Remove unnecessary allocations and Write calls.

This commit improves networking performance by 1) moving an unnecessary
allocation to outside the hot loop and by 2) eliminating an unnecessary
Write call.

This result in ~7% faster throughput as measured via iperf3 and a custom
Web service to measure requests per second.  Note that this is *despite*
the append call.

Signed-off-by: Philipp Winter <phw@brave.com>

cmd/vm/main_linux.go

@@ -152,6 +152,7 @@ func dhcp() error {
 
 func rx(conn net.Conn, tap *water.Interface, errCh chan error, mtu int) {
 	log.Info("waiting for packets...")
+	size := make([]byte, 2)
 	var frame ethernet.Frame
 	for {
 		frame.Resize(mtu)
@@ -167,15 +168,9 @@ func rx(conn net.Conn, tap *water.Interface, errCh chan error, mtu int) {
 			log.Info(packet.String())
 		}
 
-		size := make([]byte, 2)
 		binary.LittleEndian.PutUint16(size, uint16(n))
-
-		if _, err := conn.Write(size); err != nil {
-			errCh <- errors.Wrap(err, "cannot write size to socket")
-			return
-		}
-		if _, err := conn.Write(frame); err != nil {
-			errCh <- errors.Wrap(err, "cannot write packet to socket")
+		if _, err := conn.Write(append(size, frame...)); err != nil {
+			errCh <- errors.Wrap(err, "cannot write size and packet to socket")
 			return
 		}
 	}

pkg/tap/switch.go

@@ -168,14 +168,15 @@ func (e *Switch) txBuf(id int, conn protocolConn, buf []byte) error {
 		size := conn.protocolImpl.(streamProtocol).Buf()
 		conn.protocolImpl.(streamProtocol).Write(size, len(buf))
 
-		if _, err := conn.Write(size); err != nil {
+		if _, err := conn.Write(append(size, buf...)); err != nil {
+			e.disconnect(id, conn)
+			return err
+		}
+	} else {
+		if _, err := conn.Write(buf); err != nil {
 			e.disconnect(id, conn)
 			return err
 		}
-	}
-	if _, err := conn.Write(buf); err != nil {
-		e.disconnect(id, conn)
-		return err
 	}
 	return nil
 }