Techniques for debugging threading issues with Virtual Threads

[Dr Heinz M. Kabutz]

With platform threads, we have had good techniques for finding threading 
issues. For example, a thread dump would reveal deadlocks, livelocks and 
contention.

However, with virtual threads, most of these techniques won't work 
anymore. For example, here is a SimpleDeadlock:

import java.util.concurrent.locks.*;

public class SimpleDeadlock {
     public static void main(String... args) throws InterruptedException {
         var monitor1 = new Object();
         var monitor2 = new Object();
         Thread.startVirtualThread(() -> {
             synchronized (monitor1) {
                 LockSupport.parkNanos(10_000_000);
                 synchronized (monitor2) {
                     System.out.println("Got both locks");
                 }
             }
         });
         Thread.startVirtualThread(() -> {
             synchronized (monitor2) {
                 LockSupport.parkNanos(10_000_000);
                 synchronized (monitor1) {
                     System.out.println("Got both locks");
                 }
             }
         }).join();
     }
}

If we run this with Java 21-23 and -Djdk.trackAllThreads=false, the 
carrier thread indicate that they are carrying some virtual threads, but 
these do not appear in the full dump. In Java 24+24, the virtual threads 
are parked instead of pinned, and thus they vanish completely from the 
thread dumps.

Is there any project or workgroup where debugging of virtual threads is 
being looked at?

Regards

Heinz
-- 
Dr Heinz M. Kabutz (PhD CompSci)
Author of "The Java™ Specialists' Newsletter" - www.javaspecialists.eu
Java Champion - www.javachampions.org
JavaOne Rock Star Speaker
Tel: +30 69 75 595 262
Skype: kabutz