Add WithAlloc interface and stub implementations with base benchmarks

staging/src/k8s.io/apimachinery/pkg/api/meta/help.go

@@ -113,8 +113,27 @@ func getItemsPtr(list runtime.Object) (interface{}, error) {
 
 // EachListItem invokes fn on each runtime.Object in the list. Any error immediately terminates
 // the loop.
+//
+// If items passed to fn are retained for different durations, and you want to avoid
+// retaining all items in obj as long as any item is referenced, use EachListItemWithAlloc instead.
 func EachListItem(obj runtime.Object, fn func(runtime.Object) error) error {
+	return eachListItem(obj, fn, false)
+}
+
+// EachListItemWithAlloc works like EachListItem, but avoids retaining references to the items slice in obj.
+// It does this by making a shallow copy of non-pointer items in obj.
+//
+// If the items passed to fn are not retained, or are retained for the same duration, use EachListItem instead for memory efficiency.
+func EachListItemWithAlloc(obj runtime.Object, fn func(runtime.Object) error) error {
+	return eachListItem(obj, fn, true)
+}
+
+// allocNew: Whether shallow copy is required when the elements in Object.Items are struct
+func eachListItem(obj runtime.Object, fn func(runtime.Object) error, allocNew bool) error {
 	if unstructured, ok := obj.(runtime.Unstructured); ok {
+		if allocNew {
+			return unstructured.EachListItemWithAlloc(fn)
+		}
 		return unstructured.EachListItem(fn)
 	}
 	// TODO: Change to an interface call?
@@ -167,7 +186,23 @@ func EachListItem(obj runtime.Object, fn func(runtime.Object) error) error {
 
 // ExtractList returns obj's Items element as an array of runtime.Objects.
 // Returns an error if obj is not a List type (does not have an Items member).
+//
+// If items in the returned list are retained for different durations, and you want to avoid
+// retaining all items in obj as long as any item is referenced, use ExtractListWithAlloc instead.
 func ExtractList(obj runtime.Object) ([]runtime.Object, error) {
+	return extractList(obj, false)
+}
+
+// ExtractListWithAlloc works like ExtractList, but avoids retaining references to the items slice in obj.
+// It does this by making a shallow copy of non-pointer items in obj.
+//
+// If the items in the returned list are not retained, or are retained for the same duration, use ExtractList instead for memory efficiency.
+func ExtractListWithAlloc(obj runtime.Object) ([]runtime.Object, error) {
+	return extractList(obj, true)
+}
+
+// allocNew: Whether shallow copy is required when the elements in Object.Items are struct
+func extractList(obj runtime.Object, allocNew bool) ([]runtime.Object, error) {
 	itemsPtr, err := GetItemsPtr(obj)
 	if err != nil {
 		return nil, err

