Merge pull request #118601 from andrewsykim/apf-tune-max-seats

priority & fairness: support dynamic max seats

staging/src/k8s.io/apiserver/pkg/server/config.go

@@ -904,7 +904,7 @@ func DefaultBuildHandlerChain(apiHandler http.Handler, c *Config) http.Handler {
 	if c.FlowControl != nil {
 		workEstimatorCfg := flowcontrolrequest.DefaultWorkEstimatorConfig()
 		requestWorkEstimator := flowcontrolrequest.NewWorkEstimator(
-			c.StorageObjectCountTracker.Get, c.FlowControl.GetInterestedWatchCount, workEstimatorCfg)
+			c.StorageObjectCountTracker.Get, c.FlowControl.GetInterestedWatchCount, workEstimatorCfg, c.FlowControl.GetMaxSeats)
 		handler = filterlatency.TrackCompleted(handler)
 		handler = genericfilters.WithPriorityAndFairness(handler, c.LongRunningFunc, c.FlowControl, requestWorkEstimator)
 		handler = filterlatency.TrackStarted(handler, c.TracerProvider, "priorityandfairness")

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

@@ -80,6 +80,7 @@ type fakeApfFilter struct {
 	postDequeue  func()
 
 	utilflowcontrol.WatchTracker
+	utilflowcontrol.MaxSeatsTracker
 }
 
 func (t fakeApfFilter) Handle(ctx context.Context,
@@ -150,6 +151,7 @@ func newApfServerWithHooks(t *testing.T, decision mockDecision, onExecute, postE
 		postEnqueue:     postEnqueue,
 		postDequeue:     postDequeue,
 		WatchTracker:    utilflowcontrol.NewWatchTracker(),
+		MaxSeatsTracker: utilflowcontrol.NewMaxSeatsTracker(),
 	}
 	return newApfServerWithFilter(t, fakeFilter, onExecute, postExecute)
 }
@@ -349,12 +351,14 @@ type fakeWatchApfFilter struct {
 	preExecutePanic  bool
 
 	utilflowcontrol.WatchTracker
+	utilflowcontrol.MaxSeatsTracker
 }
 
 func newFakeWatchApfFilter(capacity int) *fakeWatchApfFilter {
 	return &fakeWatchApfFilter{
 		capacity:        capacity,
 		WatchTracker:    utilflowcontrol.NewWatchTracker(),
+		MaxSeatsTracker: utilflowcontrol.NewMaxSeatsTracker(),
 	}
 }
 

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

@@ -58,6 +58,11 @@ import (
 
 const timeFmt = "2006-01-02T15:04:05.999"
 
+const (
+	// priorityLevelMaxSeatsPercent is the percentage of the nominalCL used as max seats allocatable from work estimator
+	priorityLevelMaxSeatsPercent = float64(0.15)
+)
+
 // This file contains a simple local (to the apiserver) controller
 // that digests API Priority and Fairness config objects (FlowSchema
 // and PriorityLevelConfiguration) into the data structure that the
@@ -151,6 +156,12 @@ type configController struct {
 	// watchTracker implements the necessary WatchTracker interface.
 	WatchTracker
 
