8261838: Shenandoah: reconsider heap region iterators memory ordering

[shipilev]

We use CASes to distributed workers between regions. 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 seems to be excessive for region distribution code, and "relaxed" is enough, since we don't piggyback memory ordering on these.

This also calls for some refactoring in the code itself.

Additional testing:

• hotspot_gc_shenandoah
• Ad-hoc performance runs

---

Progress

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

Issue

• JDK-8261838: Shenandoah: reconsider heap region iterators memory ordering

Reviewers

• Roman Kennke (@rkennke - Reviewer)

Download

$ git fetch https://git.openjdk.java.net/jdk pull/2593/head:pull/2593
$ git checkout pull/2593

[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 (1e68734)

[rkennke]

Looks good!

[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:

8261838: Shenandoah: reconsider heap region iterators 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 5 new commits pushed to the master branch:

• 0a50688: 8241372: Several test failures due to javax.net.ssl.SSLException: Connection reset
• 61a659f: 8260415: Remove unused class ReferenceProcessorMTProcMutator
• 6b6f794: 8248223: KeyAgreement spec update on multi-party key exchange support
• 8ba390d: 8261753: Test java/lang/System/OsVersionTest.java still failing on BigSur patch versions after JDK-8253702
• 16bd7d3: 8261336: IGV: enhance default filters

Please see this link for an up-to-date comparison between the source branch of this pull request and the master branch.
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 33 commits pushed to the master branch:

• f94a845: 8261600: NMT: Relax memory order for updating MemoryCounter and fix racy updating of peak values
• 1a7adc8: 8260416: Remove unused method ReferenceProcessor::is_mt_processing_set_up()
• 3a21e1d: 8260653: Unreachable nodes keep speculative types alive
• b695c7e: 8261925: ProblemList com/sun/jdi/AfterThreadDeathTest.java on Linux
• 97e1657: 8261846: [JVMCI] c2v_iterateFrames can get out of sync with the StackFrameStream
• b8fd614: 8261939: os::strdup_check_oom() should be used in os::same_files() in os_windows.cpp
• 5f30829: 8202750: Reduce the use of get_canonical_path() in CDS
• ea5bf45: 8261621: Delegate Unicode history from JLS to j.l.Character
• d5a4d22: 8261843: incorrect info in docs/building.html
• bf75a3a: 8261851: update ReflectionCallerCacheTest.java test to use ForceGC from test library
• ... and 23 more: https://git.openjdk.java.net/jdk/compare/3f8819c66640dff98added4340ea30d9740a9fc2...master

Your commit was automatically rebased without conflicts.

Pushed as commit fd098e7.

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