8071277: G1: Merge commits and uncommits of contiguous memory

[tschatzl]

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

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Progress

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Issue

• JDK-8071277: G1: Merge commits and uncommits of contiguous memory (Enhancement - P4)

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