staging/src/k8s.io/apimachinery/pkg/api/meta/help_test.go

@@ -0,0 +1,539 @@
+/*
+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 meta
+
+import (
+	"reflect"
+	"strconv"
+	"testing"
+
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+)
+
+const (
+	fakeObjectItemsNum = 1000
+	exemptObjectIndex  = fakeObjectItemsNum / 4
+)
+
+type SampleSpec struct {
+	Flied int
+}
+
+type FooSpec struct {
+	Flied int
+}
+
+type FooList struct {
+	metav1.TypeMeta
+	metav1.ListMeta
+	Items []Foo
+}
+
+func (s *FooList) DeepCopyObject() runtime.Object { panic("unimplemented"); return nil }
+
+type SampleList struct {
+	metav1.TypeMeta
+	metav1.ListMeta
+	Items []Sample
+}
+
+func (s *SampleList) DeepCopyObject() runtime.Object { panic("unimplemented"); return nil }
+
+type RawExtensionList struct {
+	metav1.TypeMeta
+	metav1.ListMeta
+
+	Items []runtime.RawExtension
+}
+
+func (l RawExtensionList) DeepCopyObject() runtime.Object { panic("unimplemented"); return nil }
+
+// NOTE: Foo struct itself is the implementer of runtime.Object.
+type Foo struct {
+	metav1.TypeMeta
+	metav1.ObjectMeta
+	Spec FooSpec
+}
+
+func (f Foo) GetObjectKind() schema.ObjectKind {
+	tm := f.TypeMeta
+	return &tm
+}
+
+func (f Foo) DeepCopyObject() runtime.Object { panic("unimplemented"); return nil }
+
+// NOTE: the pointer of Sample that is the implementer of runtime.Object.
+// the behavior is similar to our corev1.Pod. corev1.Node
+type Sample struct {
+	metav1.TypeMeta
+	metav1.ObjectMeta
+	Spec SampleSpec
+}
+
+func (s *Sample) GetObjectKind() schema.ObjectKind {
+	tm := s.TypeMeta
+	return &tm
+}
+
+func (s *Sample) DeepCopyObject() runtime.Object { panic("unimplemented"); return nil }
+
+func fakeSampleList(numItems int) *SampleList {
+	out := &SampleList{
+		Items: make([]Sample, numItems),
+	}
+
+	for i := range out.Items {
+		out.Items[i] = Sample{
+			TypeMeta: metav1.TypeMeta{
+				APIVersion: "foo.org/v1",
+				Kind:       "Sample",
+			},
+			ObjectMeta: metav1.ObjectMeta{
+				Name:      strconv.Itoa(i),
+				Namespace: "default",
+				Labels: map[string]string{
+					"label-key-1": "label-value-1",
+				},
+				Annotations: map[string]string{
+					"annotations-key-1": "annotations-value-1",
+				},
+			},
+			Spec: SampleSpec{
+				Flied: i,
+			},
+		}
+	}
+	return out
+}
+
+func fakeExtensionList(numItems int) *RawExtensionList {
+	out := &RawExtensionList{
+		Items: make([]runtime.RawExtension, numItems),
+	}
+
+	for i := range out.Items {
+		out.Items[i] = runtime.RawExtension{
+			Object: &Foo{
+				TypeMeta: metav1.TypeMeta{
+					APIVersion: "sample.org/v1",
+					Kind:       "Foo",
+				},
+				ObjectMeta: metav1.ObjectMeta{
+					Name:      strconv.Itoa(i),
+					Namespace: "default",
+					Labels: map[string]string{
+						"label-key-1": "label-value-1",
+					},
+					Annotations: map[string]string{
+						"annotations-key-1": "annotations-value-1",
+					},
+				},
+				Spec: FooSpec{
+					Flied: i,
+				},
+			},
+		}
+	}
+	return out
+}
+
+func fakeUnstructuredList(numItems int) runtime.Unstructured {
+	out := &unstructured.UnstructuredList{
+		Items: make([]unstructured.Unstructured, numItems),
+	}
+
+	for i := range out.Items {
+		out.Items[i] = unstructured.Unstructured{
+			Object: map[string]interface{}{
+				"apiVersion": "v1",
+				"kind":       "Pod",
+				"metadata": map[string]interface{}{
+					"creationTimestamp": nil,
+					"name":              strconv.Itoa(i),
+				},
+				"spec": map[string]interface{}{
+					"hostname": "example.com",
+				},
+				"status": map[string]interface{}{},
+			},
+		}
+	}
+	return out
+}
+
+func fakeFooList(numItems int) *FooList {
+	out := &FooList{
+		Items: make([]Foo, numItems),
+	}
+
+	for i := range out.Items {
+		out.Items[i] = Foo{
+			TypeMeta: metav1.TypeMeta{
+				APIVersion: "sample.org/v1",
+				Kind:       "Foo",
+			},
+			ObjectMeta: metav1.ObjectMeta{
+				Name:      strconv.Itoa(i),
+				Namespace: "default",
+				Labels: map[string]string{
+					"label-key-1": "label-value-1",
+				},
+				Annotations: map[string]string{
+					"annotations-key-1": "annotations-value-1",
+				},
+			},
+			Spec: FooSpec{
+				Flied: i,
+			},
+		}
+	}
+	return out
+}
+
+func TestEachList(t *testing.T) {
+	tests := []struct {
+		name            string
+		generateFunc    func(num int) (list runtime.Object)
+		expectObjectNum int
+	}{
+		{
+			name: "StructReceiverList",
+			generateFunc: func(num int) (list runtime.Object) {
+				return fakeFooList(num)
+			},
+			expectObjectNum: fakeObjectItemsNum,
+		},
+		{
+			name: "PointerReceiverList",
+			generateFunc: func(num int) (list runtime.Object) {
+				return fakeSampleList(num)
+			},
+			expectObjectNum: fakeObjectItemsNum,
+		},
+		{
+			name: "RawExtensionList",
+			generateFunc: func(num int) (list runtime.Object) {
+				return fakeExtensionList(num)
+			},
+			expectObjectNum: fakeObjectItemsNum,
+		},
+		{
+			name: "UnstructuredList",
+			generateFunc: func(num int) (list runtime.Object) {
+				return fakeUnstructuredList(fakeObjectItemsNum)
+			},
+			expectObjectNum: fakeObjectItemsNum,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Run("EachListItem", func(t *testing.T) {
+				expectObjectNames := map[string]struct{}{}
+				for i := 0; i < tc.expectObjectNum; i++ {
+					expectObjectNames[strconv.Itoa(i)] = struct{}{}
+				}
+				list := tc.generateFunc(tc.expectObjectNum)
+				err := EachListItem(list, func(object runtime.Object) error {
+					o, err := Accessor(object)
+					if err != nil {
+						return err
+					}
+					delete(expectObjectNames, o.GetName())
+					return nil
+				})
+				if err != nil {
+					t.Errorf("each list item %#v: %v", list, err)
+				}
+				if len(expectObjectNames) != 0 {
+					t.Fatal("expectObjectNames should be empty")
+				}
+			})
+			t.Run("EachListItemWithAlloc", func(t *testing.T) {
+				expectObjectNames := map[string]struct{}{}
+				for i := 0; i < tc.expectObjectNum; i++ {
+					expectObjectNames[strconv.Itoa(i)] = struct{}{}
