8261501: Shenandoah: reconsider heap statistics memory ordering

[shipilev]

ShenandoahHeap collects heap-wide statistics (used, committed, etc). It does so by atomically updating them with default CASes. Unfortunately, Hotspot's default for atomic operations is memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64.

This is excessive for statistics gathering, and "relaxed" should be just as good.

Additional testing:

• Linux x86_64 hotspot_gc_shenandoah
• Linux AArch64 hotspot_gc_shenandoah
• Linux AArch64 tier1 with Shenandoah

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed

Issue

• JDK-8261501: Shenandoah: reconsider heap statistics memory ordering

Reviewers

• Roman Kennke (@rkennke - Reviewer)

Download

$ git fetch https://git.openjdk.java.net/jdk pull/2504/head:pull/2504
$ git checkout pull/2504

[bridgekeeper]

👋 Welcome back shade! 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]

@shipilev The following labels will be automatically applied to this pull request:

• hotspot-gc
• shenandoah

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

[mlbridge]

Webrevs

• 00: Full (69d5ddf)

[shipilev]

Friendly reminder.

[rkennke]

Looks good to me! Thanks!

[openjdk]

@shipilev 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:

8261501: Shenandoah: reconsider heap statistics memory ordering

Reviewed-by: rkennke

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 90 new commits pushed to the master branch:

• 3cbd16d: 8259668: Make SubTasksDone use-once
• 219b115: 8261422: Adjust problematic String.format calls in jdk/internal/util/Preconditions.java outOfBoundsMessage
• cdc874d: 8261601: free memory in early return in Java_sun_nio_ch_sctp_SctpChannelImpl_receive0
• e2d52ae: 8261413: Shenandoah: Disable class-unloading in I-U mode
• 34ae7ae: 8261609: remove remnants of XML-driven builders
• 6badd22: 8261351: Create implementation for NSAccessibilityRadioButton protocol
• 849f4c0: 8260291: The case instruction is not visible in dark mode
• 2e610f5: 8260687: Inherited font size is smaller than expected when using StyleSheet to add styles
• 3882fda: 8260414: Remove unused set_single_threaded_mode() method in task executor
• c6eedda: 8261500: Shenandoah: reconsider region live data memory ordering
• ... and 80 more: https://git.openjdk.java.net/jdk/compare/5183d8ae1eec86202eace2c4770f81edbc73cb68...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.

[shipilev]

/integrate

[openjdk]

@shipilev Since your change was applied there have been 90 commits pushed to the master branch:

• 3cbd16d: 8259668: Make SubTasksDone use-once
• 219b115: 8261422: Adjust problematic String.format calls in jdk/internal/util/Preconditions.java outOfBoundsMessage
• cdc874d: 8261601: free memory in early return in Java_sun_nio_ch_sctp_SctpChannelImpl_receive0
• e2d52ae: 8261413: Shenandoah: Disable class-unloading in I-U mode
• 34ae7ae: 8261609: remove remnants of XML-driven builders
• 6badd22: 8261351: Create implementation for NSAccessibilityRadioButton protocol
• 849f4c0: 8260291: The case instruction is not visible in dark mode
• 2e610f5: 8260687: Inherited font size is smaller than expected when using StyleSheet to add styles
• 3882fda: 8260414: Remove unused set_single_threaded_mode() method in task executor
• c6eedda: 8261500: Shenandoah: reconsider region live data memory ordering
• ... and 80 more: https://git.openjdk.java.net/jdk/compare/5183d8ae1eec86202eace2c4770f81edbc73cb68...master

Your commit was automatically rebased without conflicts.

Pushed as commit 3f8819c.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.