Remove unnecessary allocations and Write calls.

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

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
 }
@@ -223,6 +224,9 @@ loop:
 func (e *Switch) rxStream(ctx context.Context, id int, conn net.Conn, sProtocol streamProtocol) error {
 	reader := bufio.NewReader(conn)
 	sizeBuf := sProtocol.Buf()
+	// start with a 65k receive buffer and extend if needed
+	buf := make([]byte, 1<<16-1)
+
 loop:
 	for {
 		select {
@@ -237,8 +241,10 @@ loop:
 		}
 		size := sProtocol.Read(sizeBuf)
 
-		buf := make([]byte, size)
-		_, err = io.ReadFull(reader, buf)
+		if size > len(buf) {
+			buf = make([]byte, size)
+		}
+		_, err = io.ReadFull(reader, buf[:size])
 		if err != nil {
 			return errors.Wrap(err, "cannot read packet from socket")
 		}