wgengine/bench: improved rate selection.

The old decay-based one took a while to converge. This new one (based
very loosely on TCP BBR) seems to converge quickly on what seems to be
the best speed.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

wgengine/bench/bench.go

@@ -90,7 +90,7 @@ func main() {
 	traf.Start(Addr1.IP, Addr2.IP, PayloadSize+ICMPMinSize, 0)
 
 	var cur, prev Snapshot
-	var pps float64
+	var pps int64
 	i := 0
 	for {
 		i += 1
@@ -104,7 +104,7 @@ func main() {
 			if prev.WhenNsec == 0 {
 				logf("tx=%-6d rx=%-6d", d.TxPackets, d.RxPackets)
 			} else {
-				logf("%v @%7.0f pkt/sec", d, pps)
+				logf("%v @%7d pkt/s", d, pps)
 			}
 		}
 

wgengine/bench/bench_test.go

@@ -81,7 +81,7 @@ func runOnce(b *testing.B, setup SetupFunc, payload int) {
 	traf.Start(Addr1.IP, Addr2.IP, payload, int64(b.N))
 
 	var cur, prev Snapshot
-	var pps float64
+	var pps int64
 	i := 0
 	for traf.Running() {
 		i += 1
@@ -93,7 +93,7 @@ func runOnce(b *testing.B, setup SetupFunc, payload int) {
 			d := cur.Sub(prev)
 
 			if prev.WhenNsec != 0 {
-				logf("%v @%7.0f pkt/sec", d, pps)
+				logf("%v @%7d pkt/sec", d, pps)
 			}
 		}
 

wgengine/bench/trafficgen.go

@@ -74,9 +74,9 @@ type TrafficGen struct {
 	// caller wants to go.
 	nsPerPacket int64
 
-	// bestPPS is the "best observed packets-per-second" in recent
+	// ppsHistory is the observed packets-per-second from recent
-	// memory.
+	// samples.
-	bestPPS float64
+	ppsHistory [5]int64
 }
 
 // NewTrafficGen creates a new, initially locked, TrafficGen.
@@ -221,28 +221,42 @@ func (t *TrafficGen) GotPacket(b []byte, ofs int) {
 // 1% to receive them, leading to a misleading throughput calculation.
 //
 // Call this function multiple times per second.
-func (t *TrafficGen) Adjust() (pps float64) {
+func (t *TrafficGen) Adjust() (pps int64) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
 
+	d := t.cur.Sub(t.prev)
+
 	// don't adjust rate until the first full period *after* receiving
 	// the first packet. This skips any handshake time in the underlying
 	// transport.
-	if t.prev.LastSeqRx == 0 {
+	if t.prev.LastSeqRx == 0 || d.DurationNsec == 0 {
 		t.prev = t.cur
 		return 0 // no estimate yet, continue at max speed
 	}
 
-	d := t.cur.Sub(t.prev)
+	pps = int64(d.RxPackets) * 1e9 / int64(d.DurationNsec)
-	t.bestPPS *= 0.97
+
-	pps = float64(d.RxPackets) * 1e9 / float64(d.DurationNsec)
+	// We use a rate selection algorithm based loosely on TCP BBR.
+	// Basically, we set the transmit rate to be a bit higher than
+	// the best observed transmit rate in the last several time
+	// periods. This guarantees some packet loss, but should converge
+	// quickly on a rate near the sustainable maximum.
+	bestPPS := pps
+	for _, p := range t.ppsHistory {
+		if p > bestPPS {
+			bestPPS = p
+		}
+	}
 	if pps > 0 && t.prev.WhenNsec > 0 {
-		if pps > t.bestPPS {
+		copy(t.ppsHistory[1:], t.ppsHistory[0:len(t.ppsHistory)-1])
-			t.bestPPS = pps
+		t.ppsHistory[0] = pps
 	}
-		t.nsPerPacket = int64(1e9 / t.bestPPS)
+	if bestPPS > 0 {
+		pps = bestPPS * 103 / 100
+		t.nsPerPacket = int64(1e9 / pps)
 	}
 	t.prev = t.cur
 
-	return t.bestPPS
+	return pps
 }