crypto/tls: adjust dynamic record sizes to grow arithmetically

The current code, introduced after Go 1.6 to improve latency on
low-bandwidth connections, sends 1 kB packets until 1 MB has been sent,
and then sends 16 kB packets (the maximum record size).

Unfortunately this decreases throughput for 1-16 MB responses by 20% or so.

Following discussion on #15713, change cutoff to 128 kB sent
and also grow the size allowed for successive packets:
1 kB, 2 kB, 3 kB, ..., 15 kB, 16 kB.
This fixes the throughput problems: the overhead is now closer to 2%.

I hope this still helps with latency but I don't have a great way to test it.
At the least, it's not worse than Go 1.6.

Comparing MaxPacket vs DynamicPacket benchmarks:

name              maxpkt time/op  dyn. time/op delta
Throughput/1MB-8    5.07ms ± 7%   5.21ms ± 7%  +2.73%  (p=0.023 n=16+16)
Throughput/2MB-8   15.7ms ±201%    8.4ms ± 5%    ~     (p=0.604 n=20+16)
Throughput/4MB-8    14.3ms ± 1%   14.5ms ± 1%  +1.53%  (p=0.000 n=16+16)
Throughput/8MB-8    26.6ms ± 1%   26.8ms ± 1%  +0.47%  (p=0.003 n=19+18)
Throughput/16MB-8   51.0ms ± 1%   51.3ms ± 1%  +0.47%  (p=0.000 n=20+20)
Throughput/32MB-8    100ms ± 1%    100ms ± 1%  +0.24%  (p=0.033 n=20+20)
Throughput/64MB-8    197ms ± 0%    198ms ± 0%  +0.56%   (p=0.000 n=18+7)

The small MB runs are bimodal in both cases, probably GC pauses.
But there's clearly no general slowdown anymore.

Fixes #15713.

Change-Id: I5fc44680ba71812d24baac142bceee0e23f2e382
Reviewed-on: https://go-review.googlesource.com/23487Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/crypto/tls/conn.go

@@ -76,9 +76,10 @@ type Conn struct {
 	input    *block       // application data waiting to be read
 	hand     bytes.Buffer // handshake data waiting to be read
 
-	// bytesSent counts the number of bytes of application data that have
-	// been sent.
+	// bytesSent counts the bytes of application data sent.
+	// packetsSent counts packets.
 	bytesSent int64
+	packetsSent int64
 	
 	// activeCall is an atomic int32; the low bit is whether Close has
 	// been called. the rest of the bits are the number of goroutines
@@ -732,7 +733,7 @@ const (
 	// recordSizeBoostThreshold is the number of bytes of application data
 	// sent after which the TLS record size will be increased to the
 	// maximum.
-	recordSizeBoostThreshold = 1 * 1024 * 1024
+	recordSizeBoostThreshold = 128 * 1024
 )
 
 // maxPayloadSizeForWrite returns the maximum TLS payload size to use for the
@@ -788,7 +789,18 @@ func (c *Conn) maxPayloadSizeForWrite(typ recordType, explicitIVLen int) int {
 		}
 	}
 	
-	return payloadBytes
+	// Allow packet growth in arithmetic progression up to max.
+	pkt := c.packetsSent
+	c.packetsSent++
+	if pkt > 1000 {
+		return maxPlaintext // avoid overflow in multiply below
+	}
+	
+	n := payloadBytes * int(pkt + 1)
+	if n > maxPlaintext {
+		n = maxPlaintext
+	}
+	return n
 }
 
 // writeRecordLocked writes a TLS record with the given type and payload to the

src/crypto/tls/conn_test.go

@@ -208,13 +208,10 @@ func runDynamicRecordSizingTest(t *testing.T, config *Config) {
 	seenLargeRecord := false
 	for i, size := range recordSizes {
 		if !seenLargeRecord {
-			if size > tcpMSSEstimate {
-				if i < 100 {
+			if size > (i+1)*tcpMSSEstimate {
 				t.Fatalf("Record #%d has size %d, which is too large too soon", i, size)
 			}
-				if size <= maxPlaintext {
-					t.Fatalf("Record #%d has odd size %d", i, size)
-				}
+			if size >= maxPlaintext {
 				seenLargeRecord = true
 			}
 		} else if size <= maxPlaintext {

src/crypto/tls/tls_test.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"internal/testenv"
 	"io"
+	"math"
 	"net"
 	"strings"
 	"testing"
@@ -146,7 +147,7 @@ func TestX509MixedKeyPair(t *testing.T) {
 	}
 }
 
