util/latencyqueue: use util/ringbuffer for improved memory cost

Updates tailscale/corp#34129

Signed-off-by: James Tucker <james@tailscale.com>

util/latencyqueue/latencyqueue.go

@@ -11,6 +11,8 @@
 	"sync"
 	"sync/atomic"
 	"time"
+
+	"tailscale.com/util/ringbuffer"
 )
 
 var (
@@ -52,7 +54,7 @@ type Queue[T any] struct {
 	cancel context.CancelCauseFunc
 
 	mu      sync.Mutex
-	items   []queueItem[T]
+	items   *ringbuffer.RingBuffer[queueItem[T]]
 	wakeup  chan struct{}
 	started bool
 
@@ -93,7 +95,7 @@ func New[T any](parent context.Context, maxLag time.Duration) *Queue[T] {
 	q := &Queue[T]{
 		ctx:    ctx,
 		cancel: cancel,
-		items:  make([]queueItem[T], 0, 128),
+		items:  ringbuffer.New[queueItem[T]](),
 		wakeup: make(chan struct{}, 1),
 		maxLag: maxLag,
 		done:   make(chan struct{}),
@@ -154,7 +156,7 @@ func (q *Queue[T]) Enqueue(batch []T) bool {
 	default:
 	}
 
-	q.items = append(q.items, item)
+	q.items.Push(item)
 	q.numEnqueued.Add(uint64(len(batch)))
 	q.mu.Unlock()
 
@@ -181,7 +183,7 @@ func (q *Queue[T]) Barrier() <-chan struct{} {
 		kind:    kindBarrier,
 		barrier: ch,
 	}
-	q.items = append(q.items, item)
+	q.items.Push(item)
 	q.mu.Unlock()
 
 	q.wake()
@@ -227,12 +229,7 @@ func (q *Queue[T]) run(processor func(context.Context, T)) {
 	for {
 		if processing == nil {
 			q.mu.Lock()
-			hasItems := len(q.items) > 0
-			var item queueItem[T]
-			if hasItems {
-				item = q.items[0]
-				q.items = q.items[1:]
-			}
+			item, hasItems := q.items.Pop()
 			q.mu.Unlock()
 
 			if !hasItems {
@@ -323,10 +320,14 @@ func (q *Queue[T]) drainAndReleaseBarriers() {
 	q.mu.Lock()
 	defer q.mu.Unlock()
 
-	for _, item := range q.items {
+	for !q.items.IsEmpty() {
+		item, ok := q.items.Pop()
+		if !ok {
+			break
+		}
 		if item.kind == kindBarrier {
 			close(item.barrier)
 		}
 	}
-	q.items = nil
+	q.items.Clear()
 }

util/latencyqueue/latencyqueue_test.go

@@ -722,3 +722,101 @@ func TestZeroMaxLag(t *testing.T) {
 		}
 	})
 }
+
+// BenchmarkVariableLoad tests memory efficiency under variable load patterns.
+// The ringbuffer-based implementation should efficiently handle:
+// - Bursts of enqueues followed by processing
+// - Growing and shrinking queue sizes
+// - Memory compaction during idle periods
+func BenchmarkVariableLoad(b *testing.B) {
+	q := New[int](context.Background(), 10*time.Second)
+
+	processed := atomic.Int64{}
+	q.Start(func(ctx context.Context, val int) {
+		processed.Add(1)
+		// Simulate some processing time
+		time.Sleep(10 * time.Microsecond)
+	})
+	defer q.Close()
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		// Simulate bursty traffic - enqueue in batches
+		batchSize := 10 + (i % 50) // Variable batch sizes from 10-59
+		batch := make([]int, batchSize)
+		for j := range batch {
+			batch[j] = i*100 + j
+		}
+		q.Enqueue(batch)
+
+		// Occasionally wait for processing to catch up
+		if i%100 == 99 {
+			barrier := q.Barrier()
+			<-barrier
+		}
+	}
+
+	// Final barrier to ensure all items are processed
+	barrier := q.Barrier()
+	<-barrier
+
+	b.ReportMetric(float64(processed.Load()), "items")
+}
+
+// BenchmarkSteadyState tests performance under steady-state conditions.
+func BenchmarkSteadyState(b *testing.B) {
+	q := New[int](context.Background(), 10*time.Second)
+
+	q.Start(func(ctx context.Context, val int) {
+		// Fast processing
+	})
+	defer q.Close()
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		q.Enqueue([]int{i})
+	}
+
+	barrier := q.Barrier()
+	<-barrier
+}
+
+// BenchmarkBurstThenDrain tests memory efficiency in burst-then-drain scenarios.
+// This pattern exposes inefficiencies in slice-based implementations where
+// the underlying array never shrinks. The ringbuffer should compact efficiently.
+func BenchmarkBurstThenDrain(b *testing.B) {
+	q := New[int](context.Background(), 10*time.Second)
+
+	processDelay := atomic.Bool{}
+	q.Start(func(ctx context.Context, val int) {
+		if processDelay.Load() {
+			time.Sleep(100 * time.Microsecond)
+		}
+	})
+	defer q.Close()
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		// Burst phase: enqueue many items with slow processing
+		processDelay.Store(true)
+		largeBatch := make([]int, 1000)
+		for j := range largeBatch {
+			largeBatch[j] = i*1000 + j
+		}
+		q.Enqueue(largeBatch)
+
+		// Let queue fill up a bit
+		time.Sleep(500 * time.Microsecond)
+
+		// Drain phase: speed up processing
+		processDelay.Store(false)
+		barrier := q.Barrier()
+		<-barrier
+
+		// Allow time for compaction to potentially occur
+		time.Sleep(100 * time.Microsecond)
+	}
+}