From b26a0ebb6a9b06eb050fa234299756b159d937f1 Mon Sep 17 00:00:00 2001
From: jnbdz <jn@yaloub.com>
Date: Fri, 14 Aug 2026 19:12:56 -0400
Subject: [PATCH] Assign a distinct event loop to each virtual thread verticle
 instance

Motivation:

Deploying a verticle with setInstances(N) and ThreadingModel.VIRTUAL_THREAD
pins every instance to the same event loop, so raising the instance count
does not scale I/O: the single event loop saturates while the virtual
threads starve. Vert.x 4.x assigned an event loop per instance via
eventLoopGroup.next().

The shared event loop was introduced for worker verticle deployments, where
the work happens on the worker pool and a single event loop consumes fewer
resources. The virtual thread model was changed at the same time, although
that rationale does not apply to it.

Changes:

Build a fresh context for each virtual thread instance so that each one
round-robins over the event loop group, like the event-loop model does.
The shared event loop remains for worker deployments.
---
 .../tests/virtualthread/DeploymentTest.java    | 18 ++++++++++++++++++
 .../impl/deployment/DefaultDeployment.java     | 14 +++-----------
 2 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/vertx-core-java21-tests/src/test/java/io/vertx/tests/virtualthread/DeploymentTest.java b/vertx-core-java21-tests/src/test/java/io/vertx/tests/virtualthread/DeploymentTest.java
index 01e857abc7e..7021da21bca 100644
--- a/vertx-core-java21-tests/src/test/java/io/vertx/tests/virtualthread/DeploymentTest.java
+++ b/vertx-core-java21-tests/src/test/java/io/vertx/tests/virtualthread/DeploymentTest.java
@@ -13,6 +13,7 @@
 import io.vertx.core.*;
 import io.vertx.core.buffer.Buffer;
 import io.vertx.core.http.*;
+import io.vertx.core.internal.ContextInternal;
 import io.vertx.test.core.VertxTestBase;
 import io.vertx.test.http.HttpTestBase;
 import org.junit.Assert;
@@ -21,6 +22,7 @@
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -43,6 +45,22 @@ public void start() {
     await();
   }
 
+  @Test
+  public void testInstancesUseDistinctEventLoopThreads() {
+    int instances = 4;
+    Vertx vertx = vertx(new VertxOptions().setEventLoopPoolSize(2 * instances));
+    Set<String> eventLoops = Collections.synchronizedSet(new HashSet<>());
+    vertx.deployVerticle(() -> new AbstractVerticle() {
+      @Override
+      public void start() {
+        CompletableFuture<String> latch = new CompletableFuture<>();
+        ((ContextInternal) context).eventLoop().execute(() -> latch.complete(Thread.currentThread().getName()));
+        eventLoops.add(latch.join());
+      }
+    }, new DeploymentOptions().setInstances(instances).setThreadingModel(ThreadingModel.VIRTUAL_THREAD)).await();
+    Assert.assertEquals(instances, eventLoops.size());
+  }
+
   @Test
   public void testExecuteBlocking() {
     Promise<Void> p = Promise.promise();
diff --git a/vertx-core/src/main/java/io/vertx/core/impl/deployment/DefaultDeployment.java b/vertx-core/src/main/java/io/vertx/core/impl/deployment/DefaultDeployment.java
index f0a9704a37b..ee8f615273f 100644
--- a/vertx-core/src/main/java/io/vertx/core/impl/deployment/DefaultDeployment.java
+++ b/vertx-core/src/main/java/io/vertx/core/impl/deployment/DefaultDeployment.java
@@ -152,17 +152,9 @@ public Future<?> deploy(DeploymentContext deployment) {
           }
           break;
         case VIRTUAL_THREAD:
-          if (workerLoop == null) {
-            context = contextBuilder
-              .withThreadingModel(ThreadingModel.VIRTUAL_THREAD)
-              .build();
-            workerLoop = context.nettyEventLoop();
-          } else {
-            context = contextBuilder
-              .withThreadingModel(ThreadingModel.VIRTUAL_THREAD)
-              .withEventLoop(workerLoop)
-              .build();
-          }
+          context = contextBuilder
+            .withThreadingModel(ThreadingModel.VIRTUAL_THREAD)
+            .build();
           break;
         default:
           context = contextBuilder