Techniques for debugging threading issues with Virtual Threads

[Dr Heinz M. Kabutz]

We can discover deadlocks without a thread dump using the ThreadMXBean, 
but currently that does not work for virtual threads.

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

On 2024-11-20 09:12, Alex Otenko wrote:
>
> By the way, deadlocks specifically should be discoverable 
> automatically without a thread dump.
>
>
> On Wed, 20 Nov 2024, 07:10 Dr Heinz M. Kabutz, 
> <heinz at javaspecialists.eu> wrote:
>
>     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 <http://www.javaspecialists.eu>
>     Java Champion - www.javachampions.org <http://www.javachampions.org>
>     JavaOne Rock Star Speaker
>     Tel: +30 69 75 595 262
>     Skype: kabutz
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/loom-dev/attachments/20241120/b1eb28d4/attachment.htm>