internal/wrr: introduce order offset to edf scheduler as a tiebreaker…

… for entries with same deadline (#3800)

internal/wrr/edf.go

@@ -24,8 +24,10 @@ import (
 
 // edfWrr is a struct for EDF weighted round robin implementation.
 type edfWrr struct {
-	lock  sync.Mutex
-	items edfPriorityQueue
+	lock               sync.Mutex
+	items              edfPriorityQueue
+	currentOrderOffset uint64
+	currentTime        float64
 }
 
 // NewEDF creates Earliest Deadline First (EDF)
@@ -38,17 +40,20 @@ func NewEDF() WRR {
 
 // edfEntry is an internal wrapper for item that also stores weight and relative position in the queue.
 type edfEntry struct {
-	deadline float64
-	weight   int64
-	item     interface{}
+	deadline    float64
+	weight      int64
+	orderOffset uint64
+	item        interface{}
 }
 
 // edfPriorityQueue is a heap.Interface implementation for edfEntry elements.
 type edfPriorityQueue []*edfEntry
 
-func (pq edfPriorityQueue) Len() int           { return len(pq) }
-func (pq edfPriorityQueue) Less(i, j int) bool { return pq[i].deadline < pq[j].deadline }
-func (pq edfPriorityQueue) Swap(i, j int)      { pq[i], pq[j] = pq[j], pq[i] }
+func (pq edfPriorityQueue) Len() int { return len(pq) }
+func (pq edfPriorityQueue) Less(i, j int) bool {
+	return pq[i].deadline < pq[j].deadline || pq[i].deadline == pq[j].deadline && pq[i].orderOffset < pq[j].orderOffset
+}
+func (pq edfPriorityQueue) Swap(i, j int) { pq[i], pq[j] = pq[j], pq[i] }
 
 func (pq *edfPriorityQueue) Push(x interface{}) {
 	*pq = append(*pq, x.(*edfEntry))
@@ -60,22 +65,16 @@ func (pq *edfPriorityQueue) Pop() interface{} {
 	return old[len(old)-1]
 }
 
-// Current time in EDF scheduler.
-func (edf *edfWrr) currentTime() float64 {
-	if len(edf.items) == 0 {
-		return 0.0
-	}
-	return edf.items[0].deadline
-}
-
 func (edf *edfWrr) Add(item interface{}, weight int64) {
 	edf.lock.Lock()
 	defer edf.lock.Unlock()
 	entry := edfEntry{
-		deadline: edf.currentTime() + 1.0/float64(weight),
-		weight:   weight,
-		item:     item,
+		deadline:    edf.currentTime + 1.0/float64(weight),
+		weight:      weight,
+		item:        item,
+		orderOffset: edf.currentOrderOffset,
 	}
+	edf.currentOrderOffset++
 	heap.Push(&edf.items, &entry)
 }
 
@@ -86,7 +85,8 @@ func (edf *edfWrr) Next() interface{} {
 		return nil
 	}
 	item := edf.items[0]
-	item.deadline = edf.currentTime() + 1.0/float64(item.weight)
+	edf.currentTime = item.deadline
+	item.deadline = edf.currentTime + 1.0/float64(item.weight)
 	heap.Fix(&edf.items, 0)
 	return item.item
 }

internal/wrr/edf_test.go

@@ -0,0 +1,35 @@
+/*
+ *
+ * Copyright 2020 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package wrr
+
+import (
+	"testing"
+)
+
+func (s) TestEDFOnEndpointsWithSameWeight(t *testing.T) {
+	wrr := NewEDF()
+	wrr.Add("1", 1)
+	wrr.Add("2", 1)
+	wrr.Add("3", 1)
+	expected := []string{"1", "2", "3", "1", "2", "3", "1", "2", "3", "1", "2", "3"}
+	for i := 0; i < len(expected); i++ {
+		item := wrr.Next().(string)
+		if item != expected[i] {
+			t.Errorf("wrr Next=%s, want=%s", item, expected[i])
+		}
+	}
+}