8292194: G1 nmethod entry barrier disarm value wraps around too early

[fisk]

The disarm value for G1 nmethod entry barriers assumes that a concurrent GC can be aborted at most once. However, it can happen many times. In order to avoid ABA issues in the nmethod entry barriers, the limit for wrapping around and reusing numbers needs to be higher. This patch changes that point to INT_MAX as there is little point in using lower values.

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Issue

• JDK-8292194: G1 nmethod entry barrier disarm value wraps around too early

Reviewers

• Albert Mingkun Yang (@albertnetymk - Reviewer)
• Thomas Schatzl (@tschatzl - Reviewer)

Reviewing

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[mlbridge]

Webrevs

• 00: Full (64bcf823)

[albertnetymk]

Looks good. Though, it's unclear to me why the counter is signed in the first place.

[openjdk]

@fisk 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:

8292194: G1 nmethod entry barrier disarm value wraps around too early

Reviewed-by: ayang, tschatzl

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At the time when this comment was updated there had been 205 new commits pushed to the master branch:

• 0813a47: 8282729: Serial: Move BOT implementation to collector specific directory
• fa5cc4c: 8291878: NMT: Malloc limits
• ad2e0c4: 8292778: EncodingSupport_md.c convertUtf8ToPlatformString wrong placing of free
• 13c0369: 8242181: [Linux] Show source information when printing native stack traces in hs_err files
• fe0544f: 8292790: Remove hashtable.hpp from other files
• 926380d: 8292640: C2: Remove unused scratch register usages on x86
• f3be673: 8292638: x86: Improve scratch register handling in VM stubs
• d24b7b7: 8276651: java/lang/ProcessHandle tests fail with "RuntimeException: Input/output error" in java.lang.ProcessHandleImpl$Info.info0
• 8a45abd: 8292654: G1 remembered set memory footprint regression after JDK-8286115
• cc28783: 8292777: Remove hashtable.hpp from dependencies.hpp
• ... and 195 more: https://git.openjdk.org/jdk/compare/88c96dd3eb28a273305d07fb2b0a36122484189c...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.

[fisk]

Thanks for the review, @albertnetymk!

[fisk]

Any more takers?

[tschatzl]

Unfortunately, "something is seriously wrong" is not true. I have seen benchmarks where G1 is stuck in Concurrent Undo cycles because the concurrent start gc always can always reclaim some humongous object that gets G1 below the concurrent marking threshold again. That can last for 100's of such occurrences.
Obviously idk if before INT_MAX such concurrent undo marks there will be at least one "full" marking cycle, but there does not seem to be a guarantee.
One solution is to force completion of a marking cycle after X (where X is a very high number) subsequent concurrent undos.

[fisk]

Let's say a rather rapid back to back concurrent full GC interval might be 10 ms ish. With such time intervals, it would take a bit over 16 years worth of being constantly interrupted and not finishing a single GC cycle, before any assumption would be broken. I wouldn't wish for anyone to be stuck in a single JVM for that long, hence why I think "something is seriously wrong" if that ever happened. I'm personally okay with that, and think we have bigger fish to fry. For example, the CollectedHeap::_total_collections counter is an unsigned int and there assumptions that it never wraps around. That sounds more likely to ever happen. Yet seemingly unlikely.

[tschatzl]

🤣
Just mentioning. Lgtm.

[fisk]

Thanks for the review, @tschatzl.

[fisk]

/integrate

[openjdk]

Going to push as commit a45a4b9.
Since your change was applied there have been 207 commits pushed to the master branch:

• d3fed12: 8292864: Remove resourceHash.hpp from header files
• b653b9c: 8291969: Convert LoaderConstraintsTable to ResourceHashtable
• 0813a47: 8282729: Serial: Move BOT implementation to collector specific directory
• fa5cc4c: 8291878: NMT: Malloc limits
• ad2e0c4: 8292778: EncodingSupport_md.c convertUtf8ToPlatformString wrong placing of free
• 13c0369: 8242181: [Linux] Show source information when printing native stack traces in hs_err files
• fe0544f: 8292790: Remove hashtable.hpp from other files
• 926380d: 8292640: C2: Remove unused scratch register usages on x86
• f3be673: 8292638: x86: Improve scratch register handling in VM stubs
• d24b7b7: 8276651: java/lang/ProcessHandle tests fail with "RuntimeException: Input/output error" in java.lang.ProcessHandleImpl$Info.info0
• ... and 197 more: https://git.openjdk.org/jdk/compare/88c96dd3eb28a273305d07fb2b0a36122484189c...master

Your commit was automatically rebased without conflicts.

[openjdk]

@fisk Pushed as commit a45a4b9.

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