Stacks, safepoints, snapshotting and GC

[Stuart Monteith]

Hi Per,
   I'm curious as to how we'll implement this on Aarch64, the concern
being how this would be made efficient with a weak memory model. Are
you looking at using return barriers?

BR,
   Stuart

On Tue, 7 Jan 2020 at 12:59, Per Liden <per.liden at oracle.com> wrote:
>
> While we're on this topic I thought I could mention that part of the
> plans to make ZGC a true sub-millisecond max pause time GC includes
> removing thread stacks completely from the GC root set. I.e. in such a
> world ZGC will not scan any thread stacks (virtual or not) during STW,
> instead they will be scanned concurrently.
>
> But we're not quite there yet...
>
> cheers,
> Per
>
> On 12/19/19 1:52 PM, Ron Pressler wrote:
> >
> > This is a very good question. Virtual thread stacks (which are actually
> > continuation stacks from the VM’s perspective) are not GC roots, and so are
> > not scanned as part of the STW root-scanning. How and when they are scanned
> > is one of the core differences between the default implementation and the
> > new one, enabled with -XX:+UseContinuationChunks.
> >
> > Virtual threads shouldn’t make any impact on time-to-safepoint, and,
> > depending on the implementation, they may or may not make an impact
> > on STW young-generation collection. How the different implementations
> > impact ZGC/Shenandoah, the non-generational low-pause collectors is yet
> > to be explored and addressed. I would assume that their current impact
> > is that they simply crash them :)
> >
> > - Ron
> >
> >
> >
> > On 19 December 2019 at 11:40:03, Holger Hoffstätte (holger at applied-asynchrony.com(mailto:holger at applied-asynchrony.com)) wrote:
> >
> >> Hi,
> >>
> >> Quick question - not sure if this is an actual issue or somethign that has
> >> been addressed yet; pointers to docs welcome.
> >> How does (or will) Loom impact stack snapshotting and TTSP latency?
> >> There have been some amazing advances in GC with Shenandoah and ZGC recently,
> >> but their low pause times directly depend on the ability to quickly reach
> >> safepoints and take stack snapshots for liveliness analysis.
> >> How will this work with potentially one or two orders of magnitude more
> >> virtual thread stacks? If I understand correctly TTSP should only depend
> >> on the number of carrier threads (which fortunately should be much lower
> >> than in legacy designs), but somehow the virtual stacks stil need to be
> >> scraped..right?
> >>
> >> thanks,
> >> Holger
> >