Experiment Concurrent Pools in HTTP/2 Connection Pool

reactor-netty-core/src/main/java/reactor/netty/Metrics.java

@@ -204,6 +204,10 @@ public class Metrics {
 	 */
 	public static final String PENDING_STREAMS = ".pending.streams";
 
+	/**
+	 * The number of HTTP/2 stream acquisitions steal count.
+	 */
+	public static final String STEAL_STREAMS = ".steal.streams";
 
 	// ByteBufAllocator Metrics
 	/**

reactor-netty-core/src/main/java/reactor/netty/resources/PooledConnectionProvider.java

@@ -527,6 +527,20 @@ public InstrumentedPool<T> newPool(
 			return newPoolInternal(allocator, destroyHandler, defaultEvictionPredicate).build(poolFactory);
 		}
 
+		public InstrumentedPool<T> newPool(
+		        PoolBuilder<T, PoolConfig<T>> poolBuilder,
+				int maxConnections,
+				@Nullable AllocationStrategy<?> allocationStrategy,
+				Function<T, Publisher<Void>> destroyHandler,
+				BiPredicate<T, PooledRefMetadata> defaultEvictionPredicate,
+				Function<PoolConfig<T>, InstrumentedPool<T>> poolFactory) {
+			if (disposeTimeout != null) {
+				return newPoolInternal(poolBuilder, maxConnections, allocationStrategy, destroyHandler, defaultEvictionPredicate, null)
+					.build(poolFactory.andThen(InstrumentedPoolDecorators::gracefulShutdown));
+			}
+			return newPoolInternal(poolBuilder, maxConnections, allocationStrategy, destroyHandler, defaultEvictionPredicate, null).build(poolFactory);
+		}
+
 		public InstrumentedPool<T> newPool(
 				Publisher<T> allocator,
 				Function<T, Publisher<Void>> destroyHandler,
@@ -540,6 +554,21 @@ public InstrumentedPool<T> newPool(
 			return newPoolInternal(allocator, destroyHandler, defaultEvictionPredicate, poolMetricsRecorder).build(poolFactory);
 		}
 
+		public InstrumentedPool<T> newPool(
+				PoolBuilder<T, PoolConfig<T>> poolBuilder,
+				int maxConnections,
+				@Nullable AllocationStrategy<?> allocationStrategy,
+				Function<T, Publisher<Void>> destroyHandler,
+				BiPredicate<T, PooledRefMetadata> defaultEvictionPredicate,
+				PoolMetricsRecorder poolMetricsRecorder,
+				Function<PoolConfig<T>, InstrumentedPool<T>> poolFactory) {
+			if (disposeTimeout != null) {
+				return newPoolInternal(poolBuilder, maxConnections, allocationStrategy, destroyHandler, defaultEvictionPredicate, poolMetricsRecorder)
+						.build(poolFactory.andThen(InstrumentedPoolDecorators::gracefulShutdown));
+			}
+			return newPoolInternal(poolBuilder, maxConnections, allocationStrategy, destroyHandler, defaultEvictionPredicate, poolMetricsRecorder).build(poolFactory);
+		}
+
 		PoolBuilder<T, PoolConfig<T>> newPoolInternal(
 				Publisher<T> allocator,
 				Function<T, Publisher<Void>> destroyHandler,
@@ -552,11 +581,22 @@ PoolBuilder<T, PoolConfig<T>> newPoolInternal(
 				Function<T, Publisher<Void>> destroyHandler,
 				BiPredicate<T, PooledRefMetadata> defaultEvictionPredicate,
 				@Nullable PoolMetricsRecorder poolMetricsRecorder) {
-			PoolBuilder<T, PoolConfig<T>> poolBuilder =
-					PoolBuilder.from(allocator)
-					           .destroyHandler(destroyHandler)
-					           .maxPendingAcquire(pendingAcquireMaxCount)
-					           .evictInBackground(evictionInterval);
+			return newPoolInternal(PoolBuilder.from(allocator), -1, null, destroyHandler, defaultEvictionPredicate, poolMetricsRecorder);
+		}
+
+		PoolBuilder<T, PoolConfig<T>> newPoolInternal(
+				PoolBuilder<T, PoolConfig<T>> poolBuilder,
+				int maxConnections,
+				@Nullable AllocationStrategy<?> allocationStrategy,
+				Function<T, Publisher<Void>> destroyHandler,
+				BiPredicate<T, PooledRefMetadata> defaultEvictionPredicate,
+				@Nullable PoolMetricsRecorder poolMetricsRecorder) {
+			maxConnections = (maxConnections == -1) ? this.maxConnections : maxConnections;
+			allocationStrategy = (allocationStrategy == null) ? this.allocationStrategy : allocationStrategy;
+			poolBuilder = poolBuilder
+							.destroyHandler(destroyHandler)
+							.maxPendingAcquire(pendingAcquireMaxCount)
+							.evictInBackground(evictionInterval);
 
