8238686: G1 may waste lots of space or fail to uncommit when observing MinHeapFreeRatio during sizing after full gc

[walulyai]

Hi,

Please review this change which modifies the defaults for the MinHeapFreeRatio and MaxHeapFreeRatio flags in G1 so that they do not affect heap sizing unless explicitly set by the user.

Previously, these flags could trigger heap resizing at the end of Full GC, potentially overriding the G1 ergonomic heap sizing heuristics, which may be unexpected for the user. In particular, for workloads that frequently invoke System.gc(), the heap could shrink at the end of the Full GC and then expand aggressively during subsequent GCs (see image below).

With this update, the default values for MinHeapFreeRatio and MaxHeapFreeRatio no longer affect heap resizing. These flags will only take effect when explicitly set by the user..

Additionally, pause times from user-triggered System.gc() invocations will be excluded from the data used to update heap sizing heuristics. Because these GCs are user-initiated and do not reflect actual GC pressure, including their pause times can skew GC Pause Time ratios used for heap sizing decisions.

Testing: Tier 1 and JavaPerf.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Change requires CSR request JDK-8375300 to be approved
• Commit message must refer to an issue

Issues

• JDK-8238686: G1 may waste lots of space or fail to uncommit when observing MinHeapFreeRatio during sizing after full gc (Bug - P4)
• JDK-8375300: G1 may waste lots of space or fail to uncommit when observing MinHeapFreeRatio during sizing after full gc (CSR)

Reviewers

• Thomas Schatzl (@tschatzl - Reviewer)
• Stefan Johansson (@kstefanj - Reviewer)

Reviewing

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[bridgekeeper]

👋 Welcome back iwalulya! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

@walulyai This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8238686: G1 may waste lots of space or fail to uncommit when observing MinHeapFreeRatio during sizing after full gc

Reviewed-by: tschatzl, sjohanss

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 70 new commits pushed to the master branch:

• aaca0a2: 8375742: Test java/lang/invoke/MethodHandleProxies/Driver.java does not run Unnamed.java
• 42439eb: 8374889: C2 VectorAPI: must handle impossible combination of signed cast from float
• 5f8cb30: 8375626: G1: Convert G1CollectionSetChooser to use Atomic
• ... and 67 more: https://git.openjdk.org/jdk/compare/1b6c2bdd7b57891ed35e3c067871d2c0bf282824...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

[openjdk]

@walulyai The following label will be automatically applied to this pull request:

• hotspot-gc

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

[mlbridge]

Webrevs

• 02: Full - Incremental (15f46662)
• 01: Full - Incremental (c60fed23)
• 00: Full (c4ec4b58)

[walulyai]

Split the changes because the change in flag defaults needs to have a compatibility review.

[tschatzl]

/csr needed

[openjdk]

@tschatzl has indicated that a compatibility and specification (CSR) request is needed for this pull request.

@walulyai please create a CSR request for issue JDK-8238686 with the correct fix version. This pull request cannot be integrated until the CSR request is approved.

[walulyai]

Thanks @kstefanj and @tschatzl for the reviews!

/integrate

[openjdk]

Going to push as commit b134030.
Since your change was applied there have been 92 commits pushed to the master branch:

• 4f87fb5: 8375622: G1: Convert G1CodeRootSet to use Atomic
• 560a92a: 8375787: compiler/vectorapi/TestCastShapeBadOpc.java fails with release VMs
• b5727d2: 8375738: Fix -Wzero-as-null-pointer-constant warnings in MacOSX/bsd code
• ... and 89 more: https://git.openjdk.org/jdk/compare/1b6c2bdd7b57891ed35e3c067871d2c0bf282824...master

Your commit was automatically rebased without conflicts.

[openjdk]

@walulyai Pushed as commit b134030.

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