Merge pull request #117547 from wojtek-t/apf_dynamic_retry_after

Return dynamic RetryAfter header from APF

staging/src/k8s.io/apiserver/pkg/server/filters/maxinflight.go

@@ -34,7 +34,6 @@ import (
 
 const (
 	// Constant for the retry-after interval on rate limiting.
-	// TODO: maybe make this dynamic? or user-adjustable?
 	retryAfter = "1"
 
 	// How often inflight usage metric should be updated. Because
@@ -210,7 +209,7 @@ func WithMaxInFlightLimit(
 				// We need to split this data between buckets used for throttling.
 				metrics.RecordDroppedRequest(r, requestInfo, metrics.APIServerComponent, isMutatingRequest)
 				metrics.RecordRequestTermination(r, requestInfo, metrics.APIServerComponent, http.StatusTooManyRequests)
-				tooManyRequests(r, w)
+				tooManyRequests(r, w, retryAfter)
 			}
 		}
 	})
@@ -221,9 +220,3 @@ func WithMaxInFlightLimit(
 func StartMaxInFlightWatermarkMaintenance(stopCh <-chan struct{}) {
 	startWatermarkMaintenance(watermark, stopCh)
 }
-
-func tooManyRequests(req *http.Request, w http.ResponseWriter) {
-	// Return a 429 status indicating "Too Many Requests"
-	w.Header().Set("Retry-After", retryAfter)
-	http.Error(w, "Too many requests, please try again later.", http.StatusTooManyRequests)
-}

staging/src/k8s.io/apiserver/pkg/server/filters/priority-and-fairness.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"net/http"
 	"runtime"
+	"strconv"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -67,26 +68,19 @@ func truncateLogField(s string) string {
 
 var initAPFOnce sync.Once
 
-// WithPriorityAndFairness limits the number of in-flight
-// requests in a fine-grained way.
-func WithPriorityAndFairness(
-	handler http.Handler,
-	longRunningRequestCheck apirequest.LongRunningRequestCheck,
-	fcIfc utilflowcontrol.Interface,
-	workEstimator flowcontrolrequest.WorkEstimatorFunc,
-) http.Handler {
-	if fcIfc == nil {
-		klog.Warningf("priority and fairness support not found, skipping")
-		return handler
+type priorityAndFairnessHandler struct {
+	handler                 http.Handler
+	longRunningRequestCheck apirequest.LongRunningRequestCheck
+	fcIfc                   utilflowcontrol.Interface
+	workEstimator           flowcontrolrequest.WorkEstimatorFunc
+
+	// droppedRequests tracks the history of dropped requests for
+	// the purpose of computing RetryAfter header to avoid system
+	// overload.
+	droppedRequests utilflowcontrol.DroppedRequestsTracker
 }
-	initAPFOnce.Do(func() {
-		initMaxInFlight(0, 0)
-		// Fetching these gauges is delayed until after their underlying metric has been registered
-		// so that this latches onto the efficient implementation.
-		waitingMark.readOnlyObserver = fcmetrics.GetWaitingReadonlyConcurrency()
-		waitingMark.mutatingObserver = fcmetrics.GetWaitingMutatingConcurrency()
-	})
-	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+
+func (h *priorityAndFairnessHandler) Handle(w http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 	requestInfo, ok := apirequest.RequestInfoFrom(ctx)
 	if !ok {
@@ -102,9 +96,9 @@ func WithPriorityAndFairness(
 	isWatchRequest := watchVerbs.Has(requestInfo.Verb)
 
 	// Skip tracking long running non-watch requests.
-		if longRunningRequestCheck != nil && longRunningRequestCheck(r, requestInfo) && !isWatchRequest {
+	if h.longRunningRequestCheck != nil && h.longRunningRequestCheck(r, requestInfo) && !isWatchRequest {
 		klog.V(6).Infof("Serving RequestInfo=%#+v, user.Info=%#+v as longrunning\n", requestInfo, user)
-			handler.ServeHTTP(w, r)
+		h.handler.ServeHTTP(w, r)
 		return
 	}
 