+				}
+				list := tc.generateFunc(tc.expectObjectNum)
+				err := EachListItemWithAlloc(list, func(object runtime.Object) error {
+					o, err := Accessor(object)
+					if err != nil {
+						return err
+					}
+					delete(expectObjectNames, o.GetName())
+					return nil
+				})
+				if err != nil {
+					t.Errorf("each list %#v with alloc: %v", list, err)
+				}
+				if len(expectObjectNames) != 0 {
+					t.Fatal("expectObjectNames should be empty")
+				}
+			})
+		})
+	}
+}
+
+func TestExtractList(t *testing.T) {
+	tests := []struct {
+		name            string
+		generateFunc    func(num int) (list runtime.Object)
+		expectObjectNum int
+	}{
+		{
+			name: "StructReceiverList",
+			generateFunc: func(num int) (list runtime.Object) {
+				return fakeFooList(num)
+			},
+			expectObjectNum: fakeObjectItemsNum,
+		},
+		{
+			name: "PointerReceiverList",
+			generateFunc: func(num int) (list runtime.Object) {
+				return fakeSampleList(num)
+			},
+			expectObjectNum: fakeObjectItemsNum,
+		},
+		{
+			name: "RawExtensionList",
+			generateFunc: func(num int) (list runtime.Object) {
+				return fakeExtensionList(num)
+			},
+			expectObjectNum: fakeObjectItemsNum,
+		},
+		{
+			name: "UnstructuredList",
+			generateFunc: func(num int) (list runtime.Object) {
+				return fakeUnstructuredList(fakeObjectItemsNum)
+			},
+			expectObjectNum: fakeObjectItemsNum,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Run("ExtractList", func(t *testing.T) {
+				expectObjectNames := map[string]struct{}{}
+				for i := 0; i < tc.expectObjectNum; i++ {
+					expectObjectNames[strconv.Itoa(i)] = struct{}{}
+				}
+				list := tc.generateFunc(tc.expectObjectNum)
+				objs, err := ExtractList(list)
+				if err != nil {
+					t.Fatalf("extract list %#v: %v", list, err)
+				}
+				for i := range objs {
+					var (
+						o   metav1.Object
+						err error
+						obj = objs[i]
+					)
+
+					if reflect.TypeOf(obj).Kind() == reflect.Struct {
+						copy := reflect.New(reflect.TypeOf(obj))
+						copy.Elem().Set(reflect.ValueOf(obj))
+						o, err = Accessor(copy.Interface())
+					} else {
+						o, err = Accessor(obj)
+					}
+					if err != nil {
+						t.Fatalf("Accessor object %#v: %v", obj, err)
+					}
+					delete(expectObjectNames, o.GetName())
+				}
+				if len(expectObjectNames) != 0 {
+					t.Fatal("expectObjectNames should be empty")
+				}
+			})
+			t.Run("ExtractListWithAlloc", func(t *testing.T) {
+				expectObjectNames := map[string]struct{}{}
+				for i := 0; i < tc.expectObjectNum; i++ {
+					expectObjectNames[strconv.Itoa(i)] = struct{}{}
+				}
+				list := tc.generateFunc(tc.expectObjectNum)
+				objs, err := ExtractListWithAlloc(list)
+				if err != nil {
+					t.Fatalf("extract list with alloc: %v", err)
+				}
+				for i := range objs {
+					var (
+						o   metav1.Object
+						err error
+						obj = objs[i]
+					)
+					if reflect.TypeOf(obj).Kind() == reflect.Struct {
+						copy := reflect.New(reflect.TypeOf(obj))
+						copy.Elem().Set(reflect.ValueOf(obj))
+						o, err = Accessor(copy.Interface())
+					} else {
+						o, err = Accessor(obj)
+					}
+					if err != nil {
+						t.Fatalf("Accessor object %#v: %v", obj, err)
+					}
+					delete(expectObjectNames, o.GetName())
+				}
+				if len(expectObjectNames) != 0 {
+					t.Fatal("expectObjectNames should be empty")
+				}
+			})
+		})
+	}
+}
+
+func BenchmarkExtractListItem(b *testing.B) {
+	tests := []struct {
+		name string
+		list runtime.Object
+	}{
+		{
+			name: "StructReceiverList",
+			list: fakeFooList(fakeObjectItemsNum),
+		},
+		{
+			name: "PointerReceiverList",
+			list: fakeSampleList(fakeObjectItemsNum),
+		},
+		{
+			name: "RawExtensionList",
+			list: fakeExtensionList(fakeObjectItemsNum),
+		},
+	}
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				_, err := ExtractList(tc.list)
+				if err != nil {
+					b.Fatalf("ExtractList: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkEachListItem(b *testing.B) {
+	tests := []struct {
+		name string
+		list runtime.Object
+	}{
+		{
+			name: "StructReceiverList",
+			list: fakeFooList(fakeObjectItemsNum),
+		},
+		{
+			name: "PointerReceiverList",
+			list: fakeSampleList(fakeObjectItemsNum),
+		},
+		{
+			name: "RawExtensionList",
+			list: fakeExtensionList(fakeObjectItemsNum),
+		},
+		{
+			name: "UnstructuredList",
+			list: fakeUnstructuredList(fakeObjectItemsNum),
+		},
+	}
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				err := EachListItem(tc.list, func(object runtime.Object) error {
+					return nil
+				})
+				if err != nil {
+					b.Fatalf("EachListItem: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkExtractListItemWithAlloc(b *testing.B) {
+	tests := []struct {
+		name string
+		list runtime.Object
+	}{
+		{
+			name: "StructReceiverList",
+			list: fakeFooList(fakeObjectItemsNum),
+		},
+		{
+			name: "PointerReceiverList",
+			list: fakeSampleList(fakeObjectItemsNum),
+		},
+		{
+			name: "RawExtensionList",
+			list: fakeExtensionList(fakeObjectItemsNum),
+		},
+	}
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				_, err := ExtractListWithAlloc(tc.list)
+				if err != nil {
+					b.Fatalf("ExtractListWithAlloc: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkEachListItemWithAlloc(b *testing.B) {
+	tests := []struct {
+		name string
+		list runtime.Object
+	}{
+		{
+			name: "StructReceiverList",
+			list: fakeFooList(fakeObjectItemsNum),
+		},
+		{
+			name: "PointerReceiverList",
+			list: fakeSampleList(fakeObjectItemsNum),
+		},
+		{
+			name: "RawExtensionList",
+			list: fakeExtensionList(fakeObjectItemsNum),
+		},
+		{
+			name: "UnstructuredList",
+			list: fakeUnstructuredList(fakeObjectItemsNum),
+		},
+	}
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				err := EachListItemWithAlloc(tc.list, func(object runtime.Object) error {
+					return nil
+				})
+				if err != nil {
+					b.Fatalf("EachListItemWithAlloc: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}

staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/unstructured/unstructured.go

@@ -101,6 +101,11 @@ func (obj *Unstructured) EachListItem(fn func(runtime.Object) error) error {
 	return nil
 }
 
+func (obj *Unstructured) EachListItemWithAlloc(fn func(runtime.Object) error) error {
+	// EachListItem has allocated a new Object for the user, we can use it directly.
+	return obj.EachListItem(fn)
+}
+
 func (obj *Unstructured) UnstructuredContent() map[string]interface{} {
 	if obj.Object == nil {
 		return make(map[string]interface{})

staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/unstructured/unstructured_list.go

@@ -52,6 +52,10 @@ func (u *UnstructuredList) EachListItem(fn func(runtime.Object) error) error {
 	return nil
 }
 
+func (u *UnstructuredList) EachListItemWithAlloc(fn func(runtime.Object) error) error {
+	return u.EachListItem(fn)
+}
+
 // NewEmptyInstance returns a new instance of the concrete type containing only kind/apiVersion and no other data.
 // This should be called instead of reflect.New() for unstructured types because the go type alone does not preserve kind/apiVersion info.
 func (u *UnstructuredList) NewEmptyInstance() runtime.Unstructured {

staging/src/k8s.io/apimachinery/pkg/runtime/interfaces.go

@@ -365,4 +365,9 @@ type Unstructured interface {
 	// error should terminate the iteration. If IsList() returns false, this method should return an error
 	// instead of calling the provided function.
 	EachListItem(func(Object) error) error
+	// EachListItemWithAlloc works like EachListItem, but avoids retaining references to a slice of items.
+	// It does this by making a shallow copy of non-pointer items before passing them to fn.
+	//
+	// If the items passed to fn are not retained, or are retained for the same duration, use EachListItem instead for memory efficiency.
+	EachListItemWithAlloc(func(Object) error) error
 }

staging/src/k8s.io/apimachinery/pkg/runtime/testing/types.go

@@ -261,6 +261,11 @@ func (obj *Unstructured) EachListItem(fn func(runtime.Object) error) error {
 	return nil
 }
 
+func (obj *Unstructured) EachListItemWithAlloc(fn func(runtime.Object) error) error {
+	// EachListItem has allocated a new Object for the user, we can use it directly.
+	return obj.EachListItem(fn)
+}
+
 func (obj *Unstructured) NewEmptyInstance() runtime.Unstructured {
 	out := new(Unstructured)
 	if obj != nil {

staging/src/k8s.io/client-go/tools/cache/reflector.go

@@ -399,7 +399,7 @@ func (r *Reflector) list(stopCh <-chan struct{}) error {
 			pager.PageSize = 0
 		}
 
