Integrated: 8071277: G1: Merge commits and uncommits of contiguous memory

[Thomas Schatzl]

On Fri, 19 Sep 2025 07:01:00 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review this change to G1 commit/uncommit to merge commit/uncommit calls to the operating system as much as possible. This significantly improves startup performanc
> 
> Performance results before/after on Zen3 processor for a "HelloWorld" application:
> 
> 
>  $ hyperfine -w 10 -r 10 "baseline/bin/java -XX:+UseG1GC -Xms128m -Xmx128m Hello"
> Benchmark 1: baseline/bin/java -XX:+UseG1GC -Xms128m -Xmx128m Hello
>   Time (mean ± σ): 16.4 ms ± 0.4 ms [User: 11.0 ms, System: 14.4 ms]
>   Range (min … max): 15.7 ms … 16.8 ms 10 runs
> 
> $ hyperfine -w 10 -r 10 "baseline/bin/java -XX:+UseG1GC -Xms2g -Xmx2g Hello"
> Benchmark 1: baseline/bin/java -XX:+UseG1GC -Xms2g -Xmx2g Hello
>   Time (mean ± σ): 24.7 ms ± 0.4 ms [User: 10.7 ms, System: 22.7 ms]
>   Range (min … max): 24.2 ms … 25.4 ms 10 runs
> 
> My reimplementation of JDK-8071277 does show improvements here:
> 
> $ hyperfine -w 10 -r 10 "changes/bin/java -XX:+UseG1GC -Xms128m -Xmx128m Hello"
> Benchmark 1: changes/bin/java -XX:+UseG1GC -Xms128m -Xmx128m Hello
>   Time (mean ± σ): 15.9 ms ± 0.4 ms [User: 11.9 ms, System: 13.1 ms]
>   Range (min … max): 15.4 ms … 16.7 ms 10 runs
> 
> $ hyperfine -w 10 -r 10 "changes/bin/java -XX:+UseG1GC -Xms2g -Xmx2g Hello"
> Benchmark 1: changes/bin/java -XX:+UseG1GC -Xms2g -Xmx2g Hello
>   Time (mean ± σ): 19.7 ms ± 0.3 ms [User: 11.3 ms, System: 17.4 ms]
>   Range (min … max): 19.2 ms … 20.1 ms 10 runs
> 
> 
> I.e., depending on actually committed heap size (`-Xms`), reduction of startup time by 20% or so in above cases.
> 
> Testing: tier1-5, gha
> 
> Thanks,
>   Thomas

This pull request has now been integrated.

Changeset: 08b677bb
Author:    Thomas Schatzl <tschatzl at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/08b677bba4b1e23feb55b104d86fe0eef543d59c
Stats:     78 lines in 3 files changed: 33 ins; 11 del; 34 mod

8071277: G1: Merge commits and uncommits of contiguous memory

Reviewed-by: iwalulya, ayang

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PR: https://git.openjdk.org/jdk/pull/27381