@@ -114,7 +108,8 @@ func WithPriorityAndFairness(
 			FlowSchemaName:    fs.Name,
 			FlowSchemaUID:     fs.UID,
 			PriorityLevelName: pl.Name,
-				PriorityLevelUID:  pl.UID}
+			PriorityLevelUID:  pl.UID,
+		}
 
 		httplog.AddKeyValue(ctx, "apf_pl", truncateLogField(pl.Name))
 		httplog.AddKeyValue(ctx, "apf_fs", truncateLogField(fs.Name))
@@ -126,11 +121,10 @@ func WithPriorityAndFairness(
 			// the request has completed, we should never be here though.
 			klog.ErrorS(fmt.Errorf("workEstimator is being invoked before classification of the request has completed"),
 				"Using empty FlowSchema and PriorityLevelConfiguration name", "verb", r.Method, "URI", r.RequestURI)
-
-				return workEstimator(r, "", "")
+			return h.workEstimator(r, "", "")
 		}
 
-			workEstimate := workEstimator(r, classification.FlowSchemaName, classification.PriorityLevelName)
+		workEstimate := h.workEstimator(r, classification.FlowSchemaName, classification.PriorityLevelName)
 
 		fcmetrics.ObserveWorkEstimatedSeats(classification.PriorityLevelName, classification.FlowSchemaName, workEstimate.MaxSeats())
 		httplog.AddKeyValue(ctx, "apf_iseats", workEstimate.InitialSeats)
@@ -190,7 +184,7 @@ func WithPriorityAndFairness(
 			}
 			// Forget the watcher if it was registered.
 			//
-				// // This is race-free because by this point, one of the following occurred:
+			// This is race-free because by this point, one of the following occurred:
 			// case <-shouldStartWatchCh: execute() completed the assignment to forgetWatch
 			// case <-resultCh: Handle() completed, and Handle() does not return
 			//   while execute() is running
@@ -209,7 +203,7 @@ func WithPriorityAndFairness(
 			served = true
 			setResponseHeaders(classification, w)
 
-				forgetWatch = fcIfc.RegisterWatch(r)
+			forgetWatch = h.fcIfc.RegisterWatch(r)
 
 			// Notify the main thread that we're ready to start the watch.
 			close(shouldStartWatchCh)
@@ -261,14 +255,14 @@ func WithPriorityAndFairness(
 			// Note that Handle will return irrespective of whether the request
 			// executes or is rejected. In the latter case, the function will return
 			// without calling the passed `execute` function.
-				fcIfc.Handle(handleCtx, digest, noteFn, estimateWork, queueNote, execute)
+			h.fcIfc.Handle(handleCtx, digest, noteFn, estimateWork, queueNote, execute)
 		}()
 
 		select {
 		case <-shouldStartWatchCh:
 			watchCtx := utilflowcontrol.WithInitializationSignal(ctx, watchInitializationSignal)
 			watchReq = r.WithContext(watchCtx)
-				handler.ServeHTTP(w, watchReq)
+			h.handler.ServeHTTP(w, watchReq)
 			// Protect from the situation when request will not reach storage layer
 			// and the initialization signal will not be send.
 			// It has to happen before waiting on the resultCh below.
@@ -289,10 +283,10 @@ func WithPriorityAndFairness(
 			served = true
 			setResponseHeaders(classification, w)
 
-				handler.ServeHTTP(w, r)
+			h.handler.ServeHTTP(w, r)
 		}
 
-			fcIfc.Handle(ctx, digest, noteFn, estimateWork, queueNote, execute)
+		h.fcIfc.Handle(ctx, digest, noteFn, estimateWork, queueNote, execute)
 	}
 
