8227106: InitiatingHeapOccupancyPercent is G1-specific but defined in shared

[lgxbslgx]

Hi all,

Please review this little fix of G1.
The command line option InitiatingHeapOccupancyPercent is G1-specific. But it is defined in shared/gc_globals.hpp rather than g1/g1_globals.hpp. This patch moves it to the proper location.
Thank you for taking the time to review.

Best Regards.

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Progress

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Testing

	Linux aarch64	Linux arm	Linux ppc64le	Linux s390x	Linux x64	Linux x86	Windows x64	macOS x64
Build	✔️ (1/1 passed)	✔️ (1/1 passed)	✔️ (1/1 passed)	✔️ (1/1 passed)	✔️ (5/5 passed)	✔️ (2/2 passed)	✔️ (2/2 passed)	✔️ (2/2 passed)
Test (tier1)					✔️ (9/9 passed)	✔️ (9/9 passed)	✔️ (9/9 passed)	✔️ (9/9 passed)

Issue

• JDK-8227106: InitiatingHeapOccupancyPercent is G1-specific but defined in shared

Download

$ git fetch https://git.openjdk.java.net/jdk pull/1217/head:pull/1217
$ git checkout pull/1217

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[tschatzl]

Hi,

first, apologies for the late reply and actually having this PR get automatically closed due to inactivity.

Unfortunately, simply moving the InitiatingHeapOccupancyPercent flag to the other file is not sufficient to complete this problem properly. The move to that file implies that the flag will not be available for VM builds without G1 any more, possibly breaking lots of existing deployments - and there are lots of them using that flag.
Further, flags in that file should all have a G1 prefix clearly indicating that this is a G1 specific flag.

So a proper path to this "simple" move would probably be:

1. Deprecate InitiatingHeapOccupancyPercent so that people can still use it the next few releases. Basically add it at the proper location in special_jvm_flags (best grep it).
2. Create a new flag "G1InitiatingHeapOccupancyPercent" and alias the other to this one (and add code handling if both are specified etc).
3. Change the code including the tests to use the new flag.
4. Fix the documentation (which we would need to do as the documentation sources are not public)

These two steps require a CSR - I think a single one is sufficient renaming the flag.

5. At some point in the future a few releases out, obsolete and another release ahead, remove the old flag.

(I will also add this information to the CR in JIRA)

Tbh it's lots of work for not a lot of gain and a certain degree of disruption for end users so it might not be worth it.
However I will support you (with improved turnaround time) if you want to continue on that path, as in the end it's work that should be done.

Thanks,
Thomas

[lgxbslgx]

@tschatzl Thank you for your reply.

Maybe the designer want other GC to reuse InitiatingHeapOccupancyPercent in the future.
We should do more investigations about the possible useful situations before we restart this work.

[tschatzl]

It seems unlikely that this flag will be reused soon: both of the other concurrent collectors, ZGC and Shenandoah, have their own heuristics with their own set of flags that do not exactly match the semantics of InitiatingHeapOccupancyPercent.

Feel free to keep this PR closed though for now.