X freelist for PktBuf

before:

BenchmarkLinkNetPipeRTT-4         200000              9763 ns/op            9664 B/op         25 allocs/op
BenchmarkLinkTCPRTT-4             100000             19069 ns/op            9664 B/op         25 allocs/op

after:

BenchmarkLinkNetPipeRTT-4         200000              7872 ns/op            1224 B/op         17 allocs/op
BenchmarkLinkTCPRTT-4             100000             18465 ns/op            1221 B/op         17 allocs/op

go/neo/connection.go

@@ -188,8 +188,8 @@ func newNodeLink(conn net.Conn, role LinkRole) *NodeLink {
 func (nl *NodeLink) newConn(connId uint32) *Conn {
 	c := &Conn{link: nl,
 		connId: connId,
-		rxq:    make(chan *PktBuf, 1), // NOTE non-blocking - see serveRecv XXX +buf
-		txerr:  make(chan error, 1),   // NOTE non-blocking - see Conn.Send
+		rxq:    make(chan *PktBuf, 1),	// NOTE non-blocking - see serveRecv XXX +buf
+		txerr:  make(chan error, 1),	// NOTE non-blocking - see Conn.Send
 		txdown: make(chan struct{}),
 		rxdown: make(chan struct{}),
 	}
@@ -667,7 +667,9 @@ func (c *Conn) errSendShutdown() error {
 	}
 }
 
-// sendPkt sends raw packet via connection
+// sendPkt sends raw packet via connection.
+//
+// on success pkt is freed.
 func (c *Conn) sendPkt(pkt *PktBuf) error {
 	err := c.sendPkt2(pkt)
 	return c.err("send", err)
@@ -746,8 +748,11 @@ func (nl *NodeLink) serveSend() {
 
 const dumpio = false
 
-// sendPkt sends raw packet to peer
-// tx error, if any, is returned as is and is analyzed in serveSend
+// sendPkt sends raw packet to peer.
+//
+// tx error, if any, is returned as is and is analyzed in serveSend.
+//
+// XXX pkt should be freed always or only on error?
 func (nl *NodeLink) sendPkt(pkt *PktBuf) error {
 	if dumpio {
 		// XXX -> log
@@ -757,18 +762,20 @@ func (nl *NodeLink) sendPkt(pkt *PktBuf) error {
 
 	// NOTE Write writes data in full, or it is error
 	_, err := nl.peerLink.Write(pkt.Data)
+	pkt.Free()
 	return err
 }
 
 var ErrPktTooBig = errors.New("packet too big")
 
-// recvPkt receives raw packet from peer
+// recvPkt receives raw packet from peer.
+//
 // rx error, if any, is returned as is and is analyzed in serveRecv
 func (nl *NodeLink) recvPkt() (*PktBuf, error) {
 	// TODO organize rx buffers management (freelist etc)
 
 	// first read to read pkt header and hopefully up to page of data in 1 syscall
-	pkt := &PktBuf{make([]byte, 4096)}
+	pkt := pktAlloc(4096)
 	// TODO reenable, but NOTE next packet can be also prefetched here -> use buffering ?
 	//n, err := io.ReadAtLeast(nl.peerLink, pkt.Data, pktHeaderLen)
 	n, err := io.ReadFull(nl.peerLink, pkt.Data[:pktHeaderLen])
@@ -784,15 +791,7 @@ func (nl *NodeLink) recvPkt() (*PktBuf, error) {
 		return nil, ErrPktTooBig
 	}
 
-	// XXX -> pkt.Data = xbytes.Resize32(pkt.Data[:n], pktLen)
-	if pktLen > uint32(cap(pkt.Data)) {
-		// grow rxbuf
-		rxbuf2 := make([]byte, pktLen)
-		copy(rxbuf2, pkt.Data[:n])
-		pkt.Data = rxbuf2
-	}
-	// cut .Data len to length of packet
-	pkt.Data = pkt.Data[:pktLen]
+	pkt.Resize(int(pktLen))
 
