HPA: only send updates when the status has changed

This commit only sends updates if the status has actually changed.
Since the HPA runs at a regular interval, this should reduce the volume
of writes, especially on short HPA intervals with relatively constant
metrics.

pkg/controller/podautoscaler/BUILD

@@ -32,6 +32,7 @@ go_library(
         "//pkg/controller:go_default_library",
         "//pkg/controller/podautoscaler/metrics:go_default_library",
         "//vendor/github.com/golang/glog:go_default_library",
+        "//vendor/k8s.io/apimachinery/pkg/api/equality:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/api/errors:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/api/resource:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/apis/meta/v1:go_default_library",

pkg/controller/podautoscaler/horizontal.go

@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"github.com/golang/glog"
+	apiequality "k8s.io/apimachinery/pkg/api/equality"
 	"k8s.io/apimachinery/pkg/api/errors"
 	"k8s.io/apimachinery/pkg/api/resource"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
@@ -371,14 +372,20 @@ func (a *HorizontalController) reconcileAutoscaler(hpav1Shared *autoscalingv1.Ho
 		return fmt.Errorf("failed to convert the given HPA to %s: %v", autoscalingv2.SchemeGroupVersion.String(), err)
 	}
 	hpa := hpaRaw.(*autoscalingv2.HorizontalPodAutoscaler)
+	hpaStatusOriginalRaw, err := api.Scheme.DeepCopy(&hpa.Status)
+	if err != nil {
+		a.eventRecorder.Event(hpav1Shared, v1.EventTypeWarning, "FailedConvertHPA", err.Error())
+		return fmt.Errorf("failed to deep-copy the HPA status: %v", err)
+	}
+	hpaStatusOriginal := hpaStatusOriginalRaw.(*autoscalingv2.HorizontalPodAutoscalerStatus)
 
 	reference := fmt.Sprintf("%s/%s/%s", hpa.Spec.ScaleTargetRef.Kind, hpa.Namespace, hpa.Spec.ScaleTargetRef.Name)
 
 	scale, err := a.scaleNamespacer.Scales(hpa.Namespace).Get(hpa.Spec.ScaleTargetRef.Kind, hpa.Spec.ScaleTargetRef.Name)
 	if err != nil {
 		a.eventRecorder.Event(hpa, v1.EventTypeWarning, "FailedGetScale", err.Error())
 		setCondition(hpa, autoscalingv2.AbleToScale, v1.ConditionFalse, "FailedGetScale", "the HPA controller was unable to get the target's current scale: %v", err)
-		a.update(hpa)
+		a.updateStatusIfNeeded(hpaStatusOriginal, hpa)
 		return fmt.Errorf("failed to query scale subresource for %s: %v", reference, err)
 	}
 	setCondition(hpa, autoscalingv2.AbleToScale, v1.ConditionTrue, "SucceededGetScale", "the HPA controller was able to get the target's current scale")
@@ -412,7 +419,10 @@ func (a *HorizontalController) reconcileAutoscaler(hpav1Shared *autoscalingv1.Ho
 	} else {
 		metricDesiredReplicas, metricName, metricStatuses, metricTimestamp, err = a.computeReplicasForMetrics(hpa, scale, hpa.Spec.Metrics)
 		if err != nil {
-			a.updateCurrentReplicasInStatus(hpa, currentReplicas)
+			a.setCurrentReplicasInStatus(hpa, currentReplicas)
+			if err := a.updateStatusIfNeeded(hpaStatusOriginal, hpa); err != nil {
+				utilruntime.HandleError(err)
+			}
 			a.eventRecorder.Event(hpa, v1.EventTypeWarning, "FailedComputeMetricsReplicas", err.Error())
 			return fmt.Errorf("failed to compute desired number of replicas based on listed metrics for %s: %v", reference, err)
 		}
@@ -489,7 +499,10 @@ func (a *HorizontalController) reconcileAutoscaler(hpav1Shared *autoscalingv1.Ho
 		if err != nil {
 			a.eventRecorder.Eventf(hpa, v1.EventTypeWarning, "FailedRescale", "New size: %d; reason: %s; error: %v", desiredReplicas, rescaleReason, err.Error())
 			setCondition(hpa, autoscalingv2.AbleToScale, v1.ConditionFalse, "FailedUpdateScale", "the HPA controller was unable to update the target scale: %v", err)
-			a.updateCurrentReplicasInStatus(hpa, currentReplicas)
+			a.setCurrentReplicasInStatus(hpa, currentReplicas)
+			if err := a.updateStatusIfNeeded(hpaStatusOriginal, hpa); err != nil {
+				utilruntime.HandleError(err)
+			}
 			return fmt.Errorf("failed to rescale %s: %v", reference, err)
 		}
 		setCondition(hpa, autoscalingv2.AbleToScale, v1.ConditionTrue, "SucceededRescale", "the HPA controller was able to update the target scale to %d", desiredReplicas)
@@ -501,7 +514,8 @@ func (a *HorizontalController) reconcileAutoscaler(hpav1Shared *autoscalingv1.Ho
 		desiredReplicas = currentReplicas
 	}
 
