RFR: 8351405: G1: Collection set early pruning causes suboptimal region selection
Ivan Walulya
iwalulya at openjdk.org
Mon Mar 17 12:40:48 UTC 2025
Hi all,
Please review this change which aims to reduce spikes in mixed GCs, especially the last mixed-gc in a mixed cycle. Currently, G1 sorts regions identified for collection by reclaimable bytes, then prunes the list removing regions that with the lowest amount of reclaimable bytes. The pruned list is then split into collection groups which are later sorted on gc-efficiency.
In the cachestress benchmark, we run into a case where some regions contain onlya few live objects but having many incoming references from other regions. These regions very expensive collect (low gc-efficiency).
This patch improves the pruning process by tracking incoming references to regions during marking. Instead of pruning based on reclaimable bytes alone, we estimate GC efficiency beforehand and prune regions with the worst GC efficiency.
This reduces the spikes in gc pause time as shown for cachestress benchmark in the image below.

Testing: Tier 1-3.
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Commit messages:
- remove double prune
- save
- revise region selection
- save
- init
Changes: https://git.openjdk.org/jdk/pull/24076/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24076&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8351405
Stats: 79 lines in 11 files changed: 66 ins; 2 del; 11 mod
Patch: https://git.openjdk.org/jdk/pull/24076.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/24076/head:pull/24076
PR: https://git.openjdk.org/jdk/pull/24076
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