+	// MaxSeatsTracker tracks the maximum seats that should be allocatable from the
+	// work estimator for a given priority level. This controller does not enforce
+	// any limits on max seats stored in this tracker, it is up to the work estimator
+	// to set lower/upper limits on max seats (currently min=1, max=10).
+	MaxSeatsTracker
+
 	// the most recent update attempts, ordered by increasing age.
 	// Consumer trims to keep only the last minute's worth of entries.
 	// The controller uses this to limit itself to at most six updates
@@ -274,6 +285,7 @@ func newTestableController(config TestableConfig) *configController {
 		flowcontrolClient:      config.FlowcontrolClient,
 		priorityLevelStates:    make(map[string]*priorityLevelState),
 		WatchTracker:           NewWatchTracker(),
+		MaxSeatsTracker:        NewMaxSeatsTracker(),
 	}
 	klog.V(2).Infof("NewTestableController %q with serverConcurrencyLimit=%d, requestWaitLimit=%s, name=%s, asFieldManager=%q", cfgCtlr.name, cfgCtlr.serverConcurrencyLimit, cfgCtlr.requestWaitLimit, cfgCtlr.name, cfgCtlr.asFieldManager)
 	// Start with longish delay because conflicts will be between
@@ -770,6 +782,7 @@ func (meal *cfgMeal) processOldPLsLocked() {
 				// draining and no use is coming from another
 				// goroutine
 				klog.V(3).Infof("Removing undesired priority level %q, Type=%v", plName, plState.pl.Spec.Type)
+				meal.cfgCtlr.MaxSeatsTracker.ForgetPriorityLevel(plName)
 				continue
 			}
 			if !plState.quiescing {
@@ -828,6 +841,17 @@ func (meal *cfgMeal) finishQueueSetReconfigsLocked() {
 		if limited := plState.pl.Spec.Limited; limited != nil {
 			if qCfg := limited.LimitResponse.Queuing; qCfg != nil {
 				meal.maxWaitingRequests += int(qCfg.Queues * qCfg.QueueLengthLimit)
+
+				// Max seats allocatable from work estimator is calculated as MAX(1, MIN(0.15 * nominalCL, nominalCL/handSize)).
+				// This is to keep max seats relative to total available concurrency with a minimum value of 1.
+				// 15% of nominal concurrency was chosen since it preserved the previous max seats of 10 for default priority levels
+				// when using apiserver's default total server concurrency of 600 (--max-requests-inflight=400, --max-mutating-requests-inflight=200).
+				// This ensures that clusters with relatively high inflight requests will continue to use a max seats of 10
+				// while clusters with lower inflight requests will use max seats no greater than nominalCL/handSize.
+				// Calculated max seats can return arbitrarily high values but work estimator currently limits max seats at 10.
+				handSize := plState.pl.Spec.Limited.LimitResponse.Queuing.HandSize
+				maxSeats := uint64(math.Max(1, math.Min(math.Ceil(float64(concurrencyLimit)*priorityLevelMaxSeatsPercent), float64(int32(concurrencyLimit)/handSize))))
+				meal.cfgCtlr.MaxSeatsTracker.SetMaxSeats(plName, maxSeats)
 			}
 		}
 		if plState.queues == nil {

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

@@ -77,6 +77,10 @@ type Interface interface {
 
 	// WatchTracker provides the WatchTracker interface.
 	WatchTracker
+
+	// MaxSeatsTracker is invoked from the work estimator to track max seats
+	// that can be occupied by a request for a priority level.
+	MaxSeatsTracker
 }
 
 // This request filter implements https://github.com/kubernetes/enhancements/blob/master/keps/sig-api-machinery/1040-priority-and-fairness/README.md

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

@@ -0,0 +1,66 @@
+/*
+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"
+)
+
+// MaxSeatsTracker is used to track max seats allocatable per priority level from the work estimator
+type MaxSeatsTracker interface {
+	// GetMaxSeats returns the maximum seats a request should occupy for a given priority level.
+	GetMaxSeats(priorityLevelName string) uint64
+
+	// SetMaxSeats configures max seats for a priority level.
+	SetMaxSeats(priorityLevelName string, maxSeats uint64)
+
+	// ForgetPriorityLevel removes max seats tracking for a priority level.
+	ForgetPriorityLevel(priorityLevelName string)
+}
+
+type maxSeatsTracker struct {
+	sync.RWMutex
+
+	maxSeats map[string]uint64
+}
+
+func NewMaxSeatsTracker() MaxSeatsTracker {
+	return &maxSeatsTracker{
+		maxSeats: make(map[string]uint64),
+	}
+}
+
+func (m *maxSeatsTracker) GetMaxSeats(plName string) uint64 {
+	m.RLock()
+	defer m.RUnlock()
+
+	return m.maxSeats[plName]
+}
+
+func (m *maxSeatsTracker) SetMaxSeats(plName string, maxSeats uint64) {
+	m.Lock()
+	defer m.Unlock()
+
+	m.maxSeats[plName] = maxSeats
+}
+
+func (m *maxSeatsTracker) ForgetPriorityLevel(plName string) {
+	m.Lock()
+	defer m.Unlock()
+
+	delete(m.maxSeats, plName)
+}

