RFR: 8317755: G1: Periodic GC interval should test for the last whole heap GC [v2]

[Albert Mingkun Yang]

On Wed, 18 Oct 2023 14:27:26 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> See the description in the bug. Fortunately, we already track the last whole-heap GC. The new regression test verifies the behavior.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 fastdebug `tier1 tier2 tier3`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8317755-g1-periodic-whole
>  - Optional flag
>  - Merge branch 'master' into JDK-8317755-g1-periodic-whole
>  - Keep the cast
>  - Fix

I believe that having periodic Garbage Collections (whether they be young, concurrent, full or mixed) may be something that user applications need in certain scenarios. However, the current JVM lacks a way for Java to invoke a specific kind of GC cycle. If the WhiteBox API (or something equivalent) were available in Java, a user application could easily implement periodic GCs, along with any custom tweaks, e.g. the behavior of master -- short-circuiting periodic concurrent GC. That way, one can avoid confusions around `The current use of “idle” is also awkward in JEP 346...`. Just my 2 cents.

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PR Comment: https://git.openjdk.org/jdk/pull/16107#issuecomment-1768889026