 	if !served {
@@ -300,9 +294,42 @@ func WithPriorityAndFairness(
 
 		epmetrics.RecordDroppedRequest(r, requestInfo, epmetrics.APIServerComponent, isMutatingRequest)
 		epmetrics.RecordRequestTermination(r, requestInfo, epmetrics.APIServerComponent, http.StatusTooManyRequests)
-			tooManyRequests(r, w)
+		h.droppedRequests.RecordDroppedRequest(classification.PriorityLevelName)
+
+		// TODO(wojtek-t): Idea from deads2k: we can consider some jittering and in case of non-int
+		//  number, just return the truncated result and sleep the remainder server-side.
+		tooManyRequests(r, w, strconv.Itoa(int(h.droppedRequests.GetRetryAfter(classification.PriorityLevelName))))
 	}
+}
+
+// WithPriorityAndFairness limits the number of in-flight
+// requests in a fine-grained way.
+func WithPriorityAndFairness(
+	handler http.Handler,
+	longRunningRequestCheck apirequest.LongRunningRequestCheck,
+	fcIfc utilflowcontrol.Interface,
+	workEstimator flowcontrolrequest.WorkEstimatorFunc,
+) http.Handler {
+	if fcIfc == nil {
+		klog.Warningf("priority and fairness support not found, skipping")
+		return handler
+	}
+	initAPFOnce.Do(func() {
+		initMaxInFlight(0, 0)
+		// Fetching these gauges is delayed until after their underlying metric has been registered
+		// so that this latches onto the efficient implementation.
+		waitingMark.readOnlyObserver = fcmetrics.GetWaitingReadonlyConcurrency()
+		waitingMark.mutatingObserver = fcmetrics.GetWaitingMutatingConcurrency()
 	})
+
+	priorityAndFairnessHandler := &priorityAndFairnessHandler{
+		handler:                 handler,
+		longRunningRequestCheck: longRunningRequestCheck,
+		fcIfc:                   fcIfc,
+		workEstimator:           workEstimator,
+		droppedRequests:         utilflowcontrol.NewDroppedRequestsTracker(),
+	}
+	return http.HandlerFunc(priorityAndFairnessHandler.Handle)
 }
 
 // StartPriorityAndFairnessWatermarkMaintenance starts the goroutines to observe and maintain watermarks for
@@ -323,3 +350,9 @@ func setResponseHeaders(classification *PriorityAndFairnessClassification, w htt
 	w.Header().Set(flowcontrol.ResponseHeaderMatchedPriorityLevelConfigurationUID, string(classification.PriorityLevelUID))
 	w.Header().Set(flowcontrol.ResponseHeaderMatchedFlowSchemaUID, string(classification.FlowSchemaUID))
 }
+
+func tooManyRequests(req *http.Request, w http.ResponseWriter, retryAfter string) {
+	// Return a 429 status indicating "Too Many Requests"
+	w.Header().Set("Retry-After", retryAfter)
+	http.Error(w, "Too many requests, please try again later.", http.StatusTooManyRequests)
+}

staging/src/k8s.io/apiserver/pkg/server/filters/priority-and-fairness_test.go

@@ -360,11 +360,12 @@ func newFakeWatchApfFilter(capacity int) *fakeWatchApfFilter {
 
 func (f *fakeWatchApfFilter) Handle(ctx context.Context,
 	requestDigest utilflowcontrol.RequestDigest,
-	_ func(fs *flowcontrol.FlowSchema, pl *flowcontrol.PriorityLevelConfiguration, flowDistinguisher string),
+	noteFn func(fs *flowcontrol.FlowSchema, pl *flowcontrol.PriorityLevelConfiguration, flowDistinguisher string),
 	_ func() fcrequest.WorkEstimate,
 	_ fq.QueueNoteFn,
 	execFn func(),
 ) {
+	noteFn(bootstrap.SuggestedFlowSchemaGlobalDefault, bootstrap.SuggestedPriorityLevelConfigurationGlobalDefault, requestDigest.User.GetName())
 	canExecute := false
 	func() {
 		f.lock.Lock()

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/dropped_requests_tracker.go

