Integrated: 8351405: G1: Collection set early pruning causes suboptimal region selection

[Ivan Walulya]

On Mon, 17 Mar 2025 11:19:02 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

> 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. 
> 
> ![mixed-gc](https://github.com/user-attachments/assets/740fb51d-eb20-4946-bf90-4eef23afe2e4)
> 
> 
> Testing: Tier 1-3.

This pull request has now been integrated.

Changeset: 6879c446
Author:    Ivan Walulya <iwalulya at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/6879c446c6e7734c162c85bd2bd3d7f3b012cca4
Stats:     77 lines in 13 files changed: 57 ins; 2 del; 18 mod

8351405: G1: Collection set early pruning causes suboptimal region selection

Reviewed-by: ayang, tschatzl

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PR: https://git.openjdk.org/jdk/pull/24076