 			if (this.evictionPredicate != null) {
 				poolBuilder = poolBuilder.evictionPredicate(

reactor-netty-http/src/main/java/reactor/netty/http/client/Http2AllocationStrategy.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022-2023 VMware, Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2022-2024 VMware, Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -66,6 +66,23 @@ public interface Builder {
 		 * @return {@code this}
 		 */
 		Builder minConnections(int minConnections);
+
+		/**
+		 * Enables or disables work stealing mode for managing HTTP2 Connection Pools.
+		 * <p>
+		 * By default, a single Connection Pool is used by multiple Netty event loop threads.
+		 * When work stealing is enabled, each Netty event loop will maintain its own
+		 * HTTP2 Connection Pool, and HTTP2 streams allocation will be distributed over all available
+		 * pools using a work stealing strategy. This approach maximizes throughput and
+		 * resource utilization in a multithreaded environment.
+		 *
+		 * @param progressive true if the HTTP2 Connection pools should be enabled gradually (when the nth pool becomes
+		 *                    is starting to get some pendingg acquisitions request, then enable one more
+		 *                    pool until all available pools are enabled).
+		 *
+		 * @return {@code this}
+		 */
+		Builder enableWorkStealing(boolean progressive);
 	}
 
 	/**
@@ -77,6 +94,18 @@ public static Http2AllocationStrategy.Builder builder() {
 		return new Http2AllocationStrategy.Build();
 	}
 
+	/**
+	 * Creates a builder for {@link Http2AllocationStrategy} and initialize it
+	 * with an existing strategy. This method can be used to create a mutated version
+	 * of an existing strategy.
+	 *
+	 * @return a new {@link Http2AllocationStrategy.Builder} initialized with an existing http2
+	 * allocation strategy.
+	 */
+	public static Http2AllocationStrategy.Builder builder(Http2AllocationStrategy existing) {
+		return new Http2AllocationStrategy.Build(existing);
+	}
+
 	@Override
 	public Http2AllocationStrategy copy() {
 		return new Http2AllocationStrategy(this);
@@ -141,9 +170,14 @@ public void returnPermits(int returned) {
 		}
 	}
 
+	public boolean enableWorkStealing() {
+		return enableWorkStealing;
+	}
+
 	final long maxConcurrentStreams;
 	final int maxConnections;
 	final int minConnections;
+	final boolean enableWorkStealing;
 
 	volatile int permits;
 	static final AtomicIntegerFieldUpdater<Http2AllocationStrategy> PERMITS = AtomicIntegerFieldUpdater.newUpdater(Http2AllocationStrategy.class, "permits");
@@ -152,13 +186,15 @@ public void returnPermits(int returned) {
 		this.maxConcurrentStreams = build.maxConcurrentStreams;
 		this.maxConnections = build.maxConnections;
 		this.minConnections = build.minConnections;
+		this.enableWorkStealing = build.enableWorkStealing;
 		PERMITS.lazySet(this, this.maxConnections);
 	}
 