-	return a.updateStatusWithReplicas(hpa, currentReplicas, desiredReplicas, metricStatuses, rescale)
+	a.setStatus(hpa, currentReplicas, desiredReplicas, metricStatuses, rescale)
+	return a.updateStatusIfNeeded(hpaStatusOriginal, hpa)
 }
 
 func (a *HorizontalController) shouldScale(hpa *autoscalingv2.HorizontalPodAutoscaler, currentReplicas, desiredReplicas int32, timestamp time.Time) bool {
@@ -528,14 +542,14 @@ func (a *HorizontalController) shouldScale(hpa *autoscalingv2.HorizontalPodAutos
 	return false
 }
 
-func (a *HorizontalController) updateCurrentReplicasInStatus(hpa *autoscalingv2.HorizontalPodAutoscaler, currentReplicas int32) {
-	err := a.updateStatusWithReplicas(hpa, currentReplicas, hpa.Status.DesiredReplicas, hpa.Status.CurrentMetrics, false)
-	if err != nil {
-		utilruntime.HandleError(err)
-	}
+// setCurrentReplicasInStatus sets the current replica count in the status of the HPA.
+func (a *HorizontalController) setCurrentReplicasInStatus(hpa *autoscalingv2.HorizontalPodAutoscaler, currentReplicas int32) {
+	a.setStatus(hpa, currentReplicas, hpa.Status.DesiredReplicas, hpa.Status.CurrentMetrics, false)
 }
 
-func (a *HorizontalController) updateStatusWithReplicas(hpa *autoscalingv2.HorizontalPodAutoscaler, currentReplicas, desiredReplicas int32, metricStatuses []autoscalingv2.MetricStatus, rescale bool) error {
+// setStatus recreates the status of the given HPA, updating the current and
+// desired replicas, as well as the metric statuses
+func (a *HorizontalController) setStatus(hpa *autoscalingv2.HorizontalPodAutoscaler, currentReplicas, desiredReplicas int32, metricStatuses []autoscalingv2.MetricStatus, rescale bool) {
 	hpa.Status = autoscalingv2.HorizontalPodAutoscalerStatus{
 		CurrentReplicas: currentReplicas,
 		DesiredReplicas: desiredReplicas,
@@ -548,11 +562,19 @@ func (a *HorizontalController) updateStatusWithReplicas(hpa *autoscalingv2.Horiz
 		now := metav1.NewTime(time.Now())
 		hpa.Status.LastScaleTime = &now
 	}
+}
 
-	return a.update(hpa)
+// updateStatusIfNeeded calls updateStatus only if the status of the new HPA is not the same as the old status
+func (a *HorizontalController) updateStatusIfNeeded(oldStatus *autoscalingv2.HorizontalPodAutoscalerStatus, newHPA *autoscalingv2.HorizontalPodAutoscaler) error {
+	// skip a write if we wouldn't need to update
+	if apiequality.Semantic.DeepEqual(oldStatus, &newHPA.Status) {
+		return nil
+	}
+	return a.updateStatus(newHPA)
 }
 