 	// read rest of pkt data, if we need to
 	if n < len(pkt.Data) {
@@ -1129,11 +1128,11 @@ func (c *Conn) err(op string, e error) error {
 // Recv receives message
 // it receives packet and decodes message from it
 func (c *Conn) Recv() (Msg, error) {
-	// TODO use freelist for PktBuf
 	pkt, err := c.recvPkt()
 	if err != nil {
 		return nil, err
 	}
+	defer pkt.Free()
 
 	// decode packet
 	pkth := pkt.Header()
@@ -1162,7 +1161,7 @@ func (c *Conn) Send(msg Msg) error {
 	traceConnSendPre(c, msg)
 
 	l := msg.neoMsgEncodedLen()
-	buf := PktBuf{make([]byte, pktHeaderLen+l)} // TODO -> freelist
+	buf := pktAlloc(pktHeaderLen+l)
 
 	h := buf.Header()
 	// h.ConnId will be set by conn.Send
@@ -1171,9 +1170,8 @@ func (c *Conn) Send(msg Msg) error {
 
 	msg.neoMsgEncode(buf.Payload())
 
-	// XXX why pointer?
 	// XXX more context in err? (msg type)
-	return c.sendPkt(&buf)
+	return c.sendPkt(buf)
 }
 
 
@@ -1185,11 +1183,11 @@ func (c *Conn) Send(msg Msg) error {
 // on error (-1, err) is returned
 func (c *Conn) Expect(msgv ...Msg) (which int, err error) {
 	// XXX a bit dup wrt Recv
-	// TODO use freelist for PktBuf
 	pkt, err := c.recvPkt()
 	if err != nil {
 		return -1, err
 	}
+	defer pkt.Free()
 
 	pkth := pkt.Header()
 	msgCode := ntoh16(pkth.MsgCode)

go/neo/pkt.go

@@ -23,29 +23,60 @@ package neo
 import (
 	"fmt"
 	"reflect"
+	"sync"
 	"unsafe"
-)
 
-// TODO organize rx buffers management (freelist etc)
+	"lab.nexedi.com/kirr/go123/xbytes"
+)
 
-// PktBuf is a buffer with full raw packet (header + data)
+// PktBuf is a buffer with full raw packet (header + data).
 //
-// variables of type PktBuf are usually named "pkb" (packet buffer), similar to "skb" in Linux
+// variables of type PktBuf are usually named "pkb" (packet buffer), similar to "skb" in Linux.
+//
+// Allocate PktBuf via allocPkt() and free via PktBuf.Free().
 type PktBuf struct {
-	Data	[]byte	// whole packet data including all headers	XXX -> Buf ?
+	Data []byte // whole packet data including all headers
 }
 
-// Header returns pointer to packet header
+// Header returns pointer to packet header.
 func (pkt *PktBuf) Header() *PktHeader {
 	// XXX check len(Data) < PktHeader ? -> no, Data has to be allocated with cap >= pktHeaderLen
 	return (*PktHeader)(unsafe.Pointer(&pkt.Data[0]))
 }
 
-// Payload returns []byte representing packet payload
+// Payload returns []byte representing packet payload.
 func (pkt *PktBuf) Payload() []byte {
 	return pkt.Data[pktHeaderLen:]
 }
 
+// Resize resizes pkt to be of length n.
+//
+// semantic = xbytes.Resize.
+func (pkt *PktBuf) Resize(n int) {
+	pkt.Data = xbytes.Resize(pkt.Data, n)
+}
+
+// ---- PktBuf freelist ----
+
+// pktBufPool is sync.Pool<pktBuf>
+var pktBufPool = sync.Pool{New: func() interface{} {
+	return &PktBuf{Data: make([]byte, 0, 4096)}
+}}
+
+// pktAlloc allocates PktBuf with len=n
+func pktAlloc(n int) *PktBuf {
+	pkt := pktBufPool.Get().(*PktBuf)
+	pkt.Data = xbytes.Realloc(pkt.Data, n)
+	return pkt
+}
+
+// Free marks pkt as no longer needed.
+func (pkt *PktBuf) Free() {
+	pktBufPool.Put(pkt)
+}
+
+
+// ---- PktBuf dump ----
 
 // Strings dumps a packet in human-readable form
 func (pkt *PktBuf) String() string {