runtime: use doubly-linked lists for channel send/recv queues.

Avoids a potential O(n^2) performance problem when dequeueing
from very popular channels.

benchmark                old ns/op     new ns/op     delta
BenchmarkChanPopular     2563782       627201        -75.54%

Change-Id: I231aaeafea0ecd93d27b268a0b2128530df3ddd6
Reviewed-on: https://go-review.googlesource.com/1200
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/select.c

@@ -336,7 +336,7 @@ selecttype(int32 size)
 	sudog = nod(OTSTRUCT, N, N);
 	sudog->list = list(sudog->list, nod(ODCLFIELD, newname(lookup("g")), typenod(ptrto(types[TUINT8]))));
 	sudog->list = list(sudog->list, nod(ODCLFIELD, newname(lookup("selectdone")), typenod(ptrto(types[TUINT8]))));
-	sudog->list = list(sudog->list, nod(ODCLFIELD, newname(lookup("link")), typenod(ptrto(types[TUINT8]))));
+	sudog->list = list(sudog->list, nod(ODCLFIELD, newname(lookup("next")), typenod(ptrto(types[TUINT8]))));
 	sudog->list = list(sudog->list, nod(ODCLFIELD, newname(lookup("prev")), typenod(ptrto(types[TUINT8]))));
 	sudog->list = list(sudog->list, nod(ODCLFIELD, newname(lookup("elem")), typenod(ptrto(types[TUINT8]))));
 	sudog->list = list(sudog->list, nod(ODCLFIELD, newname(lookup("releasetime")), typenod(types[TUINT64])));

src/runtime/chan.go

@@ -614,12 +614,15 @@ func reflect_chancap(c *hchan) int {
 
 func (q *waitq) enqueue(sgp *sudog) {
 	sgp.next = nil
-	if q.first == nil {
+	x := q.last
+	if x == nil {
+		sgp.prev = nil
 		q.first = sgp
 		q.last = sgp
 		return
 	}
-	q.last.next = sgp
+	sgp.prev = x
+	x.next = sgp
 	q.last = sgp
 }
 
@@ -629,10 +632,14 @@ func (q *waitq) dequeue() *sudog {
 		if sgp == nil {
 			return nil
 		}
-		q.first = sgp.next
-		sgp.next = nil
-		if q.last == sgp {
+		y := sgp.next
+		if y == nil {
+			q.first = nil
 			q.last = nil
+		} else {
+			y.prev = nil
+			q.first = y
+			sgp.next = nil // mark as removed (see dequeueSudog)
 		}
 
 		// if sgp participates in a select and is already signaled, ignore it

src/runtime/chan_test.go

@@ -818,3 +818,26 @@ func BenchmarkChanSem(b *testing.B) {
 		}
 	})
 }
+
+func BenchmarkChanPopular(b *testing.B) {
+	const n = 1000
+	c := make(chan bool)
+	var a []chan bool
+	for j := 0; j < n; j++ {
+		d := make(chan bool)
+		a = append(a, d)
+		go func() {
+			for i := 0; i < b.N; i++ {
+				select {
+				case <-c:
+				case <-d:
+				}
+			}
+		}()
+	}
+	for i := 0; i < b.N; i++ {
+		for _, d := range a {
+			d <- true
+		}
+	}
+}

src/runtime/select.go

@@ -389,6 +389,7 @@ loop:
 			k.releasetime = sglist.releasetime
 		}
 		if sg == sglist {
+			// sg has already been dequeued by the G that woke us up.
 			cas = k
 		} else {
 			c = k._chan
@@ -624,23 +625,36 @@ func reflect_rselect(cases []runtimeSelect) (chosen int, recvOK bool) {
 	return
 }
 
-func (q *waitq) dequeueSudoG(s *sudog) {
-	var prevsgp *sudog
-	l := &q.first
-	for {
-		sgp := *l
-		if sgp == nil {
+func (q *waitq) dequeueSudoG(sgp *sudog) {
+	x := sgp.prev
+	y := sgp.next
+	if x != nil {
+		if y != nil {
+			// middle of queue
+			x.next = y
+			y.prev = x
+			sgp.next = nil
+			sgp.prev = nil
 			return
 		}
-		if sgp == s {
-			*l = sgp.next
-			if q.last == sgp {
-				q.last = prevsgp
-			}
-			s.next = nil
-			return
-		}
-		l = &sgp.next
-		prevsgp = sgp
+		// end of queue
+		x.next = nil
+		q.last = x
+		sgp.prev = nil
+		return
+	}
+	if y != nil {
+		// start of queue
+		y.prev = nil
+		q.first = y
+		sgp.next = nil
+		return
+	}
+
+	// x==y==nil.  Either sgp is the only element in the queue,
+	// or it has already been removed.  Use q.first to disambiguate.
+	if q.first == sgp {
+		q.first = nil
+		q.last = nil
 	}
 }