fast semaphore

internal/pool/export_test.go

@@ -20,5 +20,5 @@ func (p *ConnPool) CheckMinIdleConns() {
 }
 
 func (p *ConnPool) QueueLen() int {
-	return len(p.queue)
+	return int(p.semaphore.len())
 }

internal/pool/pool.go

@@ -132,7 +132,9 @@ type ConnPool struct {
 	dialErrorsNum uint32 // atomic
 	lastDialError atomic.Value
 
-	queue chan struct{}
+	// Fast atomic semaphore for connection limiting
+	// Replaces the old channel-based queue for better performance
+	semaphore *fastSemaphore
 
 	connsMu   sync.Mutex
 	conns     map[uint64]*Conn
@@ -158,7 +160,7 @@ func NewConnPool(opt *Options) *ConnPool {
 	p := &ConnPool{
 		cfg: opt,
 
-		queue:     make(chan struct{}, opt.PoolSize),
+		semaphore: newFastSemaphore(opt.PoolSize),
 		conns:     make(map[uint64]*Conn),
 		idleConns: make([]*Conn, 0, opt.PoolSize),
 	}
@@ -223,8 +225,12 @@ func (p *ConnPool) checkMinIdleConns() {
 	// Only create idle connections if we haven't reached the total pool size limit
 	// MinIdleConns should be a subset of PoolSize, not additional connections
 	for p.poolSize.Load() < p.cfg.PoolSize && p.idleConnsLen.Load() < p.cfg.MinIdleConns {
-		select {
-		case p.queue <- struct{}{}:
+		// Try to acquire a semaphore token
+		if !p.semaphore.tryAcquire() {
+			// Semaphore is full, can't create more connections
+			return
+		}
+
 		p.poolSize.Add(1)
 		p.idleConnsLen.Add(1)
 		go func() {
@@ -245,9 +251,6 @@ func (p *ConnPool) checkMinIdleConns() {
 			}
 			p.freeTurn()
 		}()
-		default:
-			return
-		}
 	}
 }
 
@@ -528,44 +531,35 @@ func (p *ConnPool) getConn(ctx context.Context) (*Conn, error) {
 }
 
 func (p *ConnPool) waitTurn(ctx context.Context) error {
+	// Fast path: check context first
 	select {
 	case <-ctx.Done():
 		return ctx.Err()
 	default:
 	}
 
-	select {
-	case p.queue <- struct{}{}:
+	// Fast path: try to acquire without blocking
+	if p.semaphore.tryAcquire() {
 		return nil
-	default:
 	}
 
+	// Slow path: need to wait
 	start := time.Now()
-	timer := timers.Get().(*time.Timer)
-	defer timers.Put(timer)
-	timer.Reset(p.cfg.PoolTimeout)
+	err := p.semaphore.acquire(ctx, p.cfg.PoolTimeout)
 
-	select {
-	case <-ctx.Done():
-		if !timer.Stop() {
-			<-timer.C
-		}
-		return ctx.Err()
-	case p.queue <- struct{}{}:
+	if err == nil {
+		// Successfully acquired after waiting
 		p.waitDurationNs.Add(time.Now().UnixNano() - start.UnixNano())
 		atomic.AddUint32(&p.stats.WaitCount, 1)
-		if !timer.Stop() {
-			<-timer.C
-		}
-		return nil
-	case <-timer.C:
+	} else if err == ErrPoolTimeout {
 		atomic.AddUint32(&p.stats.Timeouts, 1)
-		return ErrPoolTimeout
 	}
+
+	return err
 }
 
