hakmem/core/hakmem_tiny_superslab_constants.h

// hakmem_tiny_superslab_constants.h - SuperSlab Layout Constants
// Purpose: Centralize all SuperSlab layout magic numbers
// Phase 6-2.5: Created to fix sizeof(SuperSlab) vs hardcoded offset mismatch

#ifndef HAKMEM_TINY_SUPERSLAB_CONSTANTS_H
#define HAKMEM_TINY_SUPERSLAB_CONSTANTS_H

// ============================================================================
// SuperSlab Layout Constants
// ============================================================================

// Log2 range for SuperSlab sizes:
//  - MIN:  512KB (2^19) - Phase 2 optimization: reduced from 1MB
//  - MAX:  2MB (2^21)   - unchanged
//  - DEFAULT: 512KB (2^19) - Phase 2 optimization: reduced from 2MB
//
// Phase 2-Opt2: Reduce SuperSlab size to minimize initialization cost
// Benefit: 75% reduction in allocation size (2MB → 512KB)
// Expected: +3-5% throughput improvement
// Rationale:
//   - Smaller SuperSlab = fewer page faults during allocation
//   - Better memory granularity (less wasted VA space)
//   - Memset already removed in Phase 1, so pure allocation overhead
//   - Perf analysis showed shared_pool_acquire_slab at 23.83% CPU time
#ifndef SUPERSLAB_LG_MIN
#define SUPERSLAB_LG_MIN 19
#endif
#ifndef SUPERSLAB_LG_MAX
#define SUPERSLAB_LG_MAX 21
#endif
#ifndef SUPERSLAB_LG_DEFAULT
#define SUPERSLAB_LG_DEFAULT 19
#endif

// Size of each slab within SuperSlab (fixed, never changes)
#define SLAB_SIZE (64 * 1024)  // 64KB per slab

// SuperSlab struct size (as of P1.1)
// Actual value: sizeof(SuperSlab) = 1192 bytes
// This includes: magic, lg_size, size_class, total_active_blocks,
//                remote_heads[], slabs[], slab_listed[], class_map[], etc.
// P1.1: Added class_map[32] (+32 bytes) for out-of-band class_idx lookup
#define SUPERSLAB_HEADER_SIZE 1192

// Slab 0 data offset (CRITICAL: Must be aligned to largest block size)
// Phase 6-2.5 FIX: Changed from 1024 to 2048
//
// Why 2048?
// - sizeof(SuperSlab) = 1088 bytes
// - Largest block size = 1024 bytes (class 7)
// - Must round up to next 1024-byte boundary: (1088 + 1023) & ~1023 = 2048
//
// Layout:
//   [0..1087]     SuperSlab header (1088 bytes)
//   [1088..2047]  Padding (960 bytes, unused)
//   [2048..65535] Slab 0 data (63488 bytes = 64KB - 2048)
//
// Previous value (1024) caused:
// - 64-byte overlap with SuperSlab metadata (corruption)
// - Misalignment for class 7 allocations (1024 % 1024 != 0)
#define SUPERSLAB_SLAB0_DATA_OFFSET 2048

// Slab 0 usable size (for capacity calculation)
#define SUPERSLAB_SLAB0_USABLE_SIZE (SLAB_SIZE - SUPERSLAB_SLAB0_DATA_OFFSET)  // 63488 bytes

// Regular slab (i > 0) usable size
#define SUPERSLAB_SLAB_USABLE_SIZE SLAB_SIZE  // 65536 bytes

// ============================================================================
// Validation (compile-time check)
// ============================================================================

// Ensure SLAB0_DATA_OFFSET is aligned to largest block size (1024)
#if (SUPERSLAB_SLAB0_DATA_OFFSET % 1024) != 0
#error "SUPERSLAB_SLAB0_DATA_OFFSET must be 1024-byte aligned for class 7"
#endif

// Ensure SLAB0_DATA_OFFSET is large enough to contain SuperSlab header
#if SUPERSLAB_SLAB0_DATA_OFFSET < SUPERSLAB_HEADER_SIZE
#error "SUPERSLAB_SLAB0_DATA_OFFSET must be >= sizeof(SuperSlab)"
#endif

#endif // HAKMEM_TINY_SUPERSLAB_CONSTANTS_H