Allocate small iovec arrays on the stack

tun_darwin.go

@@ -25,21 +25,20 @@ var _ DarwinTUN = (*NativeTun)(nil)
 const PacketOffset = 4
 
 type NativeTun struct {
-	tunFd               int
-	tunFile             *os.File
-	batchSize           int
-	iovecs              []iovecBuffer
-	iovecsOutput        []iovecBuffer
-	iovecsOutputDefault []unix.Iovec
-	msgHdrs             []rawfile.MsgHdrX
-	msgHdrsOutput       []rawfile.MsgHdrX
-	buffers             []*buf.Buffer
-	stopFd              stopfd.StopFD
-	options             Options
-	inet4Address        [4]byte
-	inet6Address        [16]byte
-	routeSet            bool
-	writeMsgX           bool
+	tunFd         int
+	tunFile       *os.File
+	batchSize     int
+	iovecs        []iovecBuffer
+	iovecsOutput  []iovecBuffer
+	msgHdrs       []rawfile.MsgHdrX
+	msgHdrsOutput []rawfile.MsgHdrX
+	buffers       []*buf.Buffer
+	stopFd        stopfd.StopFD
+	options       Options
+	inet4Address  [4]byte
+	inet6Address  [16]byte
+	routeSet      bool
+	writeMsgX     bool
 }
 
 type iovecBuffer struct {

tun_darwin_gvisor.go

@@ -15,14 +15,24 @@ import (
 var _ GVisorTun = (*NativeTun)(nil)
 
 func (t *NativeTun) WritePacket(pkt *stack.PacketBuffer) (int, error) {
-	iovecs := t.iovecsOutputDefault
+	views := pkt.AsSlices()
+	numIovecs := len(views)
+	numIovecs++ // for packetHeaderVec4/6
+
+	// Allocate small iovec arrays on the stack.
+	var iovecsArr [8]unix.Iovec
+	iovecs := iovecsArr[:0]
+	if numIovecs > len(iovecsArr) {
+		iovecs = make([]unix.Iovec, 0, numIovecs)
+	}
+
 	if pkt.NetworkProtocolNumber == header.IPv4ProtocolNumber {
 		iovecs = append(iovecs, packetHeaderVec4)
 	} else {
 		iovecs = append(iovecs, packetHeaderVec6)
 	}
 	var dataLen int
-	for _, packetSlice := range pkt.AsSlices() {
+	for _, packetSlice := range views {
 		dataLen += len(packetSlice)
 		iovec := unix.Iovec{
 			Base: &packetSlice[0],
@@ -30,9 +40,6 @@ func (t *NativeTun) WritePacket(pkt *stack.PacketBuffer) (int, error) {
 		iovec.SetLen(len(packetSlice))
 		iovecs = append(iovecs, iovec)
 	}
-	if cap(iovecs) > cap(t.iovecsOutputDefault) {
-		t.iovecsOutputDefault = iovecs[:0]
-	}
 	errno := rawfile.NonBlockingWriteIovec(t.tunFd, iovecs)
 	if errno == 0 {
 		return dataLen, nil

tun_linux.go

@@ -25,20 +25,19 @@ import (
 var _ LinuxTUN = (*NativeTun)(nil)
 
 type NativeTun struct {
-	tunFd               int
-	tunFile             *os.File
-	iovecsOutputDefault []unix.Iovec
-	interfaceCallback   *list.Element[DefaultInterfaceUpdateCallback]
-	options             Options
-	ruleIndex6          []int
-	gsoEnabled          bool
-	gsoBuffer           []byte
-	gsoToWrite          []int
-	gsoReadAccess       sync.Mutex
-	tcpGROAccess        sync.Mutex
-	tcp4GROTable        *tcpGROTable
-	tcp6GROTable        *tcpGROTable
-	txChecksumOffload   bool
+	tunFd             int
+	tunFile           *os.File
+	interfaceCallback *list.Element[DefaultInterfaceUpdateCallback]
+	options           Options
+	ruleIndex6        []int
+	gsoEnabled        bool
+	gsoBuffer         []byte
+	gsoToWrite        []int
+	gsoReadAccess     sync.Mutex
+	tcpGROAccess      sync.Mutex
+	tcp4GROTable      *tcpGROTable
+	tcp6GROTable      *tcpGROTable
+	txChecksumOffload bool
 }
 
 func New(options Options) (Tun, error) {

tun_linux_gvisor.go

@@ -17,9 +17,18 @@ func init() {
 var _ GVisorTun = (*NativeTun)(nil)
 
 func (t *NativeTun) WritePacket(pkt *stack.PacketBuffer) (int, error) {
-	iovecs := t.iovecsOutputDefault
+	views := pkt.AsSlices()
+	numIovecs := len(views)
+
+	// Allocate small iovec arrays on the stack.
+	var iovecsArr [8]unix.Iovec
+	iovecs := iovecsArr[:0]
+	if numIovecs > len(iovecsArr) {
+		iovecs = make([]unix.Iovec, 0, numIovecs)
+	}
+
 	var dataLen int
-	for _, packetSlice := range pkt.AsSlices() {
+	for _, packetSlice := range views {
 		dataLen += len(packetSlice)
 		iovec := unix.Iovec{
 			Base: &packetSlice[0],
@@ -27,9 +36,6 @@ func (t *NativeTun) WritePacket(pkt *stack.PacketBuffer) (int, error) {
 		iovec.SetLen(len(packetSlice))
 		iovecs = append(iovecs, iovec)
 	}
-	if cap(iovecs) > cap(t.iovecsOutputDefault) {
-		t.iovecsOutputDefault = iovecs[:0]
-	}
 	errno := rawfile.NonBlockingWriteIovec(t.tunFd, iovecs)
 	if errno == 0 {
 		return dataLen, nil