289 lines
11 KiB
C
289 lines
11 KiB
C
#ifndef HAKMEM_TINY_STATS_H
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#define HAKMEM_TINY_STATS_H
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#include <stdint.h>
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#include <stdio.h>
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// NOTE: This header must be included AFTER hakmem_tiny.h
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// Reason: Needs TinyPool definition and TINY_NUM_CLASSES
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// Global pool (defined in hakmem_tiny.c, declared in hakmem_tiny.h)
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// Assumed to be available when this header is included
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extern TinyPool g_tiny_pool;
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// Debug-only TLS/front counters (defined in hakmem_tiny.c)
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#if HAKMEM_BUILD_DEBUG
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extern uint64_t g_tls_hit_count[TINY_NUM_CLASSES];
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extern uint64_t g_tls_miss_count[TINY_NUM_CLASSES];
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extern uint64_t g_tls_spill_ss_count[TINY_NUM_CLASSES];
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extern uint64_t g_tls_spill_owner_count[TINY_NUM_CLASSES];
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extern uint64_t g_tls_spill_mag_count[TINY_NUM_CLASSES];
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extern uint64_t g_tls_spill_requeue_count[TINY_NUM_CLASSES];
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#endif
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// ============================================================================
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// Quick Win #2: Compile-Time Statistics Toggle
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// ============================================================================
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//
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// Purpose: Zero-overhead production builds by disabling stats collection
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// Usage: Build with -DHAKMEM_ENABLE_STATS to enable (default: disabled)
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// Impact: 3-5% speedup when disabled (removes 0.5ns TLS increment)
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//
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// Default: DISABLED (production performance)
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// Enable: make CFLAGS=-DHAKMEM_ENABLE_STATS
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//
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// ============================================================================
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#ifdef HAKMEM_ENABLE_STATS
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// ============================================================================
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// Tiny Pool Statistics: Batched TLS Counters (Out-of-Band)
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// ============================================================================
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//
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// Purpose: Remove statistics overhead from allocation hot path
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// Design: Per-thread batch counters, periodic flush to global
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// Cost: 0.5 ns (TLS increment) vs 10-15 ns (XOR RNG + atomic)
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//
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// Key Insight: Batching converts expensive atomic ops into cheap TLS ops
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// 256 TLS increments (0.5ns each) → 1 atomic add (10ns)
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// Amortized: 0.5 + (10/256) = 0.54 ns per operation
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//
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// ============================================================================
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// ----------------------------------------------------------------------------
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// Configuration
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// ----------------------------------------------------------------------------
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// Batch size: Flush every N operations
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// Larger = more accurate, less atomic contention
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// Smaller = more frequent updates, higher overhead
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#define TINY_STATS_BATCH_SIZE 256
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// ----------------------------------------------------------------------------
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// Per-Thread Batch Counters (TLS)
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// ----------------------------------------------------------------------------
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// Allocation counters (flushed to g_tiny_pool.alloc_count)
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static __thread uint32_t t_alloc_batch[TINY_NUM_CLASSES] = {0};
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// Free counters (flushed to g_tiny_pool.free_count)
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static __thread uint32_t t_free_batch[TINY_NUM_CLASSES] = {0};
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// ============================================================================
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// Hot Path Operations (Inlined)
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// ============================================================================
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// ----------------------------------------------------------------------------
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// Record Allocation (Hot Path: 0.5 ns)
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// ----------------------------------------------------------------------------
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//
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// Cost: Single TLS increment (0.5 ns)
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// vs XOR RNG (3 ns) + conditional atomic (2-10 ns) = 5-13 ns
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//
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// Savings: 4.5-12.5 ns per allocation
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//
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static inline void stats_record_alloc(int class_idx) __attribute__((always_inline));
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static inline void stats_record_alloc(int class_idx) {
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t_alloc_batch[class_idx]++;
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}
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// ----------------------------------------------------------------------------
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// Record Free (Hot Path: 0.5 ns)
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// ----------------------------------------------------------------------------
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//
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// Cost: Single TLS increment (0.5 ns)
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//
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static inline void stats_record_free(int class_idx) __attribute__((always_inline));
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static inline void stats_record_free(int class_idx) {
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t_free_batch[class_idx]++;
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}
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// ============================================================================
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// Cold Path Operations (Batch Flush)
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// ============================================================================
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// ----------------------------------------------------------------------------
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// Flush Batch Counters (Cold Path: 20 ns per flush)
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// ----------------------------------------------------------------------------
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//
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// Called periodically (every 256 ops) or explicitly
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//
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// Cost: 20 ns per flush (atomic add for alloc + free)
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// Amortized: 20ns / 256 = 0.08 ns per operation
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//
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// Thread Safety: Atomic add to global counters (lock-free)
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//
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static inline void stats_flush(int class_idx) {
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// Flush allocation counter
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uint32_t alloc = t_alloc_batch[class_idx];
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if (alloc > 0) {
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// Atomic add to global counter
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__atomic_fetch_add(&g_tiny_pool.alloc_count[class_idx],
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(uint64_t)alloc,
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__ATOMIC_RELAXED);
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t_alloc_batch[class_idx] = 0;
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}
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// Flush free counter
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uint32_t freed = t_free_batch[class_idx];
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if (freed > 0) {
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__atomic_fetch_add(&g_tiny_pool.free_count[class_idx],
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(uint64_t)freed,
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__ATOMIC_RELAXED);
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t_free_batch[class_idx] = 0;
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}
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}
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// ----------------------------------------------------------------------------
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// Conditional Flush (Auto Batching)
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// ----------------------------------------------------------------------------
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//
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// Flushes only when batch is full (every 256 ops)
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//
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// Usage:
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// stats_record_alloc(class_idx);
