
+---
+
+### 2. Adopt Path Overhead (Priority 2) 🔥🔥
+
+**Problem:**
+```c
+// Lines 769-825: Complex adopt logic
+SuperSlab* adopt = ss_partial_adopt(class_idx);  // ~1,000 cycles
+if (adopt && adopt->magic == SUPERSLAB_MAGIC) {
+    int best = -1;
+    uint32_t best_score = 0;
+    int adopt_cap = ss_slabs_capacity(adopt);
+
+    // Loop through ALL 32 slabs, scoring each
+    for (int s = 0; s < adopt_cap; s++) {  // ~2,000 cycles
+        TinySlabMeta* m = &adopt->slabs[s];
+        uint32_t rc = atomic_load_explicit(&adopt->remote_counts[s], ...);  // atomic!
+        int has_remote = (atomic_load_explicit(&adopt->remote_heads[s], ...));  // atomic!
+        uint32_t score = rc + (m->freelist ? (1u<<30) : 0u) + (has_remote ? 1u : 0u);
+        // ... 32 iterations of atomic loads + arithmetic
+    }
+
+    if (best >= 0) {
+        SlabHandle h = slab_try_acquire(adopt, best, self);  // CAS - ~500 cycles
+        if (slab_is_valid(&h)) {
+            slab_drain_remote_full(&h);  // Drain remote queue - ~1,500 cycles
+            // ...
+        }
+    }
+}
+```
+
+**Cost**:
+- Scoring loop: 32 slabs × (2 atomic loads + arithmetic) = ~2,000 cycles
+- CAS acquire: ~500 cycles
+- Remote drain: ~1,500 cycles
+- **Total: ~5,000 cycles** (26% of superslab_refill)
+
+**Why it's slow**:
+- Unnecessary work: scoring ALL slabs even if first one has freelist
+- Atomic loads in loop (cache line bouncing)
+- Remote drain even when not needed
+
+**Fix**: Early exit + lazy scoring
+```c
+// Option A: First-fit (exit on first freelist)
+for (int s = 0; s < adopt_cap; s++) {
+    if (adopt->slabs[s].freelist) {  // No atomic load!
+        SlabHandle h = slab_try_acquire(adopt, s, self);
+        if (slab_is_valid(&h)) {
+            // Only drain if actually adopting
+            slab_drain_remote_full(&h);
+            tiny_tls_bind_slab(tls, h.ss, h.slab_idx);
+            return h.ss;
+        }
+    }
+}
+
+// Option B: Use nonempty_mask (already computed in P0)
+uint32_t mask = adopt->nonempty_mask;
+while (mask) {
+    int s = __builtin_ctz(mask);
+    mask &= ~(1u << s);
+    // Try acquire...
+}
+```
+
+**Expected gain**: **+15-20%** (3,000-4,000 cycles saved)
+
+---
+
+### 3. Registry Scan Overhead (Priority 3) 🔥
+
+**Problem:**
+```c
+// Lines 906-939: Linear scan of registry
+extern SuperRegEntry g_super_reg[];
+int scanned = 0;
+const int scan_max = tiny_reg_scan_max();  // Default: 256
+
+for (int i = 0; i < SUPER_REG_SIZE && scanned < scan_max; i++) {  // 256 iterations!
+    SuperRegEntry* e = &g_super_reg[i];
+    uintptr_t base = atomic_load_explicit((_Atomic uintptr_t*)&e->base, ...);  // atomic!
+    if (base == 0) continue;
+    SuperSlab* ss = atomic_load_explicit(&e->ss, ...);  // atomic!
+    if (!ss || ss->magic != SUPERSLAB_MAGIC) continue;
+    if ((int)ss->size_class != class_idx) { scanned++; continue; }
+
+    // Inner loop: scan slabs
+    int reg_cap = ss_slabs_capacity(ss);
+    for (int s = 0; s < reg_cap; s++) {  // 32 iterations
+        if (ss->slabs[s].freelist) {
+            // Try acquire...
