TUN-3208: Reduce copies and allocations on h2mux write path. Pre-allocate 16KB write buffer on the first write if possible. Use explicit byte array for chunks on write thread to avoid copying through intermediate buffer due to io.CopyN.

benchmark                                    old ns/op     new ns/op     delta
BenchmarkSingleStreamLargeResponseBody-8     17786594      12163494      -31.61%

benchmark                                    old allocs     new allocs     delta
BenchmarkSingleStreamLargeResponseBody-8     17086          15869          -7.12%

benchmark                                    old bytes     new bytes     delta
BenchmarkSingleStreamLargeResponseBody-8     58215169      21604391      -62.89%

h2mux/h2mux.go

@@ -7,11 +7,12 @@ import (
 	"sync"
 	"time"
 
-	"github.com/cloudflare/cloudflared/logger"
 	"github.com/prometheus/client_golang/prometheus"
 	"golang.org/x/net/http2"
 	"golang.org/x/net/http2/hpack"
 	"golang.org/x/sync/errgroup"
+
+	"github.com/cloudflare/cloudflared/logger"
 )
 
 const (
@@ -21,6 +22,7 @@ const (
 	defaultTimeout           time.Duration = 5 * time.Second
 	defaultRetries           uint64        = 5
 	defaultWriteBufferMaxLen int           = 1024 * 1024 // 1mb
+	writeBufferInitialSize   int           = 16 * 1024 // 16KB
 
 	SettingMuxerMagic http2.SettingID = 0x42db
 	MuxerMagicOrigin  uint32          = 0xa2e43c8b
@@ -206,9 +208,9 @@ func Handshake(
 		initialStreamWindow:     m.config.DefaultWindowSize,
 		streamWindowMax:         m.config.MaxWindowSize,
 		streamWriteBufferMaxLen: m.config.StreamWriteBufferMaxLen,
-		r:              m.r,
-		metricsUpdater: m.muxMetricsUpdater,
-		bytesRead:      inBoundCounter,
+		r:                       m.r,
+		metricsUpdater:          m.muxMetricsUpdater,
+		bytesRead:               inBoundCounter,
 	}
 	m.muxWriter = &MuxWriter{
 		f:               m.f,

h2mux/muxedstream.go

@@ -138,6 +138,14 @@ func (s *MuxedStream) Write(p []byte) (int, error) {
 	if s.writeEOF {
 		return 0, io.EOF
 	}
+
+	// pre-allocate some space in the write buffer if possible
+	if buffer, ok := s.writeBuffer.(*bytes.Buffer); ok {
+		if buffer.Cap() == 0 {
+			buffer.Grow(writeBufferInitialSize)
+		}
+	}
+
 	totalWritten := 0
 	for totalWritten < len(p) {
 		// If the buffer is full, block till there is more room.
@@ -367,7 +375,9 @@ type streamChunk struct {
 	// true if data frames should be sent
 	sendData bool
 	eof      bool
-	buffer   bytes.Buffer
+
+	buffer []byte
+	offset int
 }
 
 // getChunk atomically extracts a chunk of data to be written by MuxWriter.
@@ -385,8 +395,17 @@ func (s *MuxedStream) getChunk() *streamChunk {
 		eof:          s.writeEOF && uint32(s.writeBuffer.Len()) <= s.sendWindow,
 	}
 	// Copy at most s.sendWindow bytes, adjust the sendWindow accordingly
-	writeLen, _ := io.CopyN(&chunk.buffer, s.writeBuffer, int64(s.sendWindow))
-	s.sendWindow -= uint32(writeLen)
+	toCopy := int(s.sendWindow)
+	if toCopy > s.writeBuffer.Len() {
+		toCopy = s.writeBuffer.Len()
+	}
+
+	if toCopy > 0 {
+		buf := make([]byte, toCopy)
+		writeLen, _ := s.writeBuffer.Read(buf)
+		chunk.buffer = buf[:writeLen]
+		s.sendWindow -= uint32(writeLen)
+	}
 
 	// Allow MuxedStream::Write() to continue, if needed
 	if s.writeBuffer.Len() < s.writeBufferMaxLen {
@@ -421,8 +440,15 @@ func (c *streamChunk) sendDataFrame() bool {
 }
 
 func (c *streamChunk) nextDataFrame(frameSize int) (payload []byte, endStream bool) {
-	payload = c.buffer.Next(frameSize)
-	if c.buffer.Len() == 0 {
+	bytesLeft := len(c.buffer) - c.offset
+	if frameSize > bytesLeft {
+		frameSize = bytesLeft
+	}
+	nextOffset := c.offset + frameSize
+	payload = c.buffer[c.offset:nextOffset]
+	c.offset = nextOffset
+
+	if c.offset == len(c.buffer) {
 		// this is the last data frame in this chunk
 		c.sendData = false
 		if c.eof {