@@ -0,0 +1,231 @@
+/*
+Copyright 2023 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 flowcontrol
+
+import (
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"k8s.io/utils/clock"
+)
+
+const (
+	// maxRetryAfter represents the maximum possible retryAfter.
+	maxRetryAfter = int64(32)
+)
+
+// DroppedRequestsTracker is an interface that allows tracking
+// a history od dropped requests in the system for the purpose
+// of adjusting RetryAfter header to avoid system overload.
+type DroppedRequestsTracker interface {
+	// RecordDroppedRequest records a request that was just
+	// dropped from processing.
+	RecordDroppedRequest(plName string)
+
+	// GetRetryAfter returns the current suggested value of
+	// RetryAfter value.
+	GetRetryAfter(plName string) int64
+}
+
+// unixStat keeps a statistic how many requests were dropped within
+// a single second.
+type unixStat struct {
+	unixTime int64
+	requests int64
+}
+
+type droppedRequestsStats struct {
+	lock sync.RWMutex
+
+	// history stores the history of dropped requests.
+	history []unixStat
+
+	// To reduce lock-contention, we store the information about
+	// the current second here, which we can then access under
+	// reader lock.
+	currentUnix  int64
+	currentCount atomic.Int64
+
+	retryAfter           atomic.Int64
+	retryAfterUpdateUnix int64
+}
+
+func newDroppedRequestsStats(nowUnix int64) *droppedRequestsStats {
+	result := &droppedRequestsStats{
+		// We assume that we can bump at any time after first dropped request.
+		retryAfterUpdateUnix: 0,
+	}
+	result.retryAfter.Store(1)
+	return result
+}
+
+func (s *droppedRequestsStats) recordDroppedRequest(unixTime int64) {
+	// Short path - if the current second matches passed time,
+	// just update the stats.
+	if done := func() bool {
+		s.lock.RLock()
+		defer s.lock.RUnlock()
+		if s.currentUnix == unixTime {
+			s.currentCount.Add(1)
+			return true
+		}
+		return false
+	}(); done {
+		return
+	}
+
+	// We trigger the change of <currentUnix>.
+	s.lock.Lock()
+	defer s.lock.Unlock()
+	if s.currentUnix == unixTime {
+		s.currentCount.Add(1)
+		return
+	}
+
+	s.updateHistory(s.currentUnix, s.currentCount.Load())
+	s.currentUnix = unixTime
+	s.currentCount.Store(1)
+
+	// We only consider updating retryAfter when bumping the current second.
+	// However, given that we didn't report anything for the current second,
+	// we recompute it based on statistics from the previous one.
+	s.updateRetryAfterIfNeededLocked(unixTime)
+}
+
+func (s *droppedRequestsStats) updateHistory(unixTime int64, count int64) {
+	s.history = append(s.history, unixStat{unixTime: unixTime, requests: count})
+
+	startIndex := 0
+	for ; startIndex < len(s.history) && unixTime-s.history[startIndex].unixTime > maxRetryAfter; startIndex++ {
+	}
+	if startIndex > 0 {
+		s.history = s.history[startIndex:]
+	}
+}
+
+// updateRetryAfterIfNeededLocked updates the retryAfter based on the number of
+// dropped requests in the last `retryAfter` seconds:
+//   - if there were less than `retryAfter` dropped requests, it decreases
+//     retryAfter
+//   - if there were at least 3*`retryAfter` dropped requests, it increases
