server: optimize BCE on the packetio.writePacket (#34758)

ref #34669

server/packetio.go

@@ -166,12 +166,11 @@ func (p *packetIO) writePacket(data []byte) error {
 	writePacketBytes.Add(float64(len(data)))
 
 	for length >= mysql.MaxPayloadLen {
+		data[3] = p.sequence
 		data[0] = 0xff
 		data[1] = 0xff
 		data[2] = 0xff
 
-		data[3] = p.sequence
-
 		if n, err := p.bufWriter.Write(data[:4+mysql.MaxPayloadLen]); err != nil {
 			return errors.Trace(mysql.ErrBadConn)
 		} else if n != (4 + mysql.MaxPayloadLen) {
@@ -182,11 +181,10 @@ func (p *packetIO) writePacket(data []byte) error {
 			data = data[mysql.MaxPayloadLen:]
 		}
 	}
-
+	data[3] = p.sequence
 	data[0] = byte(length)
 	data[1] = byte(length >> 8)
 	data[2] = byte(length >> 16)
-	data[3] = p.sequence
 
 	if n, err := p.bufWriter.Write(data); err != nil {
 		terror.Log(errors.Trace(err))

server/packetio_test.go

@@ -25,6 +25,15 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func BenchmarkPacketIOWrite(b *testing.B) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		var outBuffer bytes.Buffer
+		pkt := &packetIO{bufWriter: bufio.NewWriter(&outBuffer)}
+		_ = pkt.writePacket([]byte{0x6d, 0x44, 0x42, 0x3a, 0x35, 0x36, 0x0, 0x0, 0x0, 0xfc, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x68, 0x54, 0x49, 0x44, 0x3a, 0x31, 0x30, 0x38, 0x0, 0xfe})
+	}
+}
+
 func TestPacketIOWrite(t *testing.T) {
 	// Test write one packet
 	var outBuffer bytes.Buffer