-		list, paginatedResult, err = pager.List(context.Background(), options)
+		list, paginatedResult, err = pager.ListWithAlloc(context.Background(), options)
 		if isExpiredError(err) || isTooLargeResourceVersionError(err) {
 			r.setIsLastSyncResourceVersionUnavailable(true)
 			// Retry immediately if the resource version used to list is unavailable.
@@ -408,7 +408,7 @@ func (r *Reflector) list(stopCh <-chan struct{}) error {
 			// resource version it is listing at is expired or the cache may not yet be synced to the provided
 			// resource version. So we need to fallback to resourceVersion="" in all to recover and ensure
 			// the reflector makes forward progress.
-			list, paginatedResult, err = pager.List(context.Background(), metav1.ListOptions{ResourceVersion: r.relistResourceVersion()})
+			list, paginatedResult, err = pager.ListWithAlloc(context.Background(), metav1.ListOptions{ResourceVersion: r.relistResourceVersion()})
 		}
 		close(listCh)
 	}()
@@ -446,7 +446,7 @@ func (r *Reflector) list(stopCh <-chan struct{}) error {
 	}
 	resourceVersion = listMetaInterface.GetResourceVersion()
 	initTrace.Step("Resource version extracted")
-	items, err := meta.ExtractList(list)
+	items, err := meta.ExtractListWithAlloc(list)
 	if err != nil {
 		return fmt.Errorf("unable to understand list result %#v (%v)", list, err)
 	}

staging/src/k8s.io/client-go/tools/cache/reflector_test.go

@@ -17,10 +17,12 @@ limitations under the License.
 package cache
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"math/rand"
 	"reflect"
+	goruntime "runtime"
 	"strconv"
 	"syscall"
 	"testing"
@@ -28,10 +30,13 @@ import (
 
