Limit playout delay change for high jitter (#2635)

* Limit playout delay change for high jitter

* fix test

pkg/sfu/playoutdelay.go

@@ -17,6 +17,7 @@ package sfu
 import (
 	"sync"
 	"sync/atomic"
+	"time"
 
 	"github.com/livekit/livekit-server/pkg/sfu/buffer"
 	"github.com/livekit/livekit-server/pkg/sfu/rtpextension"
@@ -30,11 +31,11 @@ const (
 	PlayoutDelaySending
 	PlayoutDelayAcked
 
-	jitterLowMultiToDelay  = 10
-	jitterHighMultiToDelay = 15
-	jitterHighThreshold    = 15
-
+	jitterMultiToDelay      = 10
 	targetDelayLogThreshold = 500
+
+	// limit max delay change to make it smoother for a/v sync
+	maxDelayChangePerSec = 80
 )
 
 func (s PlayoutDelayState) String() string {
@@ -56,6 +57,7 @@ type PlayoutDelayController struct {
 	currentDelay       uint32
 	extBytes           atomic.Value //[]byte
 	sendingAtSeq       uint16
+	sendingAtTime      time.Time
 	logger             logger.Logger
 	rtpStats           *buffer.RTPStatsSender
 	snapshotID         uint32
@@ -89,20 +91,21 @@ func (c *PlayoutDelayController) SetJitter(jitter uint32) {
 	}
 
 	c.lock.Lock()
-	multi := jitterLowMultiToDelay
-	if jitter >= jitterHighThreshold {
-		multi = jitterHighMultiToDelay
-	}
-	targetDelay := jitter * uint32(multi)
+	targetDelay := jitter * jitterMultiToDelay
 	if nackPercent > 60 {
 		targetDelay += (nackPercent - 60) * 2
 	}
 
-	// increase delay quickly, decrease slowly to make fps more stable
-	if targetDelay > c.currentDelay {
-		targetDelay = (targetDelay-c.currentDelay)*3/4 + c.currentDelay
-	} else {
-		targetDelay = c.currentDelay - (c.currentDelay-targetDelay)/5
+	elapsed := time.Since(c.sendingAtTime)
+	delayChangeLimit := uint32(maxDelayChangePerSec * elapsed.Seconds())
+	if delayChangeLimit > maxDelayChangePerSec {
+		delayChangeLimit = maxDelayChangePerSec
+	}
+
+	if targetDelay > c.currentDelay+delayChangeLimit {
+		targetDelay = c.currentDelay + delayChangeLimit
+	} else if c.currentDelay > targetDelay+delayChangeLimit {
+		targetDelay = c.currentDelay - delayChangeLimit
 	}
 	if targetDelay < c.minDelay {
 		targetDelay = c.minDelay
@@ -138,6 +141,7 @@ func (c *PlayoutDelayController) GetDelayExtension(seq uint16) []byte {
 		c.lock.Lock()
 		c.state.Store(int32(PlayoutDelaySending))
 		c.sendingAtSeq = seq
+		c.sendingAtTime = time.Now()
 		c.lock.Unlock()
 		return c.extBytes.Load().([]byte)
 	case PlayoutDelaySending:

pkg/sfu/playoutdelay_test.go

@@ -16,6 +16,7 @@ package sfu
 
 import (
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 
@@ -26,23 +27,23 @@ import (
 
 func TestPlayoutDelay(t *testing.T) {
 	stats := buffer.NewRTPStatsSender(buffer.RTPStatsParams{ClockRate: 900000, Logger: logger.GetLogger()})
-	c, err := NewPlayoutDelayController(100, 1000, logger.GetLogger(), stats)
+	c, err := NewPlayoutDelayController(100, 120, logger.GetLogger(), stats)
 	require.NoError(t, err)
 
 	ext := c.GetDelayExtension(100)
-	playoutDelayEqual(t, ext, 100, 1000)
+	playoutDelayEqual(t, ext, 100, 120)
 
 	ext = c.GetDelayExtension(105)
-	playoutDelayEqual(t, ext, 100, 1000)
+	playoutDelayEqual(t, ext, 100, 120)
 
 	// seq acked before delay changed
 	c.OnSeqAcked(65534)
 	ext = c.GetDelayExtension(105)
-	playoutDelayEqual(t, ext, 100, 1000)
+	playoutDelayEqual(t, ext, 100, 120)
 
 	c.OnSeqAcked(90)
 	ext = c.GetDelayExtension(105)
-	playoutDelayEqual(t, ext, 100, 1000)
+	playoutDelayEqual(t, ext, 100, 120)
 
 	// seq acked, no extension sent for new packet
 	c.OnSeqAcked(103)
@@ -55,16 +56,19 @@ func TestPlayoutDelay(t *testing.T) {
 	require.Nil(t, ext)
 
 	// delay changed, generate new extension to send
+	time.Sleep(200 * time.Millisecond)
 	c.SetJitter(50)
+	t.Log(c.currentDelay, c.state.Load())
 	ext = c.GetDelayExtension(108)
 	var delay rtpextension.PlayOutDelay
 	require.NoError(t, delay.Unmarshal(ext))
 	require.Greater(t, delay.Min, uint16(100))
 
 	// can't go above max
+	time.Sleep(200 * time.Millisecond)
 	c.SetJitter(10000)
 	ext = c.GetDelayExtension(109)
-	playoutDelayEqual(t, ext, 1000, 1000)
+	playoutDelayEqual(t, ext, 120, 120)
 }
 
 func playoutDelayEqual(t *testing.T, data []byte, min, max uint16) {