Add watch cache capacity upper bound adjusting logic

Author: tosi3k

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

@@ -169,7 +169,7 @@ func newWatchCache(
 		getAttrsFunc:        getAttrsFunc,
 		cache:               make([]*watchCacheEvent, defaultLowerBoundCapacity),
 		lowerBoundCapacity:  defaultLowerBoundCapacity,
-		upperBoundCapacity:  defaultUpperBoundCapacity,
+		upperBoundCapacity:  capacityUpperBound(eventFreshDuration),
 		startIndex:          0,
 		endIndex:            0,
 		store:               newStoreIndexer(indexers),
@@ -189,6 +189,30 @@ func newWatchCache(
 	return wc
 }
 
+// capacityUpperBound denotes the maximum possible capacity of the watch cache
+// to which it can resize.
+func capacityUpperBound(eventFreshDuration time.Duration) int {
+	if eventFreshDuration <= DefaultEventFreshDuration {
+		return defaultUpperBoundCapacity
+	}
+	// eventFreshDuration determines how long the watch events are supposed
+	// to be stored in the watch cache.
+	// In very high churn situations, there is a need to store more events
+	// in the watch cache, hence it would have to be upsized accordingly.
+	// Because of that, for larger values of eventFreshDuration, we set the
+	// upper bound of the watch cache's capacity proportionally to the ratio
+	// between eventFreshDuration and DefaultEventFreshDuration.
+	// Given that the watch cache size can only double, we round up that
+	// proportion to the next power of two.
+	exponent := int(math.Ceil((math.Log2(eventFreshDuration.Seconds() / DefaultEventFreshDuration.Seconds()))))
+	if maxExponent := int(math.Floor((math.Log2(math.MaxInt32 / defaultUpperBoundCapacity)))); exponent > maxExponent {
+		// Making sure that the capacity's upper bound fits in a 32-bit integer.
+		exponent = maxExponent
+		klog.Warningf("Capping watch cache capacity upper bound to %v", defaultUpperBoundCapacity<<exponent)
+	}
+	return defaultUpperBoundCapacity << exponent
+}
+
 // Add takes runtime.Object as an argument.
 func (w *watchCache) Add(obj interface{}) error {
 	object, resourceVersion, err := w.objectToVersionedRuntimeObject(obj)

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

@@ -1163,6 +1163,49 @@ func TestSuggestedWatchChannelSize(t *testing.T) {
 	}
 }
 
+func TestCapacityUpperBound(t *testing.T) {
+	testCases := []struct {
+		name               string
+		eventFreshDuration time.Duration
+		expected           int
+	}{
+		{
+			name:               "default eventFreshDuration",
+			eventFreshDuration: DefaultEventFreshDuration, // 75s
+			expected:           defaultUpperBoundCapacity, // 100 * 1024
+		},
+		{
+			name:               "lower eventFreshDuration, capacity limit unchanged",
+			eventFreshDuration: 45 * time.Second,          // 45s
+			expected:           defaultUpperBoundCapacity, // 100 * 1024
+		},
+		{
+			name:               "higher eventFreshDuration, capacity limit scaled up",
+			eventFreshDuration: 4 * DefaultEventFreshDuration, // 4 * 75s
+			expected:           4 * defaultUpperBoundCapacity, // 4 * 100 * 1024
+		},
+		{
+			name:               "higher eventFreshDuration, capacity limit scaled and rounded up",
+			eventFreshDuration: 3 * DefaultEventFreshDuration, // 3 * 75s
+			expected:           4 * defaultUpperBoundCapacity, // 4 * 100 * 1024
+		},
+		{
+			name:               "higher eventFreshDuration, capacity limit scaled up and capped",
+			eventFreshDuration: DefaultEventFreshDuration << 20, // 2^20 * 75s
+			expected:           defaultUpperBoundCapacity << 14, // 2^14 * 100 * 1024
+		},
+	}
+
+	for _, test := range testCases {
+		t.Run(test.name, func(t *testing.T) {
+			capacity := capacityUpperBound(test.eventFreshDuration)
+			if test.expected != capacity {
+				t.Errorf("expected %v, got %v", test.expected, capacity)
+			}
+		})
+	}
+}
+
 func BenchmarkWatchCache_updateCache(b *testing.B) {
 	store := newTestWatchCache(defaultUpperBoundCapacity, DefaultEventFreshDuration, &cache.Indexers{})
 	defer store.Stop()