+//     retryAfter
+//
+// The rationale behind these numbers being fairly low is that APF is queuing
+// requests and rejecting (dropping) them is a last resort, which is not expected
+// unless a given priority level is actually overloaded.
+//
+// Additionally, we rate-limit the increases of retryAfter to wait at least
+// `retryAfter' seconds after the previous increase to avoid multiple bumps
+// on a single spike.
+//
+// We're working with the interval [unixTime-retryAfter, unixTime).
+func (s *droppedRequestsStats) updateRetryAfterIfNeededLocked(unixTime int64) {
+	retryAfter := s.retryAfter.Load()
+
+	droppedRequests := int64(0)
+	if len(s.history) > 0 {
+		for i := len(s.history) - 1; i >= 0; i-- {
+			if unixTime-s.history[i].unixTime > retryAfter {
+				break
+			}
+			if s.history[i].unixTime < unixTime {
+				droppedRequests += s.history[i].requests
+			}
+		}
+	}
+
+	if unixTime-s.retryAfterUpdateUnix >= retryAfter && droppedRequests >= 3*retryAfter {
+		// We try to mimic the TCP algorithm and thus are doubling
+		// the retryAfter here.
+		retryAfter *= 2
+		if retryAfter >= maxRetryAfter {
+			retryAfter = maxRetryAfter
+		}
+		s.retryAfter.Store(retryAfter)
+		s.retryAfterUpdateUnix = unixTime
+		return
+	}
+
+	if droppedRequests < retryAfter && retryAfter > 1 {
+		// We try to mimc the TCP algorithm and thus are linearly
+		// scaling down the retryAfter here.
+		retryAfter--
+		s.retryAfter.Store(retryAfter)
+		return
+	}
+}
+
+// droppedRequestsTracker implement DroppedRequestsTracker interface
+// for the purpose of adjusting RetryAfter header for newly dropped
+// requests to avoid system overload.
+type droppedRequestsTracker struct {
+	now func() time.Time
+
+	lock    sync.RWMutex
+	plStats map[string]*droppedRequestsStats
+}
+
+// NewDroppedRequestsTracker is creating a new instance of
+// DroppedRequestsTracker.
+func NewDroppedRequestsTracker() DroppedRequestsTracker {
+	return newDroppedRequestsTracker(clock.RealClock{}.Now)
+}
+
+func newDroppedRequestsTracker(now func() time.Time) *droppedRequestsTracker {
+	return &droppedRequestsTracker{
+		now:     now,
+		plStats: make(map[string]*droppedRequestsStats),
+	}
+}
+
+func (t *droppedRequestsTracker) RecordDroppedRequest(plName string) {
+	unixTime := t.now().Unix()
+
+	stats := func() *droppedRequestsStats {
+		// The list of priority levels should change very infrequently,
+		// so in almost all cases, the fast path should be enough.
+		t.lock.RLock()
+		if plStats, ok := t.plStats[plName]; ok {
+			t.lock.RUnlock()
+			return plStats
+		}
+		t.lock.RUnlock()
+
+		// Slow path taking writer lock to update the map.
+		t.lock.Lock()
+		defer t.lock.Unlock()
+		if plStats, ok := t.plStats[plName]; ok {
+			return plStats
+		}
+		stats := newDroppedRequestsStats(unixTime)
+		t.plStats[plName] = stats
+		return stats
+	}()
+
+	stats.recordDroppedRequest(unixTime)
+}
+
+func (t *droppedRequestsTracker) GetRetryAfter(plName string) int64 {
+	t.lock.RLock()
+	defer t.lock.RUnlock()
+
+	if plStats, ok := t.plStats[plName]; ok {
+		return plStats.retryAfter.Load()
+	}
+	return 1
+}

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/dropped_requests_tracker_test.go

@@ -0,0 +1,170 @@
+/*
+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 flowcontrol
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+
+	testingclock "k8s.io/utils/clock/testing"
+)
+
+func TestDroppedRequestsTracker(t *testing.T) {
+	fakeClock := testingclock.NewFakeClock(time.Now())
+	tracker := newDroppedRequestsTracker(fakeClock.Now)
+
+	// The following table represents the list over time of:
+	// - seconds elapsed (as computed since the initial time)
+	// - requests that will be recorded as dropped in a current second
+	steps := []struct {
+		secondsElapsed int
+		// droppedRequests is the number of requests to drop, after
+		// secondsElapsed.
+		droppedRequests int
+		// retryAfter is the expected retryAfter after all dropped
+		// requests are recorded via RecordDroppedRequest.
+		retryAfter int64
+	}{
+		{secondsElapsed: 0, droppedRequests: 5, retryAfter: 1},
+		{secondsElapsed: 1, droppedRequests: 11, retryAfter: 2},
+		// Check that we don't bump immediately after despite
+		// multiple dropped requests.
+		{secondsElapsed: 2, droppedRequests: 1, retryAfter: 2},
+		{secondsElapsed: 3, droppedRequests: 11, retryAfter: 4},
+		{secondsElapsed: 4, droppedRequests: 1, retryAfter: 4},
+		{secondsElapsed: 7, droppedRequests: 1, retryAfter: 8},
+		{secondsElapsed: 11, droppedRequests: 1, retryAfter: 8},
+		{secondsElapsed: 15, droppedRequests: 1, retryAfter: 7},
+		{secondsElapsed: 17, droppedRequests: 1, retryAfter: 6},
+		{secondsElapsed: 21, droppedRequests: 14, retryAfter: 5},
+		{secondsElapsed: 22, droppedRequests: 1, retryAfter: 10},
+	}
+
+	for i, step := range steps {
+		secondsToAdvance := step.secondsElapsed
+		if i > 0 {
+			secondsToAdvance -= steps[i-1].secondsElapsed
+		}
+		fakeClock.Step(time.Duration(secondsToAdvance) * time.Second)
+
+		// Record only first dropped request and recompute retryAfter.
+		for r := 0; r < step.droppedRequests; r++ {
+			tracker.RecordDroppedRequest("pl")
+		}
+		if retryAfter := tracker.GetRetryAfter("pl"); retryAfter != step.retryAfter {
+			t.Errorf("Unexpected retryAfter: %v, expected: %v", retryAfter, step.retryAfter)
+		}
+
+	}
+}
+
+func TestDroppedRequestsTrackerPLIndependent(t *testing.T) {
+	fakeClock := testingclock.NewFakeClock(time.Now())
+	tracker := newDroppedRequestsTracker(fakeClock.Now)
+
+	// Report single dropped requests in multiple PLs.
+	// Validate if RetryAfter isn't bumped next second.
+	for i := 0; i < 10; i++ {
+		tracker.RecordDroppedRequest(fmt.Sprintf("pl-%d", i))
+	}
+	fakeClock.Step(time.Second)
+	for i := 0; i < 10; i++ {
+		tracker.RecordDroppedRequest(fmt.Sprintf("pl-%d", i))
+		retryAfter := tracker.GetRetryAfter(fmt.Sprintf("pl-%d", i))
+		if retryAfter != 1 {
+			t.Errorf("Unexpected retryAfter for pl-%d: %v", i, retryAfter)
+		}
+	}
+
+	// Record few droped requests on a single PL.
+	// Validate that RetryAfter is bumped only for this PL.
+	for i := 0; i < 5; i++ {
+		tracker.RecordDroppedRequest("pl-0")
+	}
+	fakeClock.Step(time.Second)
+	for i := 0; i < 10; i++ {
+		tracker.RecordDroppedRequest(fmt.Sprintf("pl-%d", i))
+		retryAfter := tracker.GetRetryAfter(fmt.Sprintf("pl-%d", i))
+		switch i {
+		case 0:
+			if retryAfter != 2 {
+				t.Errorf("Unexpected retryAfter for pl-0: %v", retryAfter)
+			}
+		default:
+			if retryAfter != 1 {
+				t.Errorf("Unexpected retryAfter for pl-%d: %v", i, retryAfter)
+			}
+		}
+	}
+	// Validate also PL for which no dropped requests was recorded.
+	if retryAfter := tracker.GetRetryAfter("other-pl"); retryAfter != 1 {
+		t.Errorf("Unexpected retryAfter for other-pl: %v", retryAfter)
+	}
+}
+
+func BenchmarkDroppedRequestsTracker(b *testing.B) {
+	b.StopTimer()
+
+	fakeClock := testingclock.NewFakeClock(time.Now())
+	tracker := newDroppedRequestsTracker(fakeClock.Now)
+
+	startCh := make(chan struct{})
+	wg := sync.WaitGroup{}
+	numPLs := 5
+	// For all `numPLs` priority levels, create b.N workers each
+	// of which will try to record a dropped request every 100ms
+	// with a random jitter.
+	for i := 0; i < numPLs; i++ {
+		plName := fmt.Sprintf("priority-level-%d", i)
+		for i := 0; i < b.N; i++ {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				<-startCh
+
+				for a := 0; a < 5; a++ {
+					tracker.RecordDroppedRequest(plName)
+					time.Sleep(25 * time.Millisecond)
+				}
+			}()
+		}
+	}
+	// Time-advancing goroutine.
+	stopCh := make(chan struct{})
+	timeWg := sync.WaitGroup{}
+	timeWg.Add(1)
+	go func() {
+		defer timeWg.Done()
+		for {
+			select {
+			case <-stopCh:
+				return
+			case <-time.After(25 * time.Millisecond):
+				fakeClock.Step(time.Second)
+			}
+		}
+	}()
+
+	b.StartTimer()
+	close(startCh)
+	wg.Wait()
+
+	close(stopCh)
+	timeWg.Wait()
+}