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// stats_flush_if_needed(class_idx); // Flushes every 256
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//
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static inline void stats_flush_if_needed(int class_idx) {
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// Check if batch is full (bit mask is faster than modulo)
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if ((t_alloc_batch[class_idx] & (TINY_STATS_BATCH_SIZE - 1)) == 0) {
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stats_flush(class_idx);
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}
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}
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// ----------------------------------------------------------------------------
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// Flush All Classes (Diagnostic)
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// ----------------------------------------------------------------------------
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//
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// Flushes all pending counters for all classes
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// Called on shutdown or diagnostic snapshots
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//
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static inline void stats_flush_all(void) {
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for (int i = 0; i < TINY_NUM_CLASSES; i++) {
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stats_flush(i);
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}
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}
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// ============================================================================
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// Comparison to Previous Approach (XOR RNG Sampling)
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// ============================================================================
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//
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// Previous (XOR RNG Sampling):
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// t_tiny_rng ^= t_tiny_rng << 13; // 1 ns
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// t_tiny_rng ^= t_tiny_rng >> 17; // 1 ns
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// t_tiny_rng ^= t_tiny_rng << 5; // 1 ns
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// if ((t_tiny_rng & mask) == 0) // 1 ns
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// atomic_add(&g_tiny_pool.alloc_count, 1); // 2-10 ns (if taken)
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// Total: 4-14 ns per allocation (avg ~8 ns with 1/16 sampling)
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// Accuracy: ~93.75% (1/16 sampling with variance)
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//
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// New (Batched TLS):
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// t_alloc_batch[class_idx]++; // 0.5 ns (always)
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// // Every 256 ops:
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// atomic_add(&g_tiny_pool.alloc_count, 256); // 10 ns (amortized 0.04 ns)
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// Total: 0.54 ns per allocation
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// Accuracy: 100% (exact count)
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//
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// Improvement: 8 ns → 0.54 ns = 7.5 ns saved per allocation
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// 15x faster, 6.25% more accurate
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//
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// ============================================================================
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// ============================================================================
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// Usage Example
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// ============================================================================
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//
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// Hot Path (Allocation):
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// void* hak_tiny_alloc(size_t size) {
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// // ... allocation logic ...
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// stats_record_alloc(class_idx); // 0.5 ns
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// return ptr;
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// }
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//
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// Hot Path (Free):
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// void hak_tiny_free(void* ptr) {
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// // ... free logic ...
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// stats_record_free(class_idx); // 0.5 ns
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// }
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//
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// Cold Path (Periodic flush):
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// // Option 1: Explicit flush (in slow path)
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// void refill_magazine(...) {
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// // ... refill logic ...
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// stats_flush_if_needed(class_idx); // Flushes every 256
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// }
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//
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// // Option 2: Flush on diagnostics
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// void hak_tiny_get_stats(...) {
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// stats_flush_all(); // Get exact counts
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// // ... read global counters ...
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// }
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//
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// ============================================================================
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// ============================================================================
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// Design Notes
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// ============================================================================
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//
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// 1. Why Batching Works:
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// - TLS ops are 20x cheaper than atomic ops (0.5ns vs 10ns)
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// - Allocation is bursty (256 allocs in tight loops common)
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// - Flush overhead amortizes to near-zero (0.04 ns)
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//
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// 2. Why 256 Batch Size:
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// - Power of 2: Fast bit-mask check (no division)
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// - Not too large: Counters fit in 32-bit (no overflow)
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// - Not too small: Good amortization (0.04 ns overhead)
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//
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// 3. Accuracy:
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// - Exact counts (not sampled approximations)
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// - Eventual consistency (flush delay < 1 microsecond)
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// - Diagnostic flush ensures snapshot accuracy
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//
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// 4. Thread Safety:
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// - TLS counters: No locks (thread-local)
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// - Global flush: Atomic add (lock-free)
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// - No races, no contention
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//
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// 5. Memory Overhead:
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// - 2 * 8 classes * 4 bytes = 64 bytes per thread
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// - Negligible compared to TLS magazine (16 KB)
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//
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// ============================================================================
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#else // !HAKMEM_ENABLE_STATS
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// ============================================================================
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// No-Op Macros (Statistics Disabled)
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// ============================================================================
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//
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// When HAKMEM_ENABLE_STATS is not defined, all statistics functions become
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// no-ops that the compiler will optimize away (zero overhead).
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//
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// ============================================================================
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// No-op inline functions (optimized away by compiler)
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static inline void stats_record_alloc(int class_idx) __attribute__((always_inline));
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static inline void stats_record_alloc(int class_idx) {
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(void)class_idx; // Silence unused parameter warning
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}
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static inline void stats_record_free(int class_idx) __attribute__((always_inline));
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static inline void stats_record_free(int class_idx) {
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(void)class_idx; // Silence unused parameter warning
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}
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static inline void stats_flush_batch(void) __attribute__((always_inline));
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static inline void stats_flush_batch(void) {
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// No-op
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}
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static inline void stats_diagnostic_flush(void) __attribute__((always_inline));
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static inline void stats_diagnostic_flush(void) {
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// No-op
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}
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#endif // HAKMEM_ENABLE_STATS
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#endif // HAKMEM_TINY_STATS_H
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