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Third blog post from the virtual thread series (test)
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| layout: post | ||
| title: 'Testing virtual thread applications' | ||
| date: 2023-10-02 | ||
| tags: virtual threads, reactive, test | ||
| synopsis: 'How to detect pinning while running tests.' | ||
| author: cescoffier | ||
| --- | ||
| :imagesdir: /assets/images/posts/virtual-threads | ||
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| https://quarkus.io/blog/virtual-threads-2/[In a previous post], we have seen how to implement a CRUD application using virtual threads in Quarkus. | ||
| The following video shows how to test this application and, specifically, how to detect pinning. | ||
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| <iframestyle="margin-left: auto; margin-right: auto; display: block;" width="560" height="315" src="https://www.youtube.com/embed/GOHAEh3Ujh8?si=t8n8jLek0X7JC4MR" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe> | ||
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| The complete code of the application and the tests are available in the https://github.com/quarkusio/virtual-threads-demos/tree/main/crud-example[virtual threads demos repository]. | ||
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| ## Pinning and Monopolization | ||
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| Virtual threads combine an imperative development model with a reactive execution mode. | ||
| It may provide a simple way to increase the concurrency of an application. | ||
| However, this might not always be the case. | ||
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| As described in https://quarkus.io/blog/virtual-thread-1/[another blog post], there are a few limitations, including monopolizing and pinning carrier threads. | ||
| When this happens, the application's performance can drastically decrease and increase memory usage. | ||
| Pinning for a short period can be tolerated, but it can be dramatic under load. | ||
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| While, at the moment, there are no reliable ways to detect monopolization, there are mechanisms to detect pinning. | ||
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| ## Printing stack traces when a carrier thread gets pinned | ||
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| Suppose you have your application, and your code base contains tests. | ||
| You can configure Surefire (or the plugin you use to execute your tests) to dump a stack trace as soon as a virtual thread is going to pin the carrier thread (instead of being unmounted smoothly). | ||
| You must set the `jdk.tracePinnedThreads` system property to achieve this. | ||
| For the Surefire Maven plugin, add the `argLine` parameter to the configuration: | ||
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| [source, xml] | ||
| ---- | ||
| <argLine>-Djdk.tracePinnedThreads</argLine> | ||
| ---- | ||
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| With this configuration, when, while running your test, a carrier thread is pinned, the stack trace is dumped in the console: | ||
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| [source,text] | ||
| ---- | ||
| 2023-09-15 07:51:18,312 INFO [org.acm.cru.TodoResource] (quarkus-virtual-thread-0) Called on VirtualThread[#140,quarkus-virtual-thread-0]/runnable@ForkJoinPool-1-worker-1 | ||
| Thread[#141,ForkJoinPool-1-worker-1,5,CarrierThreads] | ||
| java.base/java.lang.VirtualThread$VThreadContinuation.onPinned(VirtualThread.java:185) | ||
| java.base/jdk.internal.vm.Continuation.onPinned0(Continuation.java:393) | ||
| java.base/java.lang.VirtualThread.parkNanos(VirtualThread.java:631) | ||
| java.base/java.lang.VirtualThread.sleepNanos(VirtualThread.java:803) | ||
| java.base/java.lang.Thread.sleep(Thread.java:507) | ||
| org.acme.crud.TodoResource.pinTheCarrierThread(TodoResource.java:93) <== monitors:1 | ||
| org.acme.crud.TodoResource.getAll(TodoResource.java:32) | ||
| org.acme.crud.TodoResource$quarkusrestinvoker$getAll_0e9c86666ef01bafda5560c4bf86952c6cf09993.invoke(Unknown Source) | ||
| org.jboss.resteasy.reactive.server.handlers.InvocationHandler.handle(InvocationHandler.java:29) | ||
| io.quarkus.resteasy.reactive.server.runtime.QuarkusResteasyReactiveRequestContext.invokeHandler(QuarkusResteasyReactiveRequestContext.java:141) | ||
| org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.run(AbstractResteasyReactiveContext.java:147) | ||
| io.quarkus.virtual.threads.ContextPreservingExecutorService$1.run(ContextPreservingExecutorService.java:36) | ||
| java.base/java.util.concurrent.ThreadPerTaskExecutor$TaskRunner.run(ThreadPerTaskExecutor.java:314) | ||
| java.base/java.lang.VirtualThread.run(VirtualThread.java:311) | ||
| ---- | ||
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| Analyzing the application logs will tell you whether your application is pinning. | ||
| Furthermore, a closer look at the stack trace will give you the reason. | ||
| In our example, the `pinTheCarrierThread` method is taking a lock. | ||
| This is indicated by the `monitors:1` text: | ||
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| [source, text] | ||
| ---- | ||
| org.acme.crud.TodoResource.pinTheCarrierThread(TodoResource.java:93) <== monitors:1 | ||
| ---- | ||
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| This approach can also be used in production (so, not only in tests). | ||