+        }
+    }
+}
+```
+
+**Cost**:
+- Outer loop: 256 iterations × 2 atomic loads = ~2,000 cycles
+- Cache misses on registry entries = ~1,000 cycles
+- Inner loop: 32 × freelist check = ~500 cycles
+- **Total: ~4,000 cycles** (21% of superslab_refill)
+
+**Why it's slow**:
+- Linear scan of 256 entries
+- 2 atomic loads per entry (base + ss)
+- Cache pollution from scanning large array
+
+**Fix**: Per-class registry + early termination
+```c
+// Option A: Per-class registry (index by class_idx)
+SuperRegEntry g_super_reg_by_class[TINY_NUM_CLASSES][32];  // 8 classes × 32 entries
+
+// Scan only this class's registry (32 entries instead of 256)
+for (int i = 0; i < 32; i++) {
+    SuperRegEntry* e = &g_super_reg_by_class[class_idx][i];
+    // ... only 32 iterations, all same class
+}
+
+// Option B: Early termination (stop after first success)
+// Current code continues scanning even after finding a slab
+// Add: break; after successful adoption
+```
+
+**Expected gain**: **+10-12%** (2,000-2,500 cycles saved)
+
+---
+
+### 4. Freelist Scan with Excessive Drain (Priority 2) 🔥🔥
+
+**Problem:**
+```c
+// Lines 828-886: Freelist scan with O(1) ctz, but heavy drain
+while (__builtin_expect(nonempty_mask != 0, 1)) {
+    int i = __builtin_ctz(nonempty_mask);  // O(1) - good!
+    nonempty_mask &= ~(1u << i);
+
+    uint32_t self_tid = tiny_self_u32();
+    SlabHandle h = slab_try_acquire(tls->ss, i, self_tid);  // CAS - ~500 cycles
+    if (slab_is_valid(&h)) {
+        if (slab_remote_pending(&h)) {  // CHECK remote
+            slab_drain_remote_full(&h);  // ALWAYS drain - ~1,500 cycles
+            // ... then release and continue!
+            slab_release(&h);
+            continue;  // Doesn't even use this slab!
+        }
+        // ... bind
+    }
+}
+```
+
+**Cost**:
+- CAS acquire: ~500 cycles
+- Drain remote (even if not using slab): ~1,500 cycles
+- Release + retry: ~200 cycles
+- **Total per iteration: ~2,200 cycles**
+- **Worst case (32 slabs)**: ~70,000 cycles 💀
+
+**Why it's slow**:
+- Drains remote queue even when NOT adopting the slab
+- Continues to next slab after draining (wasted work)
+- No fast path for "clean" slabs (no remote pending)
+
+**Fix**: Skip drain if remote pending (lazy drain)
+```c
+// Option A: Skip slabs with remote pending
+if (slab_remote_pending(&h)) {
+    slab_release(&h);
+    continue;  // Try next slab (no drain!)
+}
+
+// Option B: Only drain if we're adopting
+SlabHandle h = slab_try_acquire(tls->ss, i, self_tid);
+if (slab_is_valid(&h) && !slab_remote_pending(&h)) {
+    // Adopt this slab
+    tiny_drain_freelist_to_sll_once(h.ss, h.slab_idx, class_idx);
+    tiny_tls_bind_slab(tls, h.ss, h.slab_idx);
+    return h.ss;
+}
+```
+
+**Expected gain**: **+20-30%** (4,000-6,000 cycles saved)
+
+---
+
+### 5. Must-Adopt Gate (Priority 4) 🟡
+
+**Problem:**
+```c
+// Line 943: Another expensive gate
+SuperSlab* gate_ss = tiny_must_adopt_gate(class_idx, tls);
+if (gate_ss) return gate_ss;
+```
+
+**Cost**: ~2,000 cycles (10% of superslab_refill)
+
+**Why it's slow**:
+- Calls into complex multi-layer scan (sticky/hot/bench/mailbox/registry)
+- Likely duplicates work from earlier adopt/registry paths
+
+**Fix**: Consolidate or skip if earlier paths attempted
+```c
+// Skip gate if we already scanned adopt + registry
+if (attempted_adopt && attempted_registry) {
+    // Skip gate, go directly to mmap
+}
+```
+
+**Expected gain**: **+5-8%** (1,000-1,500 cycles saved)
+
+---
+
+## Optimization Roadmap
+
+### Phase 1: Quick Wins (1-2 days) - **+30-40% expected**
+
+**1.1 Cache getenv() results** ⚡
