wrr: improve randomWRR.Next performance

* use binary search when weights are not equal
* random pick one when weights are equal

internal/wrr/random.go

@@ -19,15 +19,21 @@ package wrr
 
 import (
 	"fmt"
+	"sort"
 	"sync"
 
 	"google.golang.org/grpc/internal/grpcrand"
 )
 
 // weightedItem is a wrapped weighted item that is used to implement weighted random algorithm.
 type weightedItem struct {
-	Item   interface{}
-	Weight int64
+	Item interface{}
+	// TODO Delete Weight? This field is not necessary for randomWRR to work.
+	// But without this field, if we want to know an item's weight in randomWRR.Add , we have to
+	// calculate it (i.e. weight = items.AccumulatedWeight - previousItem.AccumulatedWeight)
+	// which is a bit less concise than items.Weight
+	Weight            int64
+	AccumulatedWeight int64
 }
 
 func (w *weightedItem) String() string {
@@ -38,7 +44,7 @@ func (w *weightedItem) String() string {
 type randomWRR struct {
 	mu           sync.RWMutex
 	items        []*weightedItem
-	sumOfWeights int64
+	equalWeights bool
 }
 
 // NewRandom creates a new WRR with random.
@@ -47,31 +53,40 @@ func NewRandom() WRR {
 }
 
 var grpcrandInt63n = grpcrand.Int63n
+var grpcrandIntn = grpcrand.Intn
 
 func (rw *randomWRR) Next() (item interface{}) {
 	rw.mu.RLock()
 	defer rw.mu.RUnlock()
-	if rw.sumOfWeights == 0 {
+	sumOfWeights := rw.items[len(rw.items)-1].AccumulatedWeight
+	if sumOfWeights == 0 {
 		return nil
 	}
-	// Random number in [0, sum).
-	randomWeight := grpcrandInt63n(rw.sumOfWeights)
-	for _, item := range rw.items {
-		randomWeight = randomWeight - item.Weight
-		if randomWeight < 0 {
-			return item.Item
-		}
+	if rw.equalWeights {
+		return rw.items[grpcrandIntn(len(rw.items))].Item
 	}
-
-	return rw.items[len(rw.items)-1].Item
+	// Random number in [0, sumOfWeights).
+	randomWeight := grpcrandInt63n(sumOfWeights)
+	// Item's accumulated weights are in ascending order, because item's weight >= 0.
+	// Binary search rw.items to find first item whose AccumulatedWeight > randomWeight
+	// The return i is guaranteed to be in range [0, len(rw.items)) because randomWeight < last item's AccumulatedWeight
+	i := sort.Search(len(rw.items), func(i int) bool { return rw.items[i].AccumulatedWeight > randomWeight })
+	return rw.items[i].Item
 }
 
 func (rw *randomWRR) Add(item interface{}, weight int64) {
 	rw.mu.Lock()
 	defer rw.mu.Unlock()
-	rItem := &weightedItem{Item: item, Weight: weight}
+	accumulatedWeight := weight
+	equalWeights := true
+	if len(rw.items) > 0 {
+		lastItem := rw.items[len(rw.items)-1]
+		accumulatedWeight = lastItem.AccumulatedWeight + weight
+		equalWeights = rw.equalWeights && weight == lastItem.Weight
+	}
+	rw.equalWeights = equalWeights
+	rItem := &weightedItem{Item: item, Weight: weight, AccumulatedWeight: accumulatedWeight}
 	rw.items = append(rw.items, rItem)
-	rw.sumOfWeights += weight
 }
 
 func (rw *randomWRR) String() string {

internal/wrr/wrr_test.go

@@ -115,4 +115,5 @@ func (s) TestEdfWrrNext(t *testing.T) {
 func init() {
 	r := rand.New(rand.NewSource(0))
 	grpcrandInt63n = r.Int63n
+	grpcrandIntn = r.Intn
 }