Significant degradation of Thread.sleep with virtual threads in JDK 25 vs JDK 21

[Fredriksson, Christian]

Hey,

I believe I have found a bug with JDK 25 and virtual threads, but will need some guidance in what information I need to provide in order to file a bug.

Short version: after changing from JDK 21 to JDK 25, doing Thread.sleep(1) in a virtual thread now sometimes takes up to 60 seconds (!).

Long version:

I recently updated an application from Java 21 to 25 and started seeing a very negative performance impact.

The application in question runs ca 20 long running virtual threads which basically does the following:

while (true) {
    doTrivialWork();
    Thread.sleep(1);
    semaphore.acquire();
}

I.e. do trivial work, sleep 1 ms, wait on a semaphore to be woken by another thread, and then loop again. After long investigation I finally realized that it was the Thread.sleep that sometimes took significantly longer with Java 25 than Java 21.
So I timed it, basically just this:

while (true) {
    doTrivialWork();
    var start = System.nanoTime();
    Thread.sleep(1);
    var end = System.nanoTime();
    System.out.println("delay=" + (end - start));
    semaphore.acquire();
}

And let it run >1 million times.

With JDK 21 and either virtual threads or platform threads, or with latest JDK 25 and platform threads, it works fine and the Thread.sleep(1) finishes according to the following:

99,5% finish within 2 ms
0,4% finish within 4 ms
Worst case 80 ms

But with latest JDK 25 and virtual threads, it’s significantly worse:

98,0% finish within 2 ms
0,3% finish within 4 ms
0,5% taking over 1000 ms
A few hundred occurrences over 60 seconds (!)

The only change between these runs is JDK version and platform vs virtual threads.

Increasing jdk.virtualThreadScheduler.parallelism and jdk.virtualThreadScheduler.timerQueues makes no difference.
There are no jdk.VirtualThreadPinned or jdk.VirtualThreadSubmitFailed events in JFR.

Additional information:
The application is running in a pod on k8s with 1 CPU allocated to it.
No additional arguments for the JDK.
CPU utilization is less than 15%.
GC happens very infrequently (many minutes apart).


[VOLVO CAR CORPORATION]<http://volvocars.com/>
Christian
Fredriksson

T +46 734 63 71 06
christian.fredriksson.2 at volvocars.com<mailto:christian.fredriksson.2 at volvocars.com>

VOLVO CAR CORPORATION
Connected Car Cloud
volvocars.com
Registered Office Göteborg, Sweden
Registration No. 556074-3089


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/loom-dev/attachments/20251121/5c1ba373/attachment-0001.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image001.gif
Type: image/gif
Size: 2421 bytes
Desc: image001.gif
URL: <https://mail.openjdk.org/pipermail/loom-dev/attachments/20251121/5c1ba373/image001-0001.gif>