 	v1 "k8s.io/api/core/v1"
 	apierrors "k8s.io/apimachinery/pkg/api/errors"
+	"k8s.io/apimachinery/pkg/api/meta"
+	"k8s.io/apimachinery/pkg/api/resource"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/apimachinery/pkg/util/sets"
 	"k8s.io/apimachinery/pkg/util/wait"
 	"k8s.io/apimachinery/pkg/watch"
 	"k8s.io/utils/clock"
@@ -1089,3 +1094,493 @@ func TestReflectorResourceVersionUpdate(t *testing.T) {
 		t.Errorf("Expected series of resource version updates of %#v but got: %#v", expectedRVs, s.resourceVersions)
 	}
 }
+
+const (
+	fakeItemsNum      = 100
+	exemptObjectIndex = fakeItemsNum / 4
+	pageNum           = 3
+)
+
+func getPodListItems(start int, numItems int) (string, string, *v1.PodList) {
+	out := &v1.PodList{
+		Items: make([]v1.Pod, numItems),
+	}
+
+	for i := 0; i < numItems; i++ {
+
+		out.Items[i] = v1.Pod{
+			TypeMeta: metav1.TypeMeta{
+				APIVersion: "v1",
+				Kind:       "Pod",
+			},
+			ObjectMeta: metav1.ObjectMeta{
+				Name:      fmt.Sprintf("pod-%d", i+start),
+				Namespace: "default",
+				Labels: map[string]string{
+					"label-key-1": "label-value-1",
+				},
+				Annotations: map[string]string{
+					"annotations-key-1": "annotations-value-1",
+				},
+			},
+			Spec: v1.PodSpec{
+				Overhead: v1.ResourceList{
+					v1.ResourceCPU:    resource.MustParse("3"),
+					v1.ResourceMemory: resource.MustParse("8"),
+				},
+				NodeSelector: map[string]string{
+					"foo": "bar",
+					"baz": "quux",
+				},
+				Affinity: &v1.Affinity{
+					NodeAffinity: &v1.NodeAffinity{
+						RequiredDuringSchedulingIgnoredDuringExecution: &v1.NodeSelector{
+							NodeSelectorTerms: []v1.NodeSelectorTerm{
+								{MatchExpressions: []v1.NodeSelectorRequirement{{Key: `foo`}}},
+							},
+						},
+						PreferredDuringSchedulingIgnoredDuringExecution: []v1.PreferredSchedulingTerm{
+							{Preference: v1.NodeSelectorTerm{MatchExpressions: []v1.NodeSelectorRequirement{{Key: `foo`}}}},
+						},
+					},
+				},
+				TopologySpreadConstraints: []v1.TopologySpreadConstraint{
+					{TopologyKey: `foo`},
+				},
+				HostAliases: []v1.HostAlias{
+					{IP: "1.1.1.1"},
+					{IP: "2.2.2.2"},
+				},
+				ImagePullSecrets: []v1.LocalObjectReference{
+					{Name: "secret1"},
+					{Name: "secret2"},
+				},
+				Containers: []v1.Container{
+					{
+						Name:  "foobar",
+						Image: "alpine",
+						Resources: v1.ResourceRequirements{
+							Requests: v1.ResourceList{
+								v1.ResourceName(v1.ResourceCPU):    resource.MustParse("1"),
+								v1.ResourceName(v1.ResourceMemory): resource.MustParse("5"),
+							},
+							Limits: v1.ResourceList{
+								v1.ResourceName(v1.ResourceCPU):    resource.MustParse("2"),
+								v1.ResourceName(v1.ResourceMemory): resource.MustParse("10"),
+							},
+						},
+					},
+					{
+						Name:  "foobar2",
+						Image: "alpine",
+						Resources: v1.ResourceRequirements{
+							Requests: v1.ResourceList{
+								v1.ResourceName(v1.ResourceCPU):    resource.MustParse("4"),
+								v1.ResourceName(v1.ResourceMemory): resource.MustParse("12"),
+							},
+							Limits: v1.ResourceList{
+								v1.ResourceName(v1.ResourceCPU):    resource.MustParse("8"),
+								v1.ResourceName(v1.ResourceMemory): resource.MustParse("24"),
+							},
+						},
+					},
+				},
+				InitContainers: []v1.Container{
+					{
+						Name:  "small-init",
+						Image: "alpine",
+						Resources: v1.ResourceRequirements{
+							Requests: v1.ResourceList{
+								v1.ResourceName(v1.ResourceCPU):    resource.MustParse("1"),
+								v1.ResourceName(v1.ResourceMemory): resource.MustParse("5"),
+							},
+							Limits: v1.ResourceList{
+								v1.ResourceName(v1.ResourceCPU):    resource.MustParse("1"),
+								v1.ResourceName(v1.ResourceMemory): resource.MustParse("5"),
+							},
+						},
+					},
+					{
+						Name:  "big-init",
+						Image: "alpine",
+						Resources: v1.ResourceRequirements{
+							Requests: v1.ResourceList{
+								v1.ResourceName(v1.ResourceCPU):    resource.MustParse("40"),
+								v1.ResourceName(v1.ResourceMemory): resource.MustParse("120"),
+							},
+							Limits: v1.ResourceList{
+								v1.ResourceName(v1.ResourceCPU):    resource.MustParse("80"),
+								v1.ResourceName(v1.ResourceMemory): resource.MustParse("240"),
+							},
+						},
+					},
+				},
+				Hostname: fmt.Sprintf("node-%d", i),
+			},
+			Status: v1.PodStatus{
+				Phase: v1.PodRunning,
+				ContainerStatuses: []v1.ContainerStatus{
+					{
+						ContainerID: "docker://numbers",
+						Image:       "alpine",
+						Name:        "foobar",
+						Ready:       false,
+					},
+					{
+						ContainerID: "docker://numbers",
+						Image:       "alpine",
+						Name:        "foobar2",
+						Ready:       false,
+					},
+				},
+				InitContainerStatuses: []v1.ContainerStatus{
+					{
+						ContainerID: "docker://numbers",
+						Image:       "alpine",
+						Name:        "small-init",
+						Ready:       false,
+					},
+					{
+						ContainerID: "docker://numbers",
+						Image:       "alpine",
+						Name:        "big-init",
+						Ready:       false,
+					},
+				},
+				Conditions: []v1.PodCondition{
+					{
+						Type:               v1.PodScheduled,
+						Status:             v1.ConditionTrue,
+						Reason:             "successfully",
+						Message:            "sync pod successfully",
+						LastProbeTime:      metav1.Now(),
+						LastTransitionTime: metav1.Now(),
+					},
+				},
+			},
+		}
+	}
+
+	return out.Items[0].GetName(), out.Items[exemptObjectIndex].GetName(), out
+}
+
+func getConfigmapListItems(start int, numItems int) (string, string, *v1.ConfigMapList) {
+	out := &v1.ConfigMapList{
+		Items: make([]v1.ConfigMap, numItems),
+	}
+
+	for i := 0; i < numItems; i++ {
+		out.Items[i] = v1.ConfigMap{
+			TypeMeta: metav1.TypeMeta{
+				APIVersion: "v1",
+				Kind:       "ConfigMap",
+			},
+			ObjectMeta: metav1.ObjectMeta{
+				Name:      fmt.Sprintf("cm-%d", i+start),
+				Namespace: "default",
+				Labels: map[string]string{
+					"label-key-1": "label-value-1",
+				},
+				Annotations: map[string]string{
+					"annotations-key-1": "annotations-value-1",
+				},
+			},
+			Data: map[string]string{
+				"data-1": "value-1",
+				"data-2": "value-2",
+			},
+		}
+	}
+
+	return out.Items[0].GetName(), out.Items[exemptObjectIndex].GetName(), out
+}
+
+type TestPagingPodsLW struct {
+	totalPageCount   int
+	fetchedPageCount int
+
+	detectedObjectNameList []string
+	exemptObjectNameList   []string
+}
+