| You can also determine how long the carrier thread was blocked by correlating the pinned stack trace with other log events (like what happened just after in the same thread). | ||
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| ## Failing tests | ||
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| Dumping the stack trace may not be very convenient when your logs are already long. | ||
| Fortunately, we released a small Junit 5 extension that allows you to fail the tests when pinning is detected. | ||
| It's advantageous when you integrate a third-party library, and you need to know how virtual-thread-friendly it is (to decide between regular worker threads and virtual threads) | ||
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| The loom-unit Junit5 extension is currently a separated project. | ||
| We are integrating it into the Quarkus test framework (under the `junit5-virtual-threads` name), so some of the steps mentioned below won't be necessary anymore or will be changed slightly. | ||
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| To use this extension, make sure you have the loom-unit extension in your project: | ||
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| [source, xml] | ||
| ---- | ||
| <dependency> | ||
| <groupId>me.escoffier.loom</groupId> <!-- Will become io.quarkus.junit5 --> | ||
| <artifactId>loom-unit</artifactId> <!-- Will become junit5-virtual-threads --> | ||
| <version>0.3.0</version> <!-- Will become unnecessary --> | ||
| <scope>test</scope> | ||
| </dependency> | ||
| ---- | ||
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| Then, in your test, use `@ExtendWith` to enable the extension: | ||
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| [source, java] | ||
| ---- | ||
| @QuarkusTest | ||
| @TestMethodOrder(MethodOrderer.OrderAnnotation.class) | ||
| @ExtendWith(LoomUnitExtension.class) // <--- Enable the extension (will become @VirtualThreadUnit) | ||
| @ShouldNotPin // <--- Detect pinning | ||
| class TodoResourceTest { | ||
| // ... | ||
| } | ||
| ---- | ||
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| Finally, use the `@ShouldNotPin` annotation to indicate to fail the test if any of the methods of the test case pins the carrier thread. | ||
| You can also use the `@ShouldNotPin` annotation on methods. | ||
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| If, during the execution of a test, a pinning event is captured, the test fails. | ||
| The stack trace of the event is attached to the test failure: | ||
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| [source, text] | ||
| ---- | ||
| java.lang.AssertionError: The test testInitialItems() was expected to NOT pin the carrier thread, but we collected 1 event(s) | ||
| * Pinning event captured: | ||
| java.lang.VirtualThread.parkOnCarrierThread(java.lang.VirtualThread.java:687) | ||
| java.lang.VirtualThread.parkNanos(java.lang.VirtualThread.java:646) | ||
| java.lang.VirtualThread.sleepNanos(java.lang.VirtualThread.java:803) | ||
| java.lang.Thread.sleep(java.lang.Thread.java:507) | ||
| org.acme.crud.TodoResource.pinTheCarrierThread(org.acme.crud.TodoResource.java:93) | ||
| org.acme.crud.TodoResource.getAll(org.acme.crud.TodoResource.java:32) | ||
| org.acme.crud.TodoResource$quarkusrestinvoker$getAll_0e9c86666ef01bafda5560c4bf86952c6cf09993.invoke(org.acme.crud.TodoResource$quarkusrestinvoker$getAll_0e9c86666ef01bafda5560c4bf86952c6cf09993.java:-1) | ||
| org.jboss.resteasy.reactive.server.handlers.InvocationHandler.handle(org.jboss.resteasy.reactive.server.handlers.InvocationHandler.java:29) | ||
| io.quarkus.resteasy.reactive.server.runtime.QuarkusResteasyReactiveRequestContext.invokeHandler(io.quarkus.resteasy.reactive.server.runtime.QuarkusResteasyReactiveRequestContext.java:141) | ||
| org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.run(org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.java:147) | ||
| io.quarkus.virtual.threads.ContextPreservingExecutorService$1.run(io.quarkus.virtual.threads.ContextPreservingExecutorService$1.java:36) | ||
| java.util.concurrent.ThreadPerTaskExecutor$TaskRunner.run(java.util.concurrent.ThreadPerTaskExecutor$TaskRunner.java:314) | ||
| java.lang.VirtualThread.runWith(java.lang.VirtualThread.java:341) | ||
| java.lang.VirtualThread.run(java.lang.VirtualThread.java:311) | ||
| java.lang.VirtualThread$VThreadContinuation$1.run(java.lang.VirtualThread$VThreadContinuation$1.java:192) | ||
| jdk.internal.vm.Continuation.enter0(jdk.internal.vm.Continuation.java:320) | ||
| jdk.internal.vm.Continuation.enter(jdk.internal.vm.Continuation.java:312) | ||
| jdk.internal.vm.Continuation.enterSpecial(jdk.internal.vm.Continuation.java:-1) | ||
| ---- | ||
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| Find more about the loom-unit extension on https://github.com/cescoffier/loom-unit[the project repository] or its https://github.com/quarkusio/quarkus/tree/main/independent-projects/junit5-virtual-threads[Quarkus sibling]. | ||
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| ## Summary | ||
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| This blog explains how you can detect pinning events while running your tests. | ||
| First, you can dump the stack trace in the log. | ||
| Second, you can use the `@ShouldNotPin` annotation to fail the tests if a pinning event is captured. | ||
| Thanks to this https://github.com/quarkusio/quarkus/pull/35992[PR], the loom-unit extension will be integrated into the `@QuarkusTest` to provide a simpler developer experience. | ||
| It will be part of Quarkus in the next release (3.5.x). |