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

@@ -0,0 +1,140 @@
+/*
+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 (
+	"testing"
+	"time"
+
+	"k8s.io/api/flowcontrol/v1beta3"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	fqs "k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/queueset"
+	"k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/testing/eventclock"
+	"k8s.io/apiserver/pkg/util/flowcontrol/metrics"
+	"k8s.io/client-go/informers"
+	clientsetfake "k8s.io/client-go/kubernetes/fake"
+)
+
+// Test_GetMaxSeats tests max seats retrieved from MaxSeatsTracker
+func Test_GetMaxSeats(t *testing.T) {
+	testcases := []struct {
+		name             string
+		nominalCL        int
+		handSize         int32
+		expectedMaxSeats uint64
+	}{
+		{
+			name:             "nominalCL=5, handSize=6",
+			nominalCL:        5,
+			handSize:         6,
+			expectedMaxSeats: 1,
+		},
+		{
+			name:             "nominalCL=10, handSize=6",
+			nominalCL:        10,
+			handSize:         6,
+			expectedMaxSeats: 1,
+		},
+		{
+			name:             "nominalCL=15, handSize=6",
+			nominalCL:        15,
+			handSize:         6,
+			expectedMaxSeats: 2,
+		},
+		{
+			name:             "nominalCL=20, handSize=6",
+			nominalCL:        20,
+			handSize:         6,
+			expectedMaxSeats: 3,
+		},
+		{
+			name:             "nominalCL=35, handSize=6",
+			nominalCL:        35,
+			handSize:         6,
+			expectedMaxSeats: 5,
+		},
+		{
+			name:             "nominalCL=100, handSize=6",
+			nominalCL:        100,
+			handSize:         6,
+			expectedMaxSeats: 15,
+		},
+		{
+			name:             "nominalCL=200, handSize=6",
+			nominalCL:        200,
+			handSize:         6,
+			expectedMaxSeats: 30,
+		},
+		{
+			name:             "nominalCL=10, handSize=1",
+			nominalCL:        10,
+			handSize:         1,
+			expectedMaxSeats: 2,
+		},
+		{
+			name:             "nominalCL=100, handSize=20",
+			nominalCL:        100,
+			handSize:         20,
+			expectedMaxSeats: 5,
+		},
+	}
+
+	for _, testcase := range testcases {
+		t.Run(testcase.name, func(t *testing.T) {
+			clientset := clientsetfake.NewSimpleClientset()
+			informerFactory := informers.NewSharedInformerFactory(clientset, time.Second)
+			flowcontrolClient := clientset.FlowcontrolV1beta3()
+			startTime := time.Now()
+			clk, _ := eventclock.NewFake(startTime, 0, nil)
+			c := newTestableController(TestableConfig{
+				Name:              "Controller",
+				Clock:             clk,
+				InformerFactory:   informerFactory,
+				FlowcontrolClient: flowcontrolClient,
+				// for the purposes of this test, serverCL ~= nominalCL since there is
+				// only 1 PL with large concurrency shares, making mandatory PLs negligible.
+				ServerConcurrencyLimit: testcase.nominalCL,
+				RequestWaitLimit:       time.Minute,
+				ReqsGaugeVec:           metrics.PriorityLevelConcurrencyGaugeVec,
+				ExecSeatsGaugeVec:      metrics.PriorityLevelExecutionSeatsGaugeVec,
+				QueueSetFactory:        fqs.NewQueueSetFactory(clk),
+			})
+
+			testPriorityLevel := &v1beta3.PriorityLevelConfiguration{
+				ObjectMeta: metav1.ObjectMeta{
+					Name: "test-pl",
+				},
+				Spec: v1beta3.PriorityLevelConfigurationSpec{
+					Type: v1beta3.PriorityLevelEnablementLimited,
+					Limited: &v1beta3.LimitedPriorityLevelConfiguration{
+						NominalConcurrencyShares: 10000,
+						LimitResponse: v1beta3.LimitResponse{
+							Queuing: &v1beta3.QueuingConfiguration{
+								HandSize: testcase.handSize,
+							},
+						},
+					},
+				},
+			}
+			c.digestConfigObjects([]*v1beta3.PriorityLevelConfiguration{testPriorityLevel}, nil)
+			maxSeats := c.GetMaxSeats("test-pl")
+			if maxSeats != testcase.expectedMaxSeats {
+				t.Errorf("unexpected max seats, got=%d, want=%d", maxSeats, testcase.expectedMaxSeats)
+			}
+		})
+	}
+}