+func newPageTestLW(totalPageNum int) *TestPagingPodsLW {
+	return &TestPagingPodsLW{
+		totalPageCount:   totalPageNum,
+		fetchedPageCount: 0,
+	}
+}
+
+func (t *TestPagingPodsLW) List(options metav1.ListOptions) (runtime.Object, error) {
+	firstPodName, exemptPodName, list := getPodListItems(t.fetchedPageCount*fakeItemsNum, fakeItemsNum)
+	t.detectedObjectNameList = append(t.detectedObjectNameList, firstPodName)
+	t.exemptObjectNameList = append(t.exemptObjectNameList, exemptPodName)
+	t.fetchedPageCount++
+	if t.fetchedPageCount >= t.totalPageCount {
+		return list, nil
+	}
+	list.SetContinue("true")
+	return list, nil
+}
+
+func (t *TestPagingPodsLW) Watch(options metav1.ListOptions) (watch.Interface, error) {
+	return nil, nil
+}
+
+func TestReflectorListExtract(t *testing.T) {
+	store := NewStore(func(obj interface{}) (string, error) {
+		pod, ok := obj.(*v1.Pod)
+		if !ok {
+			return "", fmt.Errorf("expect *v1.Pod, but got %T", obj)
+		}
+		return pod.GetName(), nil
+	})
+
+	lw := newPageTestLW(5)
+	reflector := NewReflector(lw, &v1.Pod{}, store, 0)
+	reflector.WatchListPageSize = fakeItemsNum
+
+	// execute list to fill store
+	stopCh := make(chan struct{})
+	if err := reflector.list(stopCh); err != nil {
+		t.Fatal(err)
+	}
+
+	// We will not delete exemptPod,
+	// in order to see if the existence of this Pod causes other Pods that are not used to be unable to properly clear.
+	for _, podName := range lw.exemptObjectNameList {
+		_, exist, err := store.GetByKey(podName)
+		if err != nil || !exist {
+			t.Fatalf("%s should exist in pod store", podName)
+		}
+	}
+
+	// we will pay attention to whether the memory occupied by the first Pod is released
+	// Golang's can only be SetFinalizer for the first element of the array,
+	// so pod-0 will be the object of our attention
+	detectedPodAlreadyBeCleared := make(chan struct{}, len(lw.detectedObjectNameList))
+
+	for _, firstPodName := range lw.detectedObjectNameList {
+		_, exist, err := store.GetByKey(firstPodName)
+		if err != nil || !exist {
+			t.Fatalf("%s should exist in pod store", firstPodName)
+		}
+		firstPod, exist, err := store.GetByKey(firstPodName)
+		if err != nil || !exist {
+			t.Fatalf("%s should exist in pod store", firstPodName)
+		}
+		goruntime.SetFinalizer(firstPod, func(obj interface{}) {
+			t.Logf("%s already be gc\n", obj.(*v1.Pod).GetName())
+			detectedPodAlreadyBeCleared <- struct{}{}
+		})
+	}
+
+	storedObjectKeys := store.ListKeys()
+	for _, k := range storedObjectKeys {
+		// delete all Pods except the exempted Pods.
+		if sets.NewString(lw.exemptObjectNameList...).Has(k) {
+			continue
+		}
+		obj, exist, err := store.GetByKey(k)
+		if err != nil || !exist {
+			t.Fatalf("%s should exist in pod store", k)
+		}
+
+		if err := store.Delete(obj); err != nil {
+			t.Fatalf("delete object: %v", err)
+		}
+		goruntime.GC()
+	}
+
+	clearedNum := 0
+	for {
+		select {
+		case <-detectedPodAlreadyBeCleared:
+			clearedNum++
+			if clearedNum == len(lw.detectedObjectNameList) {
+				return
+			}
+		}
+	}
+}
+
+func BenchmarkExtractList(b *testing.B) {
+	_, _, podList := getPodListItems(0, fakeItemsNum)
+	_, _, configMapList := getConfigmapListItems(0, fakeItemsNum)
+	tests := []struct {
+		name string
+		list runtime.Object
+	}{
+		{
+			name: "PodList",
+			list: podList,
+		},
+		{
+			name: "ConfigMapList",
+			list: configMapList,
+		},
+	}
+
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				_, err := meta.ExtractList(tc.list)
+				if err != nil {
+					b.Errorf("extract list: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkEachListItem(b *testing.B) {
+	_, _, podList := getPodListItems(0, fakeItemsNum)
+	_, _, configMapList := getConfigmapListItems(0, fakeItemsNum)
+	tests := []struct {
+		name string
+		list runtime.Object
+	}{
+		{
+			name: "PodList",
+			list: podList,
+		},
+		{
+			name: "ConfigMapList",
+			list: configMapList,
+		},
+	}
+
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				err := meta.EachListItem(tc.list, func(object runtime.Object) error {
+					return nil
+				})
+				if err != nil {
+					b.Errorf("each list: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkExtractListWithAlloc(b *testing.B) {
+	_, _, podList := getPodListItems(0, fakeItemsNum)
+	_, _, configMapList := getConfigmapListItems(0, fakeItemsNum)
+	tests := []struct {
+		name string
+		list runtime.Object
+	}{
+		{
+			name: "PodList",
+			list: podList,
+		},
+		{
+			name: "ConfigMapList",
+			list: configMapList,
+		},
+	}
+
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				_, err := meta.ExtractListWithAlloc(tc.list)
+				if err != nil {
+					b.Errorf("extract list with alloc: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkEachListItemWithAlloc(b *testing.B) {
+	_, _, podList := getPodListItems(0, fakeItemsNum)
+	_, _, configMapList := getConfigmapListItems(0, fakeItemsNum)
+	tests := []struct {
+		name string
+		list runtime.Object
+	}{
+		{
+			name: "PodList",
+			list: podList,
+		},
+		{
+			name: "ConfigMapList",
+			list: configMapList,
+		},
+	}
+
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				err := meta.EachListItemWithAlloc(tc.list, func(object runtime.Object) error {
+					return nil
+				})
+				if err != nil {
+					b.Errorf("each list with alloc: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkReflectorList(b *testing.B) {
+	ctx, cancel := context.WithTimeout(context.Background(), wait.ForeverTestTimeout)
+	defer cancel()
+
+	store := NewStore(func(obj interface{}) (string, error) {
+		o, err := meta.Accessor(obj)
+		if err != nil {
+			return "", err
+		}
+		return o.GetName(), nil
+	})
+
+	_, _, podList := getPodListItems(0, fakeItemsNum)
+	_, _, configMapList := getConfigmapListItems(0, fakeItemsNum)
+	tests := []struct {
+		name   string
+		sample func() interface{}
+		list   runtime.Object
+	}{
+		{
+			name: "PodList",
+			sample: func() interface{} {
+				return v1.Pod{}
+			},
+			list: podList,
+		},
+		{
+			name: "ConfigMapList",
+			sample: func() interface{} {
+				return v1.ConfigMap{}
+			},
+			list: configMapList,
+		},
+	}
+
+	for _, tc := range tests {
+		b.Run(tc.name, func(b *testing.B) {
+
+			sample := tc.sample()
+			reflector := NewReflector(newPageTestLW(pageNum), &sample, store, 0)
+			reflector.WatchListPageSize = fakeItemsNum
+
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				err := reflector.list(ctx.Done())
+				if err != nil {
+					b.Fatalf("reflect list: %v", err)
+				}
+			}
+			b.StopTimer()
+		})
+	}
+}

