Integrated: 8212084: G1: Implement UseGCOverheadLimit

[Thomas Schatzl]

On Thu, 23 Oct 2025 07:11:16 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review these changes to implement the `UseGCOverheadLimit` functionality for G1 (and make the implementation for Parallel GC have similar output).
> 
> The `UseGCOverheadLimit` feature prematurely returns `null` from a GC if GC cpu usage limits and heap usage limits are met for some time. This is to avoid a VM limping along if garbage collection gets into an endless cycle of garbage collections or until a "real" OOME is thrown.
> 
> What is important here is how this works (derived from the Parallel GC implementation):
> 
> * check overheads at the end of the (initial) garbage collection (before upgrading) to see whether we are over the limits for a sequence of GCs.
> * keep doing GCs without actually allocating memory for the allocation request to keep on measuring gc CPU usage. This is important for measuring the correct cpu usage in case of the application being able to free memory on the OOME.
> 
> Testing: tier1-5 without any OOMEs due to this feature, test case
> 
> Thanks,
>   Thomas

This pull request has now been integrated.

Changeset: 3d2ce804
Author:    Thomas Schatzl <tschatzl at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/3d2ce8045f9ea52c6559638f9cc7e0a0544b4540
Stats:     219 lines in 7 files changed: 186 ins; 15 del; 18 mod

8212084: G1: Implement UseGCOverheadLimit

Reviewed-by: ayang, iwalulya, fandreuzzi

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PR: https://git.openjdk.org/jdk/pull/27950