change adaptive way to EMA

CHANGELOG.md

@@ -3,7 +3,7 @@
 ## master / unreleased
 * [FEATURE] Distributor: Add experimental `-distributor.enable-start-timestamp` flag for Prometheus Remote Write 2.0. When enabled, `StartTimestamp (ST)` is ingested. #7371
 * [FEATURE] Memberlist: Add `-memberlist.cluster-label` and `-memberlist.cluster-label-verification-disabled` to prevent accidental cross-cluster gossip joins and support rolling label rollout. #7385
-* [ENHANCEMENT] Distributor: Introduce dynamic `Symbols` slice capacity pooling. #7398
+* [ENHANCEMENT] Distributor: Introduce dynamic `Symbols` slice capacity pooling. #7398 #7401
 * [ENHANCEMENT] Metrics Helper: Add native histogram support for aggregating and merging, including dual-format histogram handling that exposes both native and classic bucket formats. #7359
 * [ENHANCEMENT] Cache: Add per-tenant TTL configuration for query results cache to control cache expiration on a per-tenant basis with separate TTLs for regular and out-of-order data. #7357
 * [CHANGE] Querier: Make query time range configurations per-tenant: `query_ingesters_within`, `query_store_after`, and `shuffle_sharding_ingesters_lookback_period`. Uses `model.Duration` instead of `time.Duration` to support serialization but has minimum unit of 1ms (nanoseconds/microseconds not supported). #7160

pkg/cortexpb/timeseriesv2.go

@@ -102,11 +102,15 @@ func ReuseWriteRequestV2(req *PreallocWriteRequestV2) {
 	// Update the dynamic symbol capacity.
 	for {
 		current := dynamicSymbolsCapacity.Load()
-		if symbolsCap <= current {
-			// break when other goroutines have already updated the capacity to a larger value
+		// We use an EMA to update the capacity.
+		newAvg := max((current*9+symbolsCap*1)/10, int64(initialSymbolsCapacity))
+
+		if current == newAvg {
+			// nothing to change
 			break
 		}
-		if dynamicSymbolsCapacity.CompareAndSwap(current, symbolsCap) {
+
+		if dynamicSymbolsCapacity.CompareAndSwap(current, newAvg) {
 			break
 		}
 	}

pkg/cortexpb/timeseriesv2_test.go

@@ -100,8 +100,9 @@ func TestReuseWriteRequestV2(t *testing.T) {
 
 		ReuseWriteRequestV2(req)
 
-		// Verify that the dynamic capacity has scaled up
-		assert.Equal(t, int64(newCap), dynamicSymbolsCapacity.Load())
+		// Verify that the dynamic capacity has been updated
+		expectedCap := max((currentCap*9+int64(newCap))/10, int64(initialSymbolsCapacity))
+		assert.Equal(t, expectedCap, dynamicSymbolsCapacity.Load())
 	})
 	t.Run("outlier capacity does not update dynamic capacity and is discarded", func(t *testing.T) {
 		currentCap := dynamicSymbolsCapacity.Load()