optimise generation of the initial PEX

pex.go

@@ -2,6 +2,7 @@ package torrent
 
 import (
 	"net"
+	"sync"
 
 	"github.com/anacrolix/dht/v2/krpc"
 	pp "github.com/anacrolix/torrent/peer_protocol"
@@ -143,12 +144,20 @@ type pexState struct {
 	ev        []pexEvent    // event feed, append-only
 	hold      []pexEvent    // delayed drops
 	nc        int           // net number of alive conns
+	initCache pexMsgFactory // last generated initial message
+	initSeq   int           // number of events which went into initCache
+	initLock  sync.RWMutex  // serialise access to initCache and initSeq
 }
 
+// Reset wipes the state clean, releasing resources. Called from Torrent.Close().
 func (s *pexState) Reset() {
 	s.ev = nil
 	s.hold = nil
 	s.nc = 0
+	s.initLock.Lock()
+	s.initCache = pexMsgFactory{}
+	s.initSeq = 0
+	s.initLock.Unlock()
 }
 
 func (s *pexState) Add(c *PeerConn) {
@@ -179,6 +188,10 @@ func (s *pexState) Drop(c *PeerConn) {
 // Generate a PEX message based on the event feed. Also returns an index to pass to the subsequent
 // calls, producing incremental deltas.
 func (s *pexState) Genmsg(start int) (pp.PexMsg, int) {
+	if start == 0 {
+		return s.genmsg0()
+	}
+
 	var factory pexMsgFactory
 	n := start
 	for _, e := range s.ev[start:] {
@@ -190,3 +203,17 @@ func (s *pexState) Genmsg(start int) (pp.PexMsg, int) {
 	}
 	return factory.PexMsg(), n
 }
+
+func (s *pexState) genmsg0() (pp.PexMsg, int) {
+	s.initLock.Lock()
+	for _, e := range s.ev[s.initSeq:] {
+		s.initCache.addEvent(e)
+		s.initSeq++
+	}
+	s.initLock.Unlock()
+	s.initLock.RLock()
+	n := s.initSeq
+	msg := s.initCache.PexMsg()
+	s.initLock.RUnlock()
+	return msg, n
+}

pex_test.go

@@ -307,11 +307,11 @@ func TestPexGenmsg(t *testing.T) {
 func addrgen(n int) chan net.Addr {
 	c := make(chan net.Addr)
 	go func() {
+		defer close(c)
 		for i := 4747; i < 65535 && n > 0; i++ {
 			c <- &net.TCPAddr{IP: net.IPv4(127, 0, 0, 1), Port: i}
 			n--
 		}
-		close(c)
 	}()
 	return c
 }
@@ -335,6 +335,24 @@ func TestPexInitialNoCutoff(t *testing.T) {
 	require.EqualValues(t, 0, len(m.Dropped6))
 }
 
+func benchmarkPexInitialN(b *testing.B, npeers int) {
+	for i := 0; i < b.N; i++ {
+		var s pexState
+		c := addrgen(npeers)
+		for addr := range c {
+			s.Add(&PeerConn{peer: peer{RemoteAddr: addr}})
+			s.Genmsg(0)
+		}
+	}
+}
+
+// obtain at least 5 points, e.g. to plot a graph
+func BenchmarkPexInitial4(b *testing.B)   { benchmarkPexInitialN(b, 4) }
+func BenchmarkPexInitial50(b *testing.B)  { benchmarkPexInitialN(b, 50) }
+func BenchmarkPexInitial100(b *testing.B) { benchmarkPexInitialN(b, 100) }
+func BenchmarkPexInitial200(b *testing.B) { benchmarkPexInitialN(b, 200) }
+func BenchmarkPexInitial400(b *testing.B) { benchmarkPexInitialN(b, 400) }
+
 func TestPexAdd(t *testing.T) {
 	t.Run("ipv4", func(t *testing.T) {
 		addrs := addrs4