network: implement adaptive peer requests

When the network is big enough, MinPeers may be suboptimal for good network
connectivity, but if we know the network size we can do some estimation on the
number of sufficient peers.

pkg/network/discovery.go

@@ -19,6 +19,7 @@ const (
 type Discoverer interface {
 	BackFill(...string)
 	GetFanOut() int
+	NetworkSize() int
 	PoolCount() int
 	RequestRemote(int)
 	RegisterBadAddr(string)
@@ -48,6 +49,7 @@ type DefaultDiscovery struct {
 	unconnectedAddrs map[string]int
 	attempted        map[string]bool
 	optimalFanOut    int32
+	networkSize      int32
 	requestCh        chan int
 }
 
@@ -233,6 +235,11 @@ func (d *DefaultDiscovery) GetFanOut() int {
 	return int(atomic.LoadInt32(&d.optimalFanOut))
 }
 
+// NetworkSize returns the estimated network size.
+func (d *DefaultDiscovery) NetworkSize() int {
+	return int(atomic.LoadInt32(&d.networkSize))
+}
+
 // updateNetSize updates network size estimation metric. Must be called under read lock.
 func (d *DefaultDiscovery) updateNetSize() {
 	var netsize = len(d.connectedAddrs) + len(d.unconnectedAddrs) + 1 // 1 for the node itself.
@@ -242,6 +249,7 @@ func (d *DefaultDiscovery) updateNetSize() {
 	}
 
 	atomic.StoreInt32(&d.optimalFanOut, int32(fanOut+0.5)) // Truncating conversion, hence +0.5.
+	atomic.StoreInt32(&d.networkSize, int32(netsize))
 	updateNetworkSizeMetric(netsize)
 	updatePoolCountMetric(d.poolCount())
 }

pkg/network/helper_test.go

@@ -40,6 +40,11 @@ func (d *testDiscovery) RegisterBadAddr(addr string) {
 	d.bad = append(d.bad, addr)
 }
 func (d *testDiscovery) GetFanOut() int {
+	d.Lock()
+	defer d.Unlock()
+	return (len(d.connected) + len(d.backfill)) * 2 / 3
+}
+func (d *testDiscovery) NetworkSize() int {
 	d.Lock()
 	defer d.Unlock()
 	return len(d.connected) + len(d.backfill)

pkg/network/server.go

@@ -387,10 +387,28 @@ func (s *Server) ConnectedPeers() []string {
 // while itself dealing with peers management (handling connects/disconnects).
 func (s *Server) run() {
 	go s.runProto()
-	for {
-		if s.PeerCount() < s.MinPeers {
+	for loopCnt := 0; ; loopCnt++ {
+		var (
+			netSize = s.discovery.NetworkSize()
+			// "Optimal" number of peers.
+			optimalN = s.discovery.GetFanOut() * 2
+			// Real number of peers.
+			peerN = s.PeerCount()
+		)
+
+		if peerN < s.MinPeers {
+			// Starting up or going below the minimum -> quickly get many new peers.
 			s.discovery.RequestRemote(s.AttemptConnPeers)
+		} else if s.MinPeers > 0 && loopCnt%s.MinPeers == 0 && optimalN > peerN && optimalN < s.MaxPeers && optimalN < netSize {
+			// Having some number of peers, but probably can get some more, the network is big.
+			// It also allows to start picking up new peers proactively, before we suddenly have <s.MinPeers of them.
+			var connN = s.AttemptConnPeers
+			if connN > optimalN-peerN {
+				connN = optimalN - peerN
+			}
+			s.discovery.RequestRemote(connN)
 		}
+
 		if s.discovery.PoolCount() < minPoolCount {
 			s.broadcastHPMessage(NewMessage(CMDGetAddr, payload.NewNullPayload()))
 		}