Initialize connection piece priorities lazily, and using a pool

client.go

@@ -360,7 +360,7 @@ func (cl *Client) ConfigDir() string {
 }
 
 func (t *torrent) connPendPiece(c *connection, piece int) {
-	c.pendPiece(piece, t.Pieces[piece].Priority)
+	c.pendPiece(piece, t.Pieces[piece].Priority, t)
 }
 
 func (cl *Client) raisePiecePriority(t *torrent, piece int, priority piecePriority) {
@@ -1276,7 +1276,6 @@ func (t *torrent) initRequestOrdering(c *connection) {
 	if c.pieceRequestOrder != nil || c.piecePriorities != nil {
 		panic("double init of request ordering")
 	}
-	c.piecePriorities = mathRand.Perm(t.numPieces())
 	c.pieceRequestOrder = pieceordering.New()
 	for i := range iter.N(t.Info.NumPieces()) {
 		if !c.PeerHasPiece(i) {
@@ -1754,6 +1753,12 @@ func (me *Client) deleteConnection(t *torrent, c *connection) bool {
 func (me *Client) dropConnection(t *torrent, c *connection) {
 	me.event.Broadcast()
 	c.Close()
+	if c.piecePriorities != nil {
+		t.connPiecePriorites.Put(c.piecePriorities)
+		// I wonder if it's safe to set it to nil. Probably not. Since it's
+		// only read, it doesn't particularly matter if a closing connection
+		// shares the slice with another connection.
+	}
 	if me.deleteConnection(t, c) {
 		me.openNewConns(t)
 	}

connection.go

@@ -105,11 +105,14 @@ func (cn *connection) localAddr() net.Addr {
 
 // Adjust piece position in the request order for this connection based on the
 // given piece priority.
-func (cn *connection) pendPiece(piece int, priority piecePriority) {
+func (cn *connection) pendPiece(piece int, priority piecePriority, t *torrent) {
 	if priority == PiecePriorityNone {
 		cn.pieceRequestOrder.DeletePiece(piece)
 		return
 	}
+	if cn.piecePriorities == nil {
+		cn.piecePriorities = t.newConnPiecePriorities()
+	}
 	pp := cn.piecePriorities[piece]
 	// Priority regions not to scale. Within each region, piece is randomized
 	// according to connection.

connection_test.go

@@ -71,22 +71,22 @@ func TestPieceRequestOrder(t *testing.T) {
 		piecePriorities:   []int{1, 4, 0, 3, 2},
 	}
 	testRequestOrder(nil, c.pieceRequestOrder, t)
-	c.pendPiece(2, PiecePriorityNone)
+	c.pendPiece(2, PiecePriorityNone, nil)
 	testRequestOrder(nil, c.pieceRequestOrder, t)
-	c.pendPiece(1, PiecePriorityNormal)
-	c.pendPiece(2, PiecePriorityNormal)
+	c.pendPiece(1, PiecePriorityNormal, nil)
+	c.pendPiece(2, PiecePriorityNormal, nil)
 	testRequestOrder([]int{2, 1}, c.pieceRequestOrder, t)
-	c.pendPiece(0, PiecePriorityNormal)
+	c.pendPiece(0, PiecePriorityNormal, nil)
 	testRequestOrder([]int{2, 0, 1}, c.pieceRequestOrder, t)
-	c.pendPiece(1, PiecePriorityReadahead)
+	c.pendPiece(1, PiecePriorityReadahead, nil)
 	testRequestOrder([]int{1, 2, 0}, c.pieceRequestOrder, t)
-	c.pendPiece(4, PiecePriorityNow)
+	c.pendPiece(4, PiecePriorityNow, nil)
 	// now(4), r(1), normal(0, 2)
 	testRequestOrder([]int{4, 1, 2, 0}, c.pieceRequestOrder, t)
-	c.pendPiece(2, PiecePriorityReadahead)
+	c.pendPiece(2, PiecePriorityReadahead, nil)
 	// N(4), R(1, 2), N(0)
 	testRequestOrder([]int{4, 2, 1, 0}, c.pieceRequestOrder, t)
-	c.pendPiece(1, PiecePriorityNow)
+	c.pendPiece(1, PiecePriorityNow, nil)
 	// now(4, 1), readahead(2), normal(0)
 	// in the same order, the keys will be: -15+6, -15+12, -5, 1
 	// so we test that a very low priority (for this connection), "now"
@@ -94,7 +94,7 @@ func TestPieceRequestOrder(t *testing.T) {
 	testRequestOrder([]int{4, 2, 1, 0}, c.pieceRequestOrder, t)
 	// Note this intentially sets to None a piece that's not in the order.
 	for i := range iter.N(5) {
-		c.pendPiece(i, PiecePriorityNone)
+		c.pendPiece(i, PiecePriorityNone, nil)
 	}
 	testRequestOrder(nil, c.pieceRequestOrder, t)
 }

torrent.go

@@ -2,9 +2,11 @@ package torrent
 
 import (
 	"container/heap"
+	"expvar"
 	"fmt"
 	"io"
 	"log"
+	"math/rand"
 	"net"
 	"sort"
 	"sync"
@@ -102,6 +104,23 @@ type torrent struct {
 
 	// Closed when .Info is set.
 	gotMetainfo chan struct{}
+
+	connPiecePriorites sync.Pool
+}
+
+var (
+	piecePrioritiesReused = expvar.NewInt("piecePrioritiesReused")
+	piecePrioritiesNew    = expvar.NewInt("piecePrioritiesNew")
+)
+
+func (t *torrent) newConnPiecePriorities() []int {
+	_ret := t.connPiecePriorites.Get()
+	if _ret != nil {
+		piecePrioritiesReused.Add(1)
+		return _ret.([]int)
+	}
+	piecePrioritiesNew.Add(1)
+	return rand.Perm(t.numPieces())
 }
 
 func (t *torrent) pieceComplete(piece int) bool {