 	Http2AllocationStrategy(Http2AllocationStrategy copy) {
 		this.maxConcurrentStreams = copy.maxConcurrentStreams;
 		this.maxConnections = copy.maxConnections;
 		this.minConnections = copy.minConnections;
+		this.enableWorkStealing = copy.enableWorkStealing;
 		PERMITS.lazySet(this, this.maxConnections);
 	}
 
@@ -170,6 +206,17 @@ static final class Build implements Builder {
 		long maxConcurrentStreams = DEFAULT_MAX_CONCURRENT_STREAMS;
 		int maxConnections = DEFAULT_MAX_CONNECTIONS;
 		int minConnections = DEFAULT_MIN_CONNECTIONS;
+		boolean enableWorkStealing = Boolean.getBoolean("reactor.netty.pool.h2.enableworkstealing");
+
+		Build() {
+		}
+
+		Build(Http2AllocationStrategy existing) {
+			this.maxConcurrentStreams = existing.maxConcurrentStreams;
+			this.minConnections = existing.minConnections;
+			this.maxConnections = existing.maxConnections;
+			this.enableWorkStealing = existing.enableWorkStealing;
+		}
 
 		@Override
 		public Http2AllocationStrategy build() {
@@ -206,5 +253,11 @@ public Builder minConnections(int minConnections) {
 			this.minConnections = minConnections;
 			return this;
 		}
+
+		@Override
+		public Builder enableWorkStealing(boolean progressive) {
+			this.enableWorkStealing = true;
+			return this;
+		}
 	}
 }

reactor-netty-http/src/main/java/reactor/netty/http/client/Http2ConnectionProvider.java

@@ -39,6 +39,9 @@
 import reactor.netty.ConnectionObserver;
 import reactor.netty.channel.ChannelMetricsRecorder;
 import reactor.netty.channel.ChannelOperations;
+import reactor.netty.internal.shaded.reactor.pool.PoolBuilder;
+import reactor.netty.internal.shaded.reactor.pool.PoolConfig;
+import reactor.netty.internal.shaded.reactor.pool.decorators.InstrumentedPoolDecorators;
 import reactor.netty.resources.ConnectionProvider;
 import reactor.netty.resources.PooledConnectionProvider;
 import reactor.netty.transport.ClientTransportConfig;
@@ -51,13 +54,20 @@
 import reactor.util.annotation.Nullable;
 import reactor.util.concurrent.Queues;
 import reactor.util.context.Context;
+import reactor.util.function.Tuples;
 
 import java.io.IOException;
 import java.net.SocketAddress;
 import java.time.Duration;
+import java.util.Iterator;
+import java.util.List;
 import java.util.Queue;
+import java.util.concurrent.Executor;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.BiPredicate;
 import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.StreamSupport;
 