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/request/config.go

@@ -24,7 +24,7 @@ import (
 
 const (
 	minimumSeats                = 1
-	maximumSeats                = 10
+	maximumSeatsLimit           = 10
 	objectsPerSeat              = 100.0
 	watchesPerSeat              = 10.0
 	enableMutatingWorkEstimator = true
@@ -39,12 +39,13 @@ type WorkEstimatorConfig struct {
 
 	// MinimumSeats is the minimum number of seats a request must occupy.
 	MinimumSeats uint64 `json:"minimumSeats,omitempty"`
-	// MaximumSeats is the maximum number of seats a request can occupy
+
+	// MaximumSeatsLimit is an upper limit on the max seats a request can occupy.
 	//
 	// NOTE: work_estimate_seats_samples metric uses the value of maximumSeats
 	// as the upper bound, so when we change maximumSeats we should also
 	// update the buckets of the metric.
-	MaximumSeats uint64 `json:"maximumSeats,omitempty"`
+	MaximumSeatsLimit uint64 `json:"maximumSeatsLimit,omitempty"`
 }
 
 // ListWorkEstimatorConfig holds work estimator parameters related to list requests.
@@ -66,7 +67,7 @@ type MutatingWorkEstimatorConfig struct {
 func DefaultWorkEstimatorConfig() *WorkEstimatorConfig {
 	return &WorkEstimatorConfig{
 		MinimumSeats:                minimumSeats,
-		MaximumSeats:                maximumSeats,
+		MaximumSeatsLimit:           maximumSeatsLimit,
 		ListWorkEstimatorConfig:     defaultListWorkEstimatorConfig(),
 		MutatingWorkEstimatorConfig: defaultMutatingWorkEstimatorConfig(),
 	}

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/request/list_work_estimator.go

@@ -29,10 +29,11 @@ import (
 	"k8s.io/klog/v2"
 )
 
-func newListWorkEstimator(countFn objectCountGetterFunc, config *WorkEstimatorConfig) WorkEstimatorFunc {
+func newListWorkEstimator(countFn objectCountGetterFunc, config *WorkEstimatorConfig, maxSeatsFn maxSeatsFunc) WorkEstimatorFunc {
 	estimator := &listWorkEstimator{
 		config:        config,
 		countGetterFn: countFn,
+		maxSeatsFn:    maxSeatsFn,
 	}
 	return estimator.estimate
 }
@@ -40,14 +41,21 @@ func newListWorkEstimator(countFn objectCountGetterFunc, config *WorkEstimatorCo
 type listWorkEstimator struct {
 	config        *WorkEstimatorConfig
 	countGetterFn objectCountGetterFunc
+	maxSeatsFn    maxSeatsFunc
 }
 
 func (e *listWorkEstimator) estimate(r *http.Request, flowSchemaName, priorityLevelName string) WorkEstimate {
+	minSeats := e.config.MinimumSeats
+	maxSeats := e.maxSeatsFn(priorityLevelName)
+	if maxSeats == 0 || maxSeats > e.config.MaximumSeatsLimit {
+		maxSeats = e.config.MaximumSeatsLimit
+	}
+
 	requestInfo, ok := apirequest.RequestInfoFrom(r.Context())
 	if !ok {
 		// no RequestInfo should never happen, but to be on the safe side
 		// let's return maximumSeats
-		return WorkEstimate{InitialSeats: e.config.MaximumSeats}
+		return WorkEstimate{InitialSeats: maxSeats}
 	}
 
 	if requestInfo.Name != "" {
@@ -56,7 +64,7 @@ func (e *listWorkEstimator) estimate(r *http.Request, flowSchemaName, priorityLe
 		// Example of such list requests:
 		// /apis/certificates.k8s.io/v1/certificatesigningrequests?fieldSelector=metadata.name%3Dcsr-xxs4m
 		// /api/v1/namespaces/test/configmaps?fieldSelector=metadata.name%3Dbig-deployment-1&limit=500&resourceVersion=0
-		return WorkEstimate{InitialSeats: e.config.MinimumSeats}
+		return WorkEstimate{InitialSeats: minSeats}
 	}
 
 	query := r.URL.Query()
@@ -66,7 +74,7 @@ func (e *listWorkEstimator) estimate(r *http.Request, flowSchemaName, priorityLe
 
 		// This request is destined to fail in the validation layer,
 		// return maximumSeats for this request to be consistent.
-		return WorkEstimate{InitialSeats: e.config.MaximumSeats}
+		return WorkEstimate{InitialSeats: maxSeats}
 	}
 
