[External] : Re: Virtual thread memory leak because TrackingRootContainer keeps threads

[Michal Domagala]

Thanks for the explanation.

I understand that paragraph

"Unlike platform thread stacks, virtual thread stacks are not GC roots.
Thus the references they contain are not traversed in a stop-the-world
pause by garbage collectors, such as G1, that perform concurrent heap
scanning"

can be rewritten as

"Some GC, such as G1, marks GC roots in stop-the-world pause. Unlike
platform thread stacks, virtual thread stacks are not GC roots, therefore
they do not impact stop-the-world pause."

In my opinion, the current paragraph in JEP 444 requires readers to have a
deep GC background. Usually, developers are not aware of GC root cost (at
least I was not aware). Developers could tune the number of GC roots by
changing the number of platform threads. Others, like static variables, are
rather not tunable.
But usually, the OS limits the number of platform threads much more
strictly than GC performance.

To sum up, JEP 444's message is: "Do not be afraid of the G1 initial mark
phase when using virtual threads". But I think most developers, like me,
never heard about it. Ohers, more advanced, could also never care about it,
because Oracle docs says about the initial mark phase: "This phase is
piggybacked on a normal (STW) young garbage collection.". I understand this
sentence as the phase is "for free".

To sum up again: when a developer like me reads that VT is not GC root, he
does not see G1 profit behind. He reads: VT is GC'able. And the current
state, when behavior is different, is misleading.

sob., 13 lip 2024 o 14:57 Ron Pressler <ron.pressler at oracle.com> napisał(a):

>
>
> > On 11 Jul 2024, at 19:40, Michal Domagala <outsider404 at gmail.com> wrote:
> >
> > Where is the mistake in my reasoning?
> >
>
> All objects in the heap, including virtual thread stacks, are traversed by
> the GC. GC *roots*, however, are things that some GCs, G1 in particular,
> traverse *in a stop-the-world-pause* while they don’t walk non-root objects
> in a STW pause but concurrently with the application running. Furthermore,
> roots are kept in memory even if there are no strong references to them,
> while normal objects are kept only if there are strong references to them.
>
> Virtual threads, unlike platform threads, are regular heap objects rather
> than roots. This makes a difference to *when* they are scanned by the GC,
> which has an impact on GC performance, especially as there can be lots and
> lots of virtual threads. Additionally, *if* there is no strong reference to
> the virtual thread, the GC may collect it, but I pointed out that usually
> there is a strong reference that is kept by default for observability
> purposes, but you can change that option so that such a reference is not
> kept.
>
> — Ron
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/loom-dev/attachments/20240722/2d1598af/attachment.htm>