RFR: 8347337: ZGC: String dedups short-lived strings

[Erik Österlund]

On Mon, 10 Mar 2025 15:08:16 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> When -XX:+UseStringDeduplication is turned on, ZGC requests that every single String it encounters is deduplicated. The Strings of these requests are saved in weak handles, and then processed by a separate thread. One problematic part with this is that ZGC treats these handles as strong roots for young collections. So, even if the Strings are short-lived they will be artificially kept alive until they get promoted up to the old generation.
> 
> This creates an extreme amount of Strings and weak handles to be processed by the old collection, which can result in long major collections and allocation stalls.
> 
> Other GCs have a filter to check for how old the Strings are before they get deduplicated.
> 
> The proposed fix is to move the string deduplication requests to when the Strings are promoted to the old generation.
> 
> Testing:
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> * I've tested this with a small micro that showed how ZGC got extremely long major collections with string deduplication turned on.
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> * SPECjbb2015 with a JVMTI agent that induces load and adds deduplicatable strings.
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> * Tier1-7
> 
> Note: I'm currently not aware of any non-artificial workload where string deduplication is an important optimization when running with Generational ZGC. If anyone knows of a workload that greatly benefits from it *AND* uses ZGC as a low-latency collector, then that would be highly interesting to look at.
> 
> Note 2: the branch contains two changesets. In the first changeset I added a flag to be able to test and compare the old implementation with the new implementation. For the final PR I've removed that flag and the associated code as a second changeset. If we really want we could keep that flag, but given how poorly that implementation worked for Generational ZGC, I think we should just go with this new implementation.

With the change to use _forwarding->is_promotion() this looks good. Great fix!

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Marked as reviewed by eosterlund (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23965#pullrequestreview-2673496559