 	// For watch requests, we want to adjust the cost only if they explicitly request
@@ -86,7 +94,7 @@ func (e *listWorkEstimator) estimate(r *http.Request, flowSchemaName, priorityLe
 		// be conservative here and allocate maximum seats to this list request.
 		// NOTE: if a CRD is removed, its count will go stale first and then the
 		// pruner will eventually remove the CRD from the cache.
-		return WorkEstimate{InitialSeats: e.config.MaximumSeats}
+		return WorkEstimate{InitialSeats: maxSeats}
 	case err == ObjectCountNotFoundErr:
 		// there are multiple scenarios in which we can see this error:
 		//  a. the type is truly unknown, a typo on the caller's part.
@@ -100,12 +108,12 @@ func (e *listWorkEstimator) estimate(r *http.Request, flowSchemaName, priorityLe
 		// when aggregated API calls are overestimated, we allocate the minimum
 		// possible seats (see #109106 as an example when being more conservative
 		// led to problems).
-		return WorkEstimate{InitialSeats: e.config.MinimumSeats}
+		return WorkEstimate{InitialSeats: minSeats}
 	case err != nil:
 		// we should never be here since Get returns either ObjectCountStaleErr or
 		// ObjectCountNotFoundErr, return maximumSeats to be on the safe side.
 		klog.ErrorS(err, "Unexpected error from object count tracker")
-		return WorkEstimate{InitialSeats: e.config.MaximumSeats}
+		return WorkEstimate{InitialSeats: maxSeats}
 	}
 
 	limit := numStored
@@ -134,11 +142,11 @@ func (e *listWorkEstimator) estimate(r *http.Request, flowSchemaName, priorityLe
 	seats := uint64(math.Ceil(float64(estimatedObjectsToBeProcessed) / e.config.ObjectsPerSeat))
 
 	// make sure we never return a seat of zero
-	if seats < e.config.MinimumSeats {
-		seats = e.config.MinimumSeats
+	if seats < minSeats {
+		seats = minSeats
 	}
-	if seats > e.config.MaximumSeats {
-		seats = e.config.MaximumSeats
+	if seats > maxSeats {
+		seats = maxSeats
 	}
 	return WorkEstimate{InitialSeats: seats}
 }

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/request/mutating_work_estimator.go

@@ -25,10 +25,11 @@ import (
 	"k8s.io/apiserver/pkg/util/flowcontrol/metrics"
 )
 
-func newMutatingWorkEstimator(countFn watchCountGetterFunc, config *WorkEstimatorConfig) WorkEstimatorFunc {
+func newMutatingWorkEstimator(countFn watchCountGetterFunc, config *WorkEstimatorConfig, maxSeatsFn maxSeatsFunc) WorkEstimatorFunc {
 	estimator := &mutatingWorkEstimator{
 		config:     config,
 		countFn:    countFn,
+		maxSeatsFn: maxSeatsFn,
 	}
 	return estimator.estimate
 }
@@ -36,14 +37,21 @@ func newMutatingWorkEstimator(countFn watchCountGetterFunc, config *WorkEstimato
 type mutatingWorkEstimator struct {
 	config     *WorkEstimatorConfig
 	countFn    watchCountGetterFunc
+	maxSeatsFn maxSeatsFunc
 }
 
 func (e *mutatingWorkEstimator) estimate(r *http.Request, flowSchemaName, priorityLevelName string) WorkEstimate {
+	minSeats := e.config.MinimumSeats
+	maxSeats := e.maxSeatsFn(priorityLevelName)
+	if maxSeats == 0 || maxSeats > e.config.MaximumSeatsLimit {
+		maxSeats = e.config.MaximumSeatsLimit
+	}
+
 	// TODO(wojtekt): Remove once we tune the algorithm to not fail
 	// scalability tests.
 	if !e.config.Enabled {
 		return WorkEstimate{
-			InitialSeats: 1,
+			InitialSeats: minSeats,
 		}
 	}
 
