Stacks, safepoints, snapshotting and GC

Per Liden per.liden at
Mon May 25 18:09:09 UTC 2020

This work is tracked by JEP 376 ( 
This JEP is not yet targeted to a specific release. However, you can 
already today try it out by building a JDK from, which has the latest patches for 
concurrent thread stack scanning.


On 5/25/20 3:13 PM, raell at wrote:
> Hi,
> I just wanted to ask if there are concrete release plans for
> concurrent thread scanning/root processing in ZGC?
> Regards
> Ralph
>   On 7 January 2020 at 12:58:48, Per Liden 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[](mailto:holger at[])) 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

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