 func (p *ConnPool) freeTurn() {
-	<-p.queue
+	p.semaphore.release()
 }
 
 func (p *ConnPool) popIdle() (*Conn, error) {

internal/pool/semaphore.go

@@ -0,0 +1,138 @@
+package pool
+
+import (
+	"context"
+	"sync/atomic"
+	"time"
+)
+
+// fastSemaphore is a high-performance semaphore implementation using atomic operations.
+// It's optimized for the fast path (no blocking) while still supporting timeouts and context cancellation.
+//
+// Performance characteristics:
+// - Fast path (no blocking): Single atomic CAS operation
+// - Slow path (blocking): Falls back to channel-based waiting
+// - Release: Single atomic decrement + optional channel notification
+//
+// This is significantly faster than a pure channel-based semaphore because:
+// 1. The fast path avoids channel operations entirely (no scheduler involvement)
+// 2. Atomic operations are much cheaper than channel send/receive
+// 3. Better CPU cache behavior (no channel buffer allocation)
+type fastSemaphore struct {
+	// Current number of acquired tokens (atomic)
+	count atomic.Int32
+
+	// Maximum number of tokens (capacity)
+	max int32
+
+	// Channel for blocking waiters
+	// Only used when fast path fails (semaphore is full)
+	waitCh chan struct{}
+}
+
+// newFastSemaphore creates a new fast semaphore with the given capacity.
+func newFastSemaphore(capacity int32) *fastSemaphore {
+	return &fastSemaphore{
+		max:    capacity,
+		waitCh: make(chan struct{}, capacity),
+	}
+}
+
+// tryAcquire attempts to acquire a token without blocking.
+// Returns true if successful, false if the semaphore is full.
+//
+// This is the fast path - just a single CAS operation.
+func (s *fastSemaphore) tryAcquire() bool {
+	for {
+		current := s.count.Load()
+		if current >= s.max {
+			return false // Semaphore is full
+		}
+		if s.count.CompareAndSwap(current, current+1) {
+			return true // Successfully acquired
+		}
+		// CAS failed due to concurrent modification, retry
+	}
+}
+
+// acquire acquires a token, blocking if necessary until one is available or the context is cancelled.
+// Returns an error if the context is cancelled or the timeout expires.
+//
+// Performance optimization:
+// 1. First try fast path (no blocking)
+// 2. If that fails, fall back to channel-based waiting
+func (s *fastSemaphore) acquire(ctx context.Context, timeout time.Duration) error {
+	// Fast path: try to acquire without blocking
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	default:
+	}
+
+	// Try fast acquire first
+	if s.tryAcquire() {
+		return nil
+	}
+
+	// Fast path failed, need to wait
+	// Use timer pool to avoid allocation
+	timer := timers.Get().(*time.Timer)
+	defer timers.Put(timer)
+	timer.Reset(timeout)
+
+	start := time.Now()
+
+	for {
+		select {
+		case <-ctx.Done():
+			if !timer.Stop() {
+				<-timer.C
+			}
+			return ctx.Err()
+
+		case <-s.waitCh:
+			// Someone released a token, try to acquire it
+			if s.tryAcquire() {
+				if !timer.Stop() {
+					<-timer.C
+				}
+				return nil
+			}
+			// Failed to acquire (race with another goroutine), continue waiting
+
+		case <-timer.C:
+			return ErrPoolTimeout
+		}
+
+		// Periodically check if we can acquire (handles race conditions)
+		if time.Since(start) > timeout {
+			return ErrPoolTimeout
+		}
+	}
+}
+
+// release releases a token back to the semaphore.
+// This wakes up one waiting goroutine if any are blocked.
+func (s *fastSemaphore) release() {
+	s.count.Add(-1)
+
+	// Try to wake up a waiter (non-blocking)
+	// If no one is waiting, this is a no-op
+	select {
+	case s.waitCh <- struct{}{}:
+		// Successfully notified a waiter
+	default:
+		// No waiters, that's fine
+	}
+}
+
+// len returns the current number of acquired tokens.
+func (s *fastSemaphore) len() int32 {
+	return s.count.Load()
+}
+
+// cap returns the maximum capacity of the semaphore.
+func (s *fastSemaphore) cap() int32 {
+	return s.max
+}
+