-func (a *HorizontalController) update(hpa *autoscalingv2.HorizontalPodAutoscaler) error {
+// updateStatus actually does the update request for the status of the given HPA
+func (a *HorizontalController) updateStatus(hpa *autoscalingv2.HorizontalPodAutoscaler) error {
 	// convert back to autoscalingv1
 	hpaRaw, err := UnsafeConvertToVersionVia(hpa, autoscalingv1.SchemeGroupVersion)
 	if err != nil {

pkg/controller/podautoscaler/horizontal_test.go

@@ -538,7 +538,7 @@ func (tc *testCase) verifyResults(t *testing.T) {
 	}
 }
 
-func (tc *testCase) runTest(t *testing.T) {
+func (tc *testCase) setupController(t *testing.T) (*HorizontalController, informers.SharedInformerFactory) {
 	testClient, testMetricsClient, testCMClient := tc.prepareTestClient(t)
 	if tc.testClient != nil {
 		testClient = tc.testClient
@@ -598,6 +598,10 @@ func (tc *testCase) runTest(t *testing.T) {
 	)
 	hpaController.hpaListerSynced = alwaysReady
 
+	return hpaController, informerFactory
+}
+
+func (tc *testCase) runTestWithController(t *testing.T, hpaController *HorizontalController, informerFactory informers.SharedInformerFactory) {
 	stop := make(chan struct{})
 	defer close(stop)
 	informerFactory.Start(stop)
@@ -616,6 +620,11 @@ func (tc *testCase) runTest(t *testing.T) {
 	tc.verifyResults(t)
 }
 
+func (tc *testCase) runTest(t *testing.T) {
+	hpaController, informerFactory := tc.setupController(t)
+	tc.runTestWithController(t, hpaController, informerFactory)
+}
+
 func TestScaleUp(t *testing.T) {
 	tc := testCase{
 		minReplicas:         2,
@@ -1594,4 +1603,73 @@ func TestScaleDownRCImmediately(t *testing.T) {
 	tc.runTest(t)
 }
 
+func TestAvoidUncessaryUpdates(t *testing.T) {
+	tc := testCase{
+		minReplicas:          2,
+		maxReplicas:          6,
+		initialReplicas:      3,
+		desiredReplicas:      3,
+		CPUTarget:            30,
+		CPUCurrent:           40,
+		verifyCPUCurrent:     true,
+		reportedLevels:       []uint64{400, 500, 700},
+		reportedCPURequests:  []resource.Quantity{resource.MustParse("1.0"), resource.MustParse("1.0"), resource.MustParse("1.0")},
+		reportedPodReadiness: []v1.ConditionStatus{v1.ConditionTrue, v1.ConditionFalse, v1.ConditionFalse},
+		useMetricsApi:        true,
+	}
+	testClient, _, _ := tc.prepareTestClient(t)
+	tc.testClient = testClient
+	var savedHPA *autoscalingv1.HorizontalPodAutoscaler
+	testClient.PrependReactor("list", "horizontalpodautoscalers", func(action core.Action) (handled bool, ret runtime.Object, err error) {
+		tc.Lock()
+		defer tc.Unlock()
+
+		if savedHPA != nil {
+			// fake out the verification logic and mark that we're done processing
+			go func() {
+				// wait a tick and then mark that we're finished (otherwise, we have no
+				// way to indicate that we're finished, because the function decides not to do anything)
+				time.Sleep(1 * time.Second)
+				tc.statusUpdated = true
+				tc.processed <- "test-hpa"
+			}()
+			return true, &autoscalingv1.HorizontalPodAutoscalerList{
+				Items: []autoscalingv1.HorizontalPodAutoscaler{*savedHPA},
+			}, nil
+		}
+
+		// fallthrough
+		return false, nil, nil
+	})
+	testClient.PrependReactor("update", "horizontalpodautoscalers", func(action core.Action) (handled bool, ret runtime.Object, err error) {
+		tc.Lock()
+		defer tc.Unlock()
+
+		if savedHPA == nil {
+			// save the HPA and return it
+			savedHPA = action.(core.UpdateAction).GetObject().(*autoscalingv1.HorizontalPodAutoscaler)
+			return true, savedHPA, nil
+		}
+
+		assert.Fail(t, "should not have attempted to update the HPA when nothing changed")
+		// mark that we've processed this HPA
+		tc.processed <- ""
+		return true, nil, fmt.Errorf("unexpected call")
+	})
+
+	controller, informerFactory := tc.setupController(t)
+
+	// fake an initial processing loop to populate savedHPA
+	initialHPAs, err := testClient.Autoscaling().HorizontalPodAutoscalers("test-namespace").List(metav1.ListOptions{})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if err := controller.reconcileAutoscaler(&initialHPAs.Items[0]); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	// actually run the test
+	tc.runTestWithController(t, controller, informerFactory)
+}
+
 // TODO: add more tests