staging/src/k8s.io/client-go/tools/pager/pager.go

@@ -73,7 +73,23 @@ func New(fn ListPageFunc) *ListPager {
 // List returns a single list object, but attempts to retrieve smaller chunks from the
 // server to reduce the impact on the server. If the chunk attempt fails, it will load
 // the full list instead. The Limit field on options, if unset, will default to the page size.
+//
+// If items in the returned list are retained for different durations, and you want to avoid
+// retaining the whole slice returned by p.PageFn as long as any item is referenced,
+// use ListWithAlloc instead.
 func (p *ListPager) List(ctx context.Context, options metav1.ListOptions) (runtime.Object, bool, error) {
+	return p.list(ctx, options, false)
+}
+
+// ListWithAlloc works like List, but avoids retaining references to the items slice returned by p.PageFn.
+// It does this by making a shallow copy of non-pointer items in the slice returned by p.PageFn.
+//
+// If the items in the returned list are not retained, or are retained for the same duration, use List instead for memory efficiency.
+func (p *ListPager) ListWithAlloc(ctx context.Context, options metav1.ListOptions) (runtime.Object, bool, error) {
+	return p.list(ctx, options, true)
+}
+
+func (p *ListPager) list(ctx context.Context, options metav1.ListOptions, allocNew bool) (runtime.Object, bool, error) {
 	if options.Limit == 0 {
 		options.Limit = p.PageSize
 	}
@@ -123,7 +139,11 @@ func (p *ListPager) List(ctx context.Context, options metav1.ListOptions) (runti
 			list.ResourceVersion = m.GetResourceVersion()
 			list.SelfLink = m.GetSelfLink()
 		}
-		if err := meta.EachListItem(obj, func(obj runtime.Object) error {
+		eachListItemFunc := meta.EachListItem
+		if allocNew {
+			eachListItemFunc = meta.EachListItemWithAlloc
+		}
+		if err := eachListItemFunc(obj, func(obj runtime.Object) error {
 			list.Items = append(list.Items, obj)
 			return nil
 		}); err != nil {
@@ -156,12 +176,26 @@ func (p *ListPager) List(ctx context.Context, options metav1.ListOptions) (runti
 //
 // Items are retrieved in chunks from the server to reduce the impact on the server with up to
 // ListPager.PageBufferSize chunks buffered concurrently in the background.
+//
+// If items passed to fn are retained for different durations, and you want to avoid
+// retaining the whole slice returned by p.PageFn as long as any item is referenced,
+// use EachListItemWithAlloc instead.
 func (p *ListPager) EachListItem(ctx context.Context, options metav1.ListOptions, fn func(obj runtime.Object) error) error {
 	return p.eachListChunkBuffered(ctx, options, func(obj runtime.Object) error {
 		return meta.EachListItem(obj, fn)
 	})
 }
 
+// EachListItemWithAlloc works like EachListItem, but avoids retaining references to the items slice returned by p.PageFn.
+// It does this by making a shallow copy of non-pointer items in the slice returned by p.PageFn.
+//
+// If the items passed to fn are not retained, or are retained for the same duration, use EachListItem instead for memory efficiency.
+func (p *ListPager) EachListItemWithAlloc(ctx context.Context, options metav1.ListOptions, fn func(obj runtime.Object) error) error {
+	return p.eachListChunkBuffered(ctx, options, func(obj runtime.Object) error {
+		return meta.EachListItemWithAlloc(obj, fn)
+	})
+}
+
 // eachListChunkBuffered fetches runtimeObject list chunks using this ListPager and invokes fn on
 // each list chunk.  If fn returns an error, processing stops and that error is returned. If fn does
 // not return an error, any error encountered while retrieving the list from the server is