Debugger overhead for virtual threads creation

[Chris Plummer]

The short answer is yes. The debug agent needs to deal with 
JVMTI_EVENT_VIRTUAL_THREAD_START/END events for every virtual thread. 
What makes it worse is when there are a large number of virtual threads 
that are currently alive. They are tracked on a list of ThreadNodes that 
starts to slow down debug agent performance when it gets too long. I 
have a work in progress that proactively purges these ThreadNodes so the 
list does not get too big. I've been meaning to revive this project for 
quite some time. If you have a test case I'd be willing to experiment 
with these changes some more. I could not access to the IDEA-365900 link 
you provided.

Note I think after the work is done to purge ThreadNodes proactively it 
might not be that hard of step to move to not needing 
JVMTI_EVENT_VIRTUAL_THREAD_START/END events enabled, which will help 
performance a lot more.

Chris

On 4/1/25 10:14 AM, Egor Ushakov wrote:
> Hi everyone!
>
> Is it expected that with the debugger attached creating virtual 
> threads is much slower?
> We're getting bugs like: https://youtrack.jetbrains.com/issue/IDEA-365900
> And I can reproduce it easily with jdb...
> Just attaching the debugger immediately slows down virtual threads 
> creation significantly.
>
> >java 
> -agentlib:jdwp=transport=dt_shmem,server=y,suspend=n,address=8000 app
> ...
> 6808805 (1.2046688E7 threads per second)
> ...
> after >jdb -attach 8000
> ...
> 30215 (95986.055 threads per second)
> ...
>
> Thanks,
> Egor