@@ -52,15 +60,15 @@ func (e *mutatingWorkEstimator) estimate(r *http.Request, flowSchemaName, priori
 		// no RequestInfo should never happen, but to be on the safe side
 		// let's return a large value.
 		return WorkEstimate{
-			InitialSeats:      1,
-			FinalSeats:        e.config.MaximumSeats,
+			InitialSeats:      minSeats,
+			FinalSeats:        maxSeats,
 			AdditionalLatency: e.config.eventAdditionalDuration(),
 		}
 	}
 
 	if isRequestExemptFromWatchEvents(requestInfo) {
 		return WorkEstimate{
-			InitialSeats:      e.config.MinimumSeats,
+			InitialSeats:      minSeats,
 			FinalSeats:        0,
 			AdditionalLatency: time.Duration(0),
 		}
@@ -126,8 +134,8 @@ func (e *mutatingWorkEstimator) estimate(r *http.Request, flowSchemaName, priori
 		//
 		// TODO: Confirm that the current cap of maximumSeats allow us to
 		//   achieve the above.
-		if finalSeats > e.config.MaximumSeats {
-			finalSeats = e.config.MaximumSeats
+		if finalSeats > maxSeats {
+			finalSeats = maxSeats
 		}
 		additionalLatency = finalWork.DurationPerSeat(float64(finalSeats))
 	}

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/request/width.go

@@ -64,15 +64,19 @@ type objectCountGetterFunc func(string) (int64, error)
 // number of watchers potentially interested in a given request.
 type watchCountGetterFunc func(*apirequest.RequestInfo) int
 
+// MaxSeatsFunc represents a function that returns the maximum seats
+// allowed for the work estimator for a given priority level.
+type maxSeatsFunc func(priorityLevelName string) uint64
+
 // NewWorkEstimator estimates the work that will be done by a given request,
 // if no WorkEstimatorFunc matches the given request then the default
 // work estimate of 1 seat is allocated to the request.
-func NewWorkEstimator(objectCountFn objectCountGetterFunc, watchCountFn watchCountGetterFunc, config *WorkEstimatorConfig) WorkEstimatorFunc {
+func NewWorkEstimator(objectCountFn objectCountGetterFunc, watchCountFn watchCountGetterFunc, config *WorkEstimatorConfig, maxSeatsFn maxSeatsFunc) WorkEstimatorFunc {
 	estimator := &workEstimator{
 		minimumSeats:          config.MinimumSeats,
-		maximumSeats:          config.MaximumSeats,
-		listWorkEstimator:     newListWorkEstimator(objectCountFn, config),
-		mutatingWorkEstimator: newMutatingWorkEstimator(watchCountFn, config),
+		maximumSeatsLimit:     config.MaximumSeatsLimit,
+		listWorkEstimator:     newListWorkEstimator(objectCountFn, config, maxSeatsFn),
+		mutatingWorkEstimator: newMutatingWorkEstimator(watchCountFn, config, maxSeatsFn),
 	}
 	return estimator.estimate
 }
@@ -89,8 +93,8 @@ func (e WorkEstimatorFunc) EstimateWork(r *http.Request, flowSchemaName, priorit
 type workEstimator struct {
 	// the minimum number of seats a request must occupy
 	minimumSeats uint64
-	// the maximum number of seats a request can occupy
-	maximumSeats uint64
+	// the default maximum number of seats a request can occupy
+	maximumSeatsLimit uint64
 	// listWorkEstimator estimates work for list request(s)
 	listWorkEstimator WorkEstimatorFunc
 	// mutatingWorkEstimator calculates the width of mutating request(s)
@@ -102,7 +106,7 @@ func (e *workEstimator) estimate(r *http.Request, flowSchemaName, priorityLevelN
 	if !ok {
 		klog.ErrorS(fmt.Errorf("no RequestInfo found in context"), "Failed to estimate work for the request", "URI", r.RequestURI)
 		// no RequestInfo should never happen, but to be on the safe side let's return maximumSeats
-		return WorkEstimate{InitialSeats: e.maximumSeats}
+		return WorkEstimate{InitialSeats: e.maximumSeatsLimit}
 	}
 
 	switch requestInfo.Verb {

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/request/width_test.go

@@ -32,8 +32,6 @@ func TestWorkEstimator(t *testing.T) {
 	defer featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.WatchList, true)()
 
 	defaultCfg := DefaultWorkEstimatorConfig()
-	minimumSeats := defaultCfg.MinimumSeats
-	maximumSeats := defaultCfg.MaximumSeats
 