 import static reactor.netty.ReactorNetty.format;
 import static reactor.netty.ReactorNetty.getChannelContext;
@@ -565,12 +575,56 @@ static final class PooledConnectionAllocator {
 			this.config = (HttpClientConfig) config;
 			this.remoteAddress = remoteAddress;
 			this.resolver = resolver;
-			this.pool = id == null ?
-					poolFactory.newPool(connectChannel(), null, DEFAULT_DESTROY_HANDLER, DEFAULT_EVICTION_PREDICATE,
-							poolConfig -> new Http2Pool(poolConfig, poolFactory.allocationStrategy())) :
-					poolFactory.newPool(connectChannel(), DEFAULT_DESTROY_HANDLER, DEFAULT_EVICTION_PREDICATE,
-							new MicrometerPoolMetricsRecorder(id, name, remoteAddress),
-							poolConfig -> new Http2Pool(poolConfig, poolFactory.allocationStrategy()));
+
+			Http2AllocationStrategy http2Strategy = poolFactory.allocationStrategy() instanceof Http2AllocationStrategy ?
+					(Http2AllocationStrategy) poolFactory.allocationStrategy() : null;
+
+			if (http2Strategy == null || !http2Strategy.enableWorkStealing) {
+				this.pool = id == null ?
+						poolFactory.newPool(connectChannel(), null, DEFAULT_DESTROY_HANDLER, DEFAULT_EVICTION_PREDICATE,
+								poolConfig -> new Http2Pool(poolConfig, poolFactory.allocationStrategy())) :
+						poolFactory.newPool(connectChannel(), DEFAULT_DESTROY_HANDLER, DEFAULT_EVICTION_PREDICATE,
+								new MicrometerPoolMetricsRecorder(id, name, remoteAddress),
+								poolConfig -> new Http2Pool(poolConfig, poolFactory.allocationStrategy()));
+			}
+			else {
+				// Create one connection allocator (it will be shared by all Http2Pool instances)
+				Publisher<Connection> allocator = connectChannel();
+
+				List<Executor> execs =  StreamSupport.stream(config.loopResources().onClient(true).spliterator(), false)
+						.limit(http2Strategy.maxConnections)
+						.collect(Collectors.toList());
+				Iterator<Executor> execsIter = execs.iterator();
+
+				MicrometerPoolMetricsRecorder micrometerRecorder = id == null ? null : new MicrometerPoolMetricsRecorder(id, name, remoteAddress);
+				AtomicInteger subPoolIndex = new AtomicInteger();
+
+				this.pool = InstrumentedPoolDecorators.concurrentPools(execs.size(), allocator,
+						(PoolBuilder<Connection, PoolConfig<Connection>> poolBuilder) -> {
+					int index = subPoolIndex.getAndIncrement();
+					int minDiv = http2Strategy.minConnections / execs.size();
+					int minMod = http2Strategy.minConnections % execs.size();
+					int maxDiv = http2Strategy.maxConnections / execs.size();
+					int maxMod = http2Strategy.maxConnections % execs.size();
+
+					int minConn = index < minMod ? minDiv + 1 : minDiv;
+					int maxConn = index < maxMod ? maxDiv + 1 : maxDiv;
+
+					Http2AllocationStrategy adaptedH2Strategy = Http2AllocationStrategy.builder(http2Strategy)
+							.minConnections(minConn)
+							.maxConnections(maxConn)
+							.build();
+
+					InstrumentedPool<Connection> pool =
+							id == null ?
+									poolFactory.newPool(poolBuilder, maxConn, adaptedH2Strategy, DEFAULT_DESTROY_HANDLER, DEFAULT_EVICTION_PREDICATE,
+											poolConfig -> new Http2Pool(poolConfig, adaptedH2Strategy)) :
+									poolFactory.newPool(poolBuilder, maxConn, adaptedH2Strategy, DEFAULT_DESTROY_HANDLER, DEFAULT_EVICTION_PREDICATE,
+											micrometerRecorder,
+											poolConfig -> new Http2Pool(poolConfig, adaptedH2Strategy));
+					return Tuples.of(pool, execsIter.next());
+				});
+			}
 		}
 
 		Publisher<Connection> connectChannel() {

reactor-netty-http/src/main/java/reactor/netty/http/client/Http2ConnectionProviderMeters.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022-2023 VMware, Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2022-2024 VMware, Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -67,6 +67,26 @@ public Meter.Type getType() {
 		}
 	},
 
+	/**
+	 * The number of HTTP/2 stream acquisition steal count.
+	 */
+	STEAL_STREAMS {
+		@Override
+		public String getName() {
+			return "reactor.netty.connection.provider.steal.streams";
+		}
+
+		@Override
+		public KeyName[] getKeyNames() {
+			return Http2ConnectionProviderMetersTags.values();
+		}
+
+		@Override
+		public Meter.Type getType() {
+			return Meter.Type.COUNTER;
+		}
+	},
+
 	/**
 	 * The number of the idle connections in the connection pool.
 	 */