tsdns: remove forwarding queue.

Two levels of queueing are unnecessary.
The resulting implementation performs as follows
under request bursts (`count` packets sent concurrently):

lost  count            avg latency
   0 /  256 (00.00%) - 28ms
   0 /  512 (00.00%) - 146ms
   0 /  768 (00.00%) - 166ms
   0 / 1024 (00.00%) - 416ms
  11 / 1280 (00.86%) - 430ms
 145 / 1536 (09.44%) - 715ms
 364 / 2048 (17.77%) - 836ms

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/tsdns/forwarder.go

@@ -22,11 +22,6 @@ import (
 // headerBytes is the number of bytes in a DNS message header.
 const headerBytes = 12
 
-// forwardQueueSize is the maximal number of requests that can be pending delegation.
-// Note that this is distinct from the number of requests that are pending a response,
-// which is not limited (except by txid collisions).
-const forwardQueueSize = 64
-
 // connCount is the number of UDP connections to use for forwarding.
 const connCount = 32
 
@@ -138,7 +133,6 @@ func newForwarder(logf logger.Logf, responses chan Packet) *forwarder {
 	return &forwarder{
 		logf:      logger.WithPrefix(logf, "forward: "),
 		responses: responses,
-		queue:     make(chan forwardedPacket, forwardQueueSize),
 		closed:    make(chan struct{}),
 		conns:     make([]*net.UDPConn, connCount),
 		txMap:     make(map[txid]forwardingRecord),
@@ -155,11 +149,10 @@ func (f *forwarder) Start() error {
 		}
 	}
 
-	f.wg.Add(connCount + 2)
+	f.wg.Add(connCount + 1)
 	for idx, conn := range f.conns {
 		go f.recv(uint16(idx), conn)
 	}
-	go f.send()
 	go f.cleanMap()
 
 	return nil
@@ -191,29 +184,14 @@ func (f *forwarder) setUpstreams(upstreams []net.Addr) {
 	f.mu.Unlock()
 }
 
-func (f *forwarder) send() {
-	defer f.wg.Done()
-
-	var packet forwardedPacket
-	for {
-		select {
-		case <-f.closed:
-			return
-		case packet = <-f.queue:
-			// continue
-		}
-
+func (f *forwarder) send(packet []byte, dst net.Addr) {
 	connIdx := rand.Intn(connCount)
 	conn := f.conns[connIdx]
-		_, err := conn.WriteTo(packet.payload, packet.dst)
-		if err != nil {
+	_, err := conn.WriteTo(packet, dst)
 	// Do not log errors due to expired deadline.
-			if !errors.Is(err, os.ErrDeadlineExceeded) {
+	if err != nil && !errors.Is(err, os.ErrDeadlineExceeded) {
 		f.logf("send: %v", err)
 	}
-			return
-		}
-	}
 }
 
 func (f *forwarder) recv(connIdx uint16, conn *net.UDPConn) {
@@ -308,17 +286,8 @@ func (f *forwarder) forward(query Packet) error {
 
 	f.mu.Unlock()
 
-	packet := forwardedPacket{
-		payload: query.Payload,
-	}
 	for _, upstream := range upstreams {
-		packet.dst = upstream
-		select {
-		case <-f.closed:
-			return ErrClosed
-		case f.queue <- packet:
-			// continue
-		}
+		f.send(query.Payload, upstream)
 	}
 
 	return nil

wgengine/tsdns/tsdns.go

@@ -22,10 +22,10 @@ import (
 // maxResponseBytes is the maximum size of a response from a Resolver.
 const maxResponseBytes = 512
 
-// pendingQueueSize is the maximal number of DNS requests that can await polling.
+// queueSize is the maximal number of DNS requests that can await polling.
 // If EnqueueRequest is called when this many requests are already pending,
 // the request will be dropped to avoid blocking the caller.
-const pendingQueueSize = 64
+const queueSize = 64
 
 // defaultTTL is the TTL of all responses from Resolver.
 const defaultTTL = 600 * time.Second
@@ -94,7 +94,7 @@ type ResolverConfig struct {
 func NewResolver(config ResolverConfig) *Resolver {
 	r := &Resolver{
 		logf:      logger.WithPrefix(config.Logf, "tsdns: "),
-		queue:     make(chan Packet, pendingQueueSize),
+		queue:     make(chan Packet, queueSize),
 		responses: make(chan Packet),
 		errors:    make(chan error),
 		closed:    make(chan struct{}),