 	tests := []struct {
 		name                      string
@@ -42,6 +40,7 @@ func TestWorkEstimator(t *testing.T) {
 		counts                    map[string]int64
 		countErr                  error
 		watchCount                int
+		maxSeats                  uint64
 		initialSeatsExpected      uint64
 		finalSeatsExpected        uint64
 		additionalLatencyExpected time.Duration
@@ -50,7 +49,8 @@ func TestWorkEstimator(t *testing.T) {
 			name:                 "request has no RequestInfo",
 			requestURI:           "http://server/apis/",
 			requestInfo:          nil,
-			initialSeatsExpected: maximumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 10,
 		},
 		{
 			name:       "request verb is not list",
@@ -58,7 +58,8 @@ func TestWorkEstimator(t *testing.T) {
 			requestInfo: &apirequest.RequestInfo{
 				Verb: "get",
 			},
-			initialSeatsExpected: minimumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 1,
 		},
 		{
 			name:       "request verb is list, conversion to ListOptions returns error",
@@ -71,7 +72,8 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 799,
 			},
-			initialSeatsExpected: maximumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 10,
 		},
 		{
 			name:       "request verb is list, has limit and resource version is 1",
@@ -84,6 +86,7 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 699,
 			},
+			maxSeats:             10,
 			initialSeatsExpected: 8,
 		},
 		{
@@ -97,6 +100,7 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 699,
 			},
+			maxSeats:             10,
 			initialSeatsExpected: 7,
 		},
 		{
@@ -110,6 +114,7 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 699,
 			},
+			maxSeats:             10,
 			initialSeatsExpected: 8,
 		},
 		{
@@ -123,6 +128,7 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 399,
 			},
+			maxSeats:             10,
 			initialSeatsExpected: 8,
 		},
 		{
@@ -134,7 +140,8 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "events",
 			},
 			countErr:             ObjectCountNotFoundErr,
-			initialSeatsExpected: minimumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 1,
 		},
 		{
 			name:       "request verb is list, continuation is set",
@@ -147,6 +154,7 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 699,
 			},
+			maxSeats:             10,
 			initialSeatsExpected: 8,
 		},
 		{
@@ -160,6 +168,7 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 399,
 			},
+			maxSeats:             10,
 			initialSeatsExpected: 4,
 		},
 		{
@@ -186,6 +195,7 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 699,
 			},
+			maxSeats:             10,
 			initialSeatsExpected: 8,
 		},
 		{
@@ -199,6 +209,7 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 799,
 			},
+			maxSeats:             10,
 			initialSeatsExpected: 8,
 		},
 		{
@@ -212,7 +223,22 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 1999,
 			},
-			initialSeatsExpected: maximumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 10,
+		},
+		{
+			name:       "request verb is list, maximum is capped, lower max seats",
+			requestURI: "http://server/apis/foo.bar/v1/events?resourceVersion=foo",
+			requestInfo: &apirequest.RequestInfo{
+				Verb:     "list",
+				APIGroup: "foo.bar",
+				Resource: "events",
+			},
+			counts: map[string]int64{
+				"events.foo.bar": 1999,
+			},
+			maxSeats:             5,
+			initialSeatsExpected: 5,
 		},
 		{
 			name:       "request verb is list, list from cache, count not known",
@@ -223,7 +249,8 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "events",
 			},
 			countErr:             ObjectCountNotFoundErr,
-			initialSeatsExpected: minimumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 1,
 		},
 		{
 			name:       "request verb is list, object count is stale",
@@ -237,7 +264,8 @@ func TestWorkEstimator(t *testing.T) {
 				"events.foo.bar": 799,
 			},
 			countErr:             ObjectCountStaleErr,
-			initialSeatsExpected: maximumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 10,
 		},
 		{
 			name:       "request verb is list, object count is not found",
@@ -248,7 +276,8 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "events",
 			},
 			countErr:             ObjectCountNotFoundErr,
-			initialSeatsExpected: minimumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 1,
 		},
 		{
 			name:       "request verb is list, count getter throws unknown error",
@@ -259,7 +288,8 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "events",
 			},
 			countErr:             errors.New("unknown error"),
-			initialSeatsExpected: maximumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 10,
 		},
 		{
