8261495: Shenandoah: reconsider update references memory ordering

[shipilev]

Shenandoah update heap references code uses default Atomic::cmpxchg to avoid races with mutator updates. 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 Shenandoah update references code, and "release" is enough. We do not seem to piggyback on update-references memory effects anywhere (in fact, if not for mutator, we would not even need a CAS). But, there is an interplay with concurrent evacuation and updates from self-healing.

Average time goes down, the number of GC cycles go up, since the cycles are shorter. For example, SPECjvm2008:compiler.sunflow:

# Before
[172.931s][info][gc,stats] Concurrent Update Refs         =    9.866 s (a =    60161 us)
[283.225s][info][gc,stats] Concurrent Update Refs         =    8.274 s (a =    57859 us)
[232.514s][info][gc,stats] Concurrent Update Refs         =    9.321 s (a =    58991 us)
[215.495s][info][gc,stats] Concurrent Update Refs         =    9.233 s (a =    58434 us)
[217.557s][info][gc,stats] Concurrent Update Refs         =    9.153 s (a =    58672 us)

# After
[169.752s][info][gc,stats] Concurrent Update Refs         =    8.150 s (a =    49697 us)
[168.798s][info][gc,stats] Concurrent Update Refs         =    8.430 s (a =    51092 us)
[166.353s][info][gc,stats] Concurrent Update Refs         =    8.812 s (a =    54395 us)
[236.542s][info][gc,stats] Concurrent Update Refs         =    8.215 s (a =    51026 us)
[214.006s][info][gc,stats] Concurrent Update Refs         =    8.244 s (a =    52175 us)

Additional testing:

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

---

Progress

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Issue

• JDK-8261495: Shenandoah: reconsider update references memory ordering

Reviewers

• Zhengyu Gu (@zhengyu123 - Reviewer) ⚠️ Review applies to 36bee3a9e5344e6e20cd6c179e12be931fac60dd
• Roman Kennke (@rkennke - Reviewer)

Reviewing

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[bridgekeeper]

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[openjdk]

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

• hotspot-gc
• shenandoah

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[mlbridge]

Webrevs

• 06: Full - Incremental (3adc78b4)
• 05: Full (71f419f1)
• 04: Full - Incremental (a033e37a)
• 03: Full - Incremental (0d29996885e707f7ab5dc487beac997e61b71454)
• 02: Full - Incremental (36bee3a9e5344e6e20cd6c179e12be931fac60dd)
• 01: Full - Incremental (87a609f4a9e18acd3dab8606345ffbba3330bd2e)
• 00: Full (d83b9af4b163bb0a6abfcd7325d544d5e42d29cc)

[zhengyu123]

Looks good to me.

[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:

8261495: Shenandoah: reconsider update references memory ordering

Reviewed-by: zgu, 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 19 new commits pushed to the master branch:

• ee0247f: 8263461: jdk/jfr/event/gc/detailed/TestEvacuationFailedEvent.java uses wrong mechanism to cause evacuation failure
• 3ad20fc: 8269571: NMT should print total malloc bytes and invocation count
• b969136: 8245877: assert(_value != __null) failed: resolving NULL _value in JvmtiExport::post_compiled_method_load
• ee526a2: Merge
• 0d745ae: 8269034: AccessControlException for SunPKCS11 daemon threads
• d042029: 8269529: javax/swing/reliability/HangDuringStaticInitialization.java fails in Windows debug build
• 401cb0a: 8269232: assert(!is_jweak(handle)) failed: wrong method for detroying jweak
• b8a16e9: 8268884: C2: Compile::remove_speculative_types must iterate top-down
• 25f9f19: 8249646: Runtime.exec(String, String[], File) documentation contains literal {@link ...}
• 0d83dc2: 8268699: Shenandoah: Add test for JDK-8268127
• ... and 9 more: https://git.openjdk.java.net/jdk/compare/a97715755d01b88ad9e4cf32f10ca5a3f2fda898...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.

[rkennke]

Looks good to me.

[bridgekeeper]

@shipilev This pull request has been inactive for more than 4 weeks and will be automatically closed if another 4 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

[shipilev]

Not yet, bot. Waiting for JDK-8261579 to remeasure perf differences.

[bridgekeeper]

@shipilev This pull request has been inactive for more than 4 weeks and will be automatically closed if another 4 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

[shipilev]

Not yet, bot. Still waiting.

[bridgekeeper]

@shipilev This pull request has been inactive for more than 4 weeks and will be automatically closed if another 4 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

[shipilev]

Not now, bot. Still waiting.

[shipilev]

@rkennke, @zhengyu123 -- please take a look again. Now that JDK-8261579 is there, the performance data is available (PR body), and it seems to be ready to go.

[zhengyu123]

Still good to me. Thanks.

[shipilev]

/integrate

[openjdk]

Going to push as commit a96012a.
Since your change was applied there have been 22 commits pushed to the master branch:

• 23d2996: 8269478: Shenandoah: gc/shenandoah/mxbeans tests should be more resilient
• a6b253d: 8269416: [JVMCI] capture libjvmci crash data to a file
• a0f32cb: 8268906: gc/g1/mixedgc/TestOldGenCollectionUsage.java assumes that GCs take 1ms minimum
• ee0247f: 8263461: jdk/jfr/event/gc/detailed/TestEvacuationFailedEvent.java uses wrong mechanism to cause evacuation failure
• 3ad20fc: 8269571: NMT should print total malloc bytes and invocation count
• b969136: 8245877: assert(_value != __null) failed: resolving NULL _value in JvmtiExport::post_compiled_method_load
• ee526a2: Merge
• 0d745ae: 8269034: AccessControlException for SunPKCS11 daemon threads
• d042029: 8269529: javax/swing/reliability/HangDuringStaticInitialization.java fails in Windows debug build
• 401cb0a: 8269232: assert(!is_jweak(handle)) failed: wrong method for detroying jweak
• ... and 12 more: https://git.openjdk.java.net/jdk/compare/a97715755d01b88ad9e4cf32f10ca5a3f2fda898...master

Your commit was automatically rebased without conflicts.

[openjdk]

@shipilev Pushed as commit a96012a.

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