+- Move to init-time caching
+- Files: `core/hakmem_tiny_init.c`, `core/hakmem_tiny_free.inc`
+- Expected: **+8-10%** (1,600 cycles saved)
+
+**1.2 Early exit in adopt scoring** ⚡
+- First-fit instead of best-fit
+- Stop on first freelist found
+- Files: `core/hakmem_tiny_free.inc:774-783`
+- Expected: **+15-20%** (3,000 cycles saved)
+
+**1.3 Skip drain on remote pending** ⚡
+- Only drain if actually adopting
+- Files: `core/hakmem_tiny_free.inc:860-872`
+- Expected: **+10-15%** (2,000-3,000 cycles saved)
+
+### Phase 2: Structural Improvements (3-5 days) - **+25-35% additional**
+
+**2.1 Per-class registry indexing**
+- Index registry by class_idx (256 → 32 entries scanned)
+- Files: New global array, registry management
+- Expected: **+10-12%** (2,000 cycles saved)
+
+**2.2 Consolidate gates**
+- Merge adopt + registry + must-adopt into single pass
+- Remove duplicate scanning
+- Files: `core/hakmem_tiny_free.inc`
+- Expected: **+8-10%** (1,500 cycles saved)
+
+**2.3 Batch refill optimization**
+- Increase refill count to reduce refill frequency
+- Already has env var: `HAKMEM_TINY_REFILL_COUNT_HOT`
+- Test values: 64, 96, 128
+- Expected: **+5-10%** (reduce refill calls by 2-4x)
+
+### Phase 3: Advanced (1 week) - **+15-20% additional**
+
+**3.1 TLS SuperSlab cache**
+- Keep last N superslabs per class in TLS
+- Avoid registry/adopt paths entirely
+- Expected: **+10-15%**
+
+**3.2 Lazy initialization**
+- Defer expensive checks to slow path
+- Fast path should be 1-2 cycles
+- Expected: **+5-8%**
+
+---
+
+## Expected Results
+
+| Optimization | Cycles Saved | Cumulative Gain | Throughput |
+|--------------|--------------|-----------------|------------|
+| **Baseline** | - | - | 1.59 M ops/s |
+| getenv cache | 1,600 | +8% | 1.72 M ops/s |
+| Adopt early exit | 3,000 | +24% | 1.97 M ops/s |
+| Skip remote drain | 2,500 | +37% | 2.18 M ops/s |
+| Per-class registry | 2,000 | +47% | 2.34 M ops/s |
+| Gate consolidation | 1,500 | +55% | 2.46 M ops/s |
+| Batch refill tuning | 4,000 | +75% | 2.78 M ops/s |
+| **Total (all phases)** | **~15,000** | **+75-100%** | **2.78-3.18 M ops/s** 🎯 |
+
+---
+
+## Immediate Action Items
+
+### Priority 1 (Today)
+1. ✅ Cache `getenv()` results at init time
+2. ✅ Implement early exit in adopt scoring
+3. ✅ Skip drain on remote pending
+
+### Priority 2 (This Week)
+4. ⏳ Per-class registry indexing
+5. ⏳ Consolidate adopt/registry/gate paths
+6. ⏳ Tune batch refill count (A/B test 64/96/128)
+
+### Priority 3 (Next Week)
+7. ⏳ TLS SuperSlab cache
+8. ⏳ Lazy initialization
+
+---
+
+## Conclusion
+
+The `sll_refill_small_from_ss()` bottleneck is primarily caused by **superslab_refill()** being a 298-line complexity monster with:
+
+**Top 5 Issues:**
+1. 🔥🔥🔥 **getenv() in hot path**: 1,600 cycles wasted
+2. 🔥🔥 **Adopt scoring all slabs**: 3,000 cycles, should early exit
+3. 🔥🔥 **Unnecessary remote drain**: 2,500 cycles, should be lazy
+4. 🔥 **Registry linear scan**: 2,000 cycles, should be per-class indexed
+5. 🟡 **Duplicate gates**: 1,500 cycles, should consolidate
+
+**Bottom Line**: With focused optimizations, we can reduce superslab_refill from **19,400 cycles → 4,000-5,000 cycles**, achieving **+75-100% throughput gain** (1.59M → 2.78-3.18M ops/s).
+
+**Files to modify**:
+- `/home/user/hakmem_private/core/hakmem_tiny_init.c` - Add env caching
+- `/home/user/hakmem_private/core/hakmem_tiny_free.inc` - Optimize superslab_refill
+- `/home/user/hakmem_private/core/hakmem_tiny_refill_p0.inc.h` - Tune batch refill
+
+**Start with Phase 1 (getenv + early exit + skip drain) for quick +30-40% win!** 🚀