RFR: 8071277: G1: Merge commits and uncommits of contiguous memory [v2]

[Thomas Schatzl]

On Fri, 26 Sep 2025 11:46:28 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
>
> Thomas Schatzl has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - * exploit the fact that this is always a single region to (un-)commit
>  - * ayang idea to use an iterative approach for better readability

The latest change implements @albertnetymk 's suggestion to make the loop explicit; I also noticed that actually there can be at most a single range of memory that can be committed/uncommitted for one commit/uncommit call, so the loop can actually be removed.

Testing: tier1-5

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PR Comment: https://git.openjdk.org/jdk/pull/27381#issuecomment-3338293792