Merge pull request #102843 from tkashem/apf-fix-102299

apf: always include seats in virtual time

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/queueset/queueset.go

@@ -674,8 +674,7 @@ func (qs *queueSet) selectQueueLocked() *queue {
 		qs.robinIndex = (qs.robinIndex + 1) % nq
 		queue := qs.queues[qs.robinIndex]
 		if queue.requests.Length() != 0 {
-
-			currentVirtualFinish := queue.GetVirtualFinish(0, qs.estimatedServiceTime)
+			currentVirtualFinish := queue.GetNextFinish(qs.estimatedServiceTime)
 			if currentVirtualFinish < minVirtualFinish {
 				minVirtualFinish = currentVirtualFinish
 				minQueue = queue
@@ -742,8 +741,8 @@ func (qs *queueSet) finishRequestLocked(r *request) {
 	S := now.Sub(r.startTime).Seconds()
 
 	// When a request finishes being served, and the actual service time was S,
-	// the queue’s virtual start time is decremented by G - S.
-	r.queue.virtualStart -= (qs.estimatedServiceTime * float64(r.Seats())) - S
+	// the queue’s virtual start time is decremented by (G - S)*width.
+	r.queue.virtualStart -= (qs.estimatedServiceTime - S) * float64(r.Seats())
 
 	// request has finished, remove from requests executing
 	r.queue.requestsExecuting--

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/queueset/types.go

@@ -101,15 +101,27 @@ func (q *queue) Dequeue() (*request, bool) {
 	return request, ok
 }
 
-// GetVirtualFinish returns the expected virtual finish time of the request at
-// index J in the queue with estimated finish time G
-func (q *queue) GetVirtualFinish(J int, G float64) float64 {
-	// The virtual finish time of request number J in the queue
-	// (counting from J=1 for the head) is J * G + (virtual start time).
+// GetNextFinish returns the expected virtual finish time of the
+// oldest request in the queue with estimated duration G
+func (q *queue) GetNextFinish(G float64) float64 {
+	// TODO: if we decide to generalize this function to return virtual finish time
+	//  for the Nth oldest request waiting in the queue, we need to carefully
+	//  evaluate and potentially improve the performance here.
+	var oldestReq *request
+	q.requests.Walk(func(r *request) bool {
+		oldestReq = r
+		return false
+	})
+	if oldestReq == nil {
+		// we should never be here, since the caller should ensure
+		// that this queue has request(s) waiting to be served before
+		// calling this function.
+		return q.virtualStart
+	}
 
-	// counting from J=1 for the head (eg: queue.requests[0] -> J=1) - J+1
-	jg := float64(J+1) * float64(G)
-	return jg + q.virtualStart
+	// the estimated service time of the oldest request is (G * request width)
+	estimatedServiceTime := float64(G) * float64(oldestReq.Seats())
+	return q.virtualStart + estimatedServiceTime
 }
 
 func (q *queue) dump(includeDetails bool) debug.QueueDump {

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/queueset/types_test.go

@@ -0,0 +1,62 @@
+/*
+Copyright 2021 The Kubernetes 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 queueset
+
+import (
+	"testing"
+)
+
+func TestGetNextFinish(t *testing.T) {
+	var virtualStart float64 = 100
+	var G float64 = 60
+	tests := []struct {
+		name                  string
+		requests              []*request
+		virtualFinishExpected float64
+	}{
+		{
+			name: "for the oldest request",
+			requests: []*request{
+				{width: 5},
+				{width: 6},
+				{width: 7},
+			},
+			virtualFinishExpected: virtualStart + (5 * G),
+		},
+		{
+			name:                  "queue does not have any request waiting",
+			requests:              []*request{},
+			virtualFinishExpected: virtualStart,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			requests := newRequestFIFO()
+			for i := range test.requests {
+				requests.Enqueue(test.requests[i])
+			}
+
+			q := &queue{requests: requests}
+			q.virtualStart = virtualStart
+			virtualFinishGot := q.GetNextFinish(G)
+			if test.virtualFinishExpected != virtualFinishGot {
+				t.Errorf("Expected virtual finish time: %.9fs, but got: %.9fs", test.virtualFinishExpected, virtualFinishGot)
+			}
+		})
+	}
+}