optimize watch-cache getlist (#116327)

* ftr(watch-cache): add benchmarks

* ftr(kube-apiserver): faster watch-cache getlist

* refine: testcase name

* - refine var name make it easier to convey meaning
- add comment to explain why we need to apply for a slice of runtime.Object instead of making a slice of ListObject.Items directly.

staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher.go

@@ -795,24 +795,30 @@ func (c *Cacher) GetList(ctx context.Context, key string, opts storage.ListOptio
 		return err
 	}
 	span.AddEvent("Listed items from cache", attribute.Int("count", len(objs)))
-	if len(objs) > listVal.Cap() && pred.Label.Empty() && pred.Field.Empty() {
-		// Resize the slice appropriately, since we already know that none
-		// of the elements will be filtered out.
-		listVal.Set(reflect.MakeSlice(reflect.SliceOf(c.objectType.Elem()), 0, len(objs)))
-		span.AddEvent("Resized result")
-	}
+	// store pointer of eligible objects,
+	// Why not directly put object in the items of listObj?
+	//   the elements in ListObject are Struct type, making slice will bring excessive memory consumption.
+	//   so we try to delay this action as much as possible
+	var selectedObjects []runtime.Object
 	for _, obj := range objs {
 		elem, ok := obj.(*storeElement)
 		if !ok {
 			return fmt.Errorf("non *storeElement returned from storage: %v", obj)
 		}
 		if filter(elem.Key, elem.Labels, elem.Fields) {
-			listVal.Set(reflect.Append(listVal, reflect.ValueOf(elem.Object).Elem()))
+			selectedObjects = append(selectedObjects, elem.Object)
 		}
 	}
-	if listVal.IsNil() {
+	if len(selectedObjects) == 0 {
 		// Ensure that we never return a nil Items pointer in the result for consistency.
 		listVal.Set(reflect.MakeSlice(listVal.Type(), 0, 0))
+	} else {
+		// Resize the slice appropriately, since we already know that size of result set
+		listVal.Set(reflect.MakeSlice(listVal.Type(), len(selectedObjects), len(selectedObjects)))
+		span.AddEvent("Resized result")
+		for i, o := range selectedObjects {
+			listVal.Index(i).Set(reflect.ValueOf(o).Elem())
+		}
 	}
 	span.AddEvent("Filtered items", attribute.Int("count", listVal.Len()))
 	if c.versioner != nil {

staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher_whitebox_test.go

@@ -18,6 +18,7 @@ package cacher
 
 import (
 	"context"
+	"crypto/rand"
 	"errors"
 	"fmt"
 	"reflect"
@@ -114,11 +115,22 @@ func newTestCacher(s storage.Interface) (*Cacher, storage.Versioner, error) {
 		GroupResource:  schema.GroupResource{Resource: "pods"},
 		ResourcePrefix: prefix,
 		KeyFunc:        func(obj runtime.Object) (string, error) { return storage.NamespaceKeyFunc(prefix, obj) },
-		GetAttrsFunc:   storage.DefaultNamespaceScopedAttr,
-		NewFunc:        func() runtime.Object { return &example.Pod{} },
-		NewListFunc:    func() runtime.Object { return &example.PodList{} },
-		Codec:          codecs.LegacyCodec(examplev1.SchemeGroupVersion),
-		Clock:          clock.RealClock{},
+		GetAttrsFunc: func(obj runtime.Object) (labels.Set, fields.Set, error) {
+			pod, ok := obj.(*example.Pod)
+			if !ok {
+				return storage.DefaultNamespaceScopedAttr(obj)
+			}
+			labelsSet, fieldsSet, err := storage.DefaultNamespaceScopedAttr(obj)
+			if err != nil {
+				return nil, nil, err
+			}
+			fieldsSet["spec.nodeName"] = pod.Spec.NodeName
+			return labelsSet, fieldsSet, nil
+		},
+		NewFunc:     func() runtime.Object { return &example.Pod{} },
+		NewListFunc: func() runtime.Object { return &example.PodList{} },
+		Codec:       codecs.LegacyCodec(examplev1.SchemeGroupVersion),
+		Clock:       clock.RealClock{},
 	}
 	cacher, err := NewCacherFromConfig(config)
 	return cacher, testVersioner{}, err
@@ -1830,3 +1842,107 @@ func TestWaitUntilWatchCacheFreshAndForceAllEvents(t *testing.T) {
 	require.NoError(t, err)
 	require.True(t, forceAllEvents, "the target method should instruct the caller to ask for all events in the cache (full state)")
 }
+
+type fakeStorage struct {
+	pods []example.Pod
+	storage.Interface
+}
+
+func newObjectStorage(fakePods []example.Pod) *fakeStorage {
+	return &fakeStorage{
+		pods: fakePods,
+	}
+}
+
+func (m fakeStorage) GetList(ctx context.Context, key string, opts storage.ListOptions, listObj runtime.Object) error {
+	podList := listObj.(*example.PodList)
+	podList.ListMeta = metav1.ListMeta{ResourceVersion: "12345"}
+	podList.Items = m.pods
+	return nil
+}
+func (m fakeStorage) Watch(_ context.Context, _ string, _ storage.ListOptions) (watch.Interface, error) {
+	return newDummyWatch(), nil
+}
+
+func BenchmarkCacher_GetList(b *testing.B) {
+	testCases := []struct {
+		totalObjectNum  int
+		expectObjectNum int
+	}{
+		{
+			totalObjectNum:  5000,
+			expectObjectNum: 50,
+		},
+		{
+			totalObjectNum:  5000,
+			expectObjectNum: 500,
+		},
+		{
+			totalObjectNum:  5000,
+			expectObjectNum: 1000,
+		},
+		{
+			totalObjectNum:  5000,
+			expectObjectNum: 2500,
+		},
+		{
+			totalObjectNum:  5000,
+			expectObjectNum: 5000,
+		},
+	}
+	for _, tc := range testCases {
+		b.Run(
+			fmt.Sprintf("totalObjectNum=%d, expectObjectNum=%d", tc.totalObjectNum, tc.expectObjectNum),
+			func(b *testing.B) {
+				// create sample pods
+				fakePods := make([]example.Pod, tc.totalObjectNum, tc.totalObjectNum)
+				for i := range fakePods {
+					fakePods[i].Namespace = "default"
+					fakePods[i].Name = fmt.Sprintf("pod-%d", i)
+					fakePods[i].ResourceVersion = strconv.Itoa(i)
+					if i%(tc.totalObjectNum/tc.expectObjectNum) == 0 {
+						fakePods[i].Spec.NodeName = "node-0"
+					}
+					data := make([]byte, 1024*2, 1024*2) // 2k labels
+					rand.Read(data)
+					fakePods[i].Spec.NodeSelector = map[string]string{
+						"key": string(data),
+					}
+				}
+
+				// build test cacher
+				cacher, _, err := newTestCacher(newObjectStorage(fakePods))
+				if err != nil {
+					b.Fatalf("new cacher: %v", err)
+				}
+				defer cacher.Stop()
+
+				// prepare result and pred
+				parsedField, err := fields.ParseSelector("spec.nodeName=node-0")
+				if err != nil {
+					b.Fatalf("parse selector: %v", err)
+				}
+				pred := storage.SelectionPredicate{
+					Label: labels.Everything(),
+					Field: parsedField,
+				}
+
+				// now we start benchmarking
+				b.ResetTimer()
+				for i := 0; i < b.N; i++ {
+					result := &example.PodList{}
+					err = cacher.GetList(context.TODO(), "pods", storage.ListOptions{
+						Predicate:       pred,
+						Recursive:       true,
+						ResourceVersion: "12345",
+					}, result)
+					if err != nil {
+						b.Fatalf("GetList cache: %v", err)
+					}
+					if len(result.Items) != tc.expectObjectNum {
+						b.Fatalf("expect %d but got %d", tc.expectObjectNum, len(result.Items))
+					}
+				}
+			})
+	}
+}