RFR: 8307378: Allow collectors to provide specific values for GC notifications' actions [v3]

[Stefan Karlsson]

On Thu, 4 May 2023 18:30:17 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> At the end of a GC pause, a `GarbageCollectionNotificationInfo` may be emitted. The notification has a `gcAction` field which presently originates from the field `_gc_end_message` in `GCMemoryManager`. Concurrent collectors such as Shenandoah, ZGC and G1 may have more (brief) pauses in their cycle than they have memory managers. This makes