-func newLocalListener(t *testing.T) net.Listener {
+func newLocalListener(t testing.TB) net.Listener {
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		ln, err = net.Listen("tcp6", "[::1]:0")
@@ -473,3 +474,153 @@ func (w *changeImplConn) Close() error {
 	}
 	return w.Conn.Close()
 }
+
+func throughput(b *testing.B, totalBytes int64, dynamicRecordSizingDisabled bool) {
+	ln := newLocalListener(b)
+	defer ln.Close()
+
+	var serr error
+	go func() {
+		for i := 0; i < b.N; i++ {
+			sconn, err := ln.Accept()
+			if err != nil {
+				serr = err
+				return
+			}
+			serverConfig := *testConfig
+			serverConfig.DynamicRecordSizingDisabled = dynamicRecordSizingDisabled
+			srv := Server(sconn, &serverConfig)
+			if err := srv.Handshake(); err != nil {
+				serr = fmt.Errorf("handshake: %v", err)
+				return
+			}
+			io.Copy(srv, srv)
+		}
+	}()
+
+	b.SetBytes(totalBytes)
+	clientConfig := *testConfig
+	clientConfig.DynamicRecordSizingDisabled = dynamicRecordSizingDisabled
+
+	buf := make([]byte, 1<<16)
+	chunks := int(math.Ceil(float64(totalBytes) / float64(len(buf))))
+	for i := 0; i < b.N; i++ {
+		conn, err := Dial("tcp", ln.Addr().String(), &clientConfig)
+		if err != nil {
+			b.Fatal(err)
+		}
+		for j := 0; j < chunks; j++ {
+			_, err := conn.Write(buf)
+			if err != nil {
+				b.Fatal(err)
+			}
+			_, err = io.ReadFull(conn, buf)
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+		conn.Close()
+	}
+}
+
+func BenchmarkThroughput(b *testing.B) {
+	for _, mode := range []string{"Max", "Dynamic"} {
+		for size := 1; size <= 64; size<<=1{
+			name := fmt.Sprintf("%sPacket/%dMB", mode, size)
+			b.Run(name, func(b *testing.B) {
+				throughput(b, int64(size<<20), mode == "Max")
+			})
+		}
+	}
+}
+
+type slowConn struct {
+	net.Conn
+	bps int
+}
+
+func (c *slowConn) Write(p []byte) (int, error) {
+	if c.bps == 0 {
+		panic("too slow")
+	}
+	t0 := time.Now()
+	wrote := 0
+	for wrote < len(p) {
+		time.Sleep(100*time.Microsecond)
+		allowed := int(time.Since(t0).Seconds() * float64(c.bps)) / 8
+		if allowed > len(p) {
+			allowed = len(p)
+		}
+		if wrote < allowed {
+			n, err := c.Conn.Write(p[wrote:allowed])
+			wrote += n
+			if err != nil {
+				return wrote, err
+			}
+		}
+	}
+	return len(p), nil
+}
+
+func latency(b *testing.B, bps int, dynamicRecordSizingDisabled bool) {
+	ln := newLocalListener(b)
+	defer ln.Close()
+
+	var serr error
+	go func() {
+		for i := 0; i < b.N; i++ {
+			sconn, err := ln.Accept()
+			if err != nil {
+				serr = err
+				return
+			}
+			serverConfig := *testConfig
+			serverConfig.DynamicRecordSizingDisabled = dynamicRecordSizingDisabled
+			srv := Server(&slowConn{sconn, bps}, &serverConfig)
+			if err := srv.Handshake(); err != nil {
+				serr = fmt.Errorf("handshake: %v", err)
+				return
+			}
+			io.Copy(srv, srv)
+		}
+	}()
+
+	clientConfig := *testConfig
+	clientConfig.DynamicRecordSizingDisabled = dynamicRecordSizingDisabled
+
+	buf := make([]byte, 16384)
+	peek := make([]byte, 1)
+
+	for i := 0; i < b.N; i++ {
+		conn, err := Dial("tcp", ln.Addr().String(), &clientConfig)
+		if err != nil {
+			b.Fatal(err)
+		}
+		// make sure we're connected and previous connection has stopped
+		if _, err := conn.Write(buf[:1]); err != nil {
+			b.Fatal(err)
+		}
+		if _, err := io.ReadFull(conn, peek); err != nil {
+			b.Fatal(err)
+		}
+		if _, err := conn.Write(buf); err != nil {
+			b.Fatal(err)
+		}
+		if _, err = io.ReadFull(conn, peek); err != nil {
+			b.Fatal(err)
+		}
+		conn.Close()
+	}
+}
+
+
+func BenchmarkLatency(b *testing.B) {
+	for _, mode := range []string{"Max", "Dynamic"} {
+		for _, kbps := range []int{200, 500, 1000, 2000, 5000} {
+			name := fmt.Sprintf("%sPacket/%dkbps", mode, kbps)
+			b.Run(name, func(b *testing.B) {
+				latency(b, kbps*1000, mode == "Max")
+			})
+		}
+	}
+}