wrr: use binary search to improve randomWRR performance

internal/wrr/random.go

@@ -19,15 +19,20 @@ 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{}
+	// Delete Weight? This field is not necessary for randomWRR to work properly.
+	// But without this field, if we want to know an item's weight, we have to
+	// calculate it. weight = items.AccumulatedWeight - previousItem.AccumulatedWeight
+	Weight            int64
+	AccumulatedWeight int64
 }
 
 func (w *weightedItem) String() string {
@@ -36,9 +41,8 @@ func (w *weightedItem) String() string {
 
 // randomWRR is a struct that contains weighted items implement weighted random algorithm.
 type randomWRR struct {
-	mu           sync.RWMutex
-	items        []*weightedItem
-	sumOfWeights int64
+	mu    sync.RWMutex
+	items []*weightedItem
 }
 
 // NewRandom creates a new WRR with random.
@@ -51,27 +55,28 @@ var grpcrandInt63n = grpcrand.Int63n
 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
-		}
-	}
-
-	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
+	if len(rw.items) > 0 {
+		accumulatedWeight = rw.items[len(rw.items)-1].AccumulatedWeight + weight
+	}
+	rItem := &weightedItem{Item: item, Weight: weight, AccumulatedWeight: accumulatedWeight}
 	rw.items = append(rw.items, rItem)
-	rw.sumOfWeights += weight
 }
 
 func (rw *randomWRR) String() string {