 			name:       "request verb is list, metadata.name specified",
@@ -273,7 +303,8 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 799,
 			},
-			initialSeatsExpected: minimumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 1,
 		},
 		{
 			name:       "request verb is list, metadata.name, resourceVersion and limit specified",
@@ -287,7 +318,8 @@ func TestWorkEstimator(t *testing.T) {
 			counts: map[string]int64{
 				"events.foo.bar": 799,
 			},
-			initialSeatsExpected: minimumSeats,
+			maxSeats:             10,
+			initialSeatsExpected: 1,
 		},
 		{
 			name:       "request verb is watch, sendInitialEvents is nil",
@@ -336,6 +368,7 @@ func TestWorkEstimator(t *testing.T) {
 				APIGroup: "foo.bar",
 				Resource: "foos",
 			},
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        0,
 			additionalLatencyExpected: 0,
@@ -349,6 +382,7 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "foos",
 			},
 			watchCount:                29,
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        3,
 			additionalLatencyExpected: 5 * time.Millisecond,
@@ -362,6 +396,7 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "foos",
 			},
 			watchCount:                5,
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        0,
 			additionalLatencyExpected: 0,
@@ -375,6 +410,7 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "foos",
 			},
 			watchCount:                199,
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        10,
 			additionalLatencyExpected: 10 * time.Millisecond,
@@ -387,6 +423,7 @@ func TestWorkEstimator(t *testing.T) {
 				APIGroup: "foo.bar",
 				Resource: "foos",
 			},
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        0,
 			additionalLatencyExpected: 0,
@@ -400,6 +437,7 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "foos",
 			},
 			watchCount:                29,
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        3,
 			additionalLatencyExpected: 5 * time.Millisecond,
@@ -412,6 +450,7 @@ func TestWorkEstimator(t *testing.T) {
 				APIGroup: "foo.bar",
 				Resource: "foos",
 			},
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        0,
 			additionalLatencyExpected: 0,
@@ -425,10 +464,25 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "foos",
 			},
 			watchCount:                29,
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        3,
 			additionalLatencyExpected: 5 * time.Millisecond,
 		},
+		{
+			name:       "request verb is patch, watches registered, lower max seats",
+			requestURI: "http://server/apis/foo.bar/v1/foos/myfoo",
+			requestInfo: &apirequest.RequestInfo{
+				Verb:     "patch",
+				APIGroup: "foo.bar",
+				Resource: "foos",
+			},
+			watchCount:                100,
+			maxSeats:                  5,
+			initialSeatsExpected:      1,
+			finalSeatsExpected:        5,
+			additionalLatencyExpected: 10 * time.Millisecond,
+		},
 		{
 			name:       "request verb is delete, no watches",
 			requestURI: "http://server/apis/foo.bar/v1/foos/myfoo",
@@ -437,6 +491,7 @@ func TestWorkEstimator(t *testing.T) {
 				APIGroup: "foo.bar",
 				Resource: "foos",
 			},
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        0,
 			additionalLatencyExpected: 0,
@@ -450,6 +505,7 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "foos",
 			},
 			watchCount:                29,
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
 			finalSeatsExpected:        3,
 			additionalLatencyExpected: 5 * time.Millisecond,
@@ -464,7 +520,8 @@ func TestWorkEstimator(t *testing.T) {
 				Subresource: "token",
 			},
 			watchCount:                5777,
-			initialSeatsExpected:      minimumSeats,
+			maxSeats:                  10,
+			initialSeatsExpected:      1,
 			finalSeatsExpected:        0,
 			additionalLatencyExpected: 0,
 		},
@@ -477,8 +534,9 @@ func TestWorkEstimator(t *testing.T) {
 				Resource: "serviceaccounts",
 			},
 			watchCount:                1000,
+			maxSeats:                  10,
 			initialSeatsExpected:      1,
-			finalSeatsExpected:        maximumSeats,
+			finalSeatsExpected:        10,
 			additionalLatencyExpected: 50 * time.Millisecond,
 		},
 	}
@@ -495,8 +553,11 @@ func TestWorkEstimator(t *testing.T) {
 			watchCountsFn := func(_ *apirequest.RequestInfo) int {
 				return test.watchCount
 			}
+			maxSeatsFn := func(_ string) uint64 {
+				return test.maxSeats
+			}
 
-			estimator := NewWorkEstimator(countsFn, watchCountsFn, defaultCfg)
+			estimator := NewWorkEstimator(countsFn, watchCountsFn, defaultCfg, maxSeatsFn)
 
 			req, err := http.NewRequest("GET", test.requestURI, nil)
 			if err != nil {