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

[Stefan Karlsson]

On Mon, 10 Mar 2025 21:35:35 GMT, Kim Barrett <kbarrett 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:
>> 
>> * I've tested this with a small micro that showed how ZGC got extremely long major collections with string deduplication turned on.
>> 
>> * SPECjbb2015 with a JVMTI agent that induces load and adds deduplicatable strings.
>> 
>> * 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.
>
> src/hotspot/share/gc/z/zRelocate.cpp line 802:
> 
>> 800: 
>> 801:   void maybe_string_dedup(zaddress to_addr) {
>> 802:     const bool is_promotion = _forwarding->to_age() == ZPageAge::old && _forwarding->from_age() != ZPageAge::old;
> 
> It seems like this could be computed once at `ZRelocateWork` construction time?  Unless `_forwarding`
> can change.  Also, isn't this the same as `_forwarding->is_promotion()`?

`_forwarding` changes whenever we move to the next ZPage to relocate. We could probably add an `_is_promotion` variable/constant to ZForwarding, but we're already reading `_to_age` and `_from_age` in `update_remset_for_fields` so I don't expect a noticeable performance difference if we would do that.

Great point about `_forwarding->is_promotion()`. I'm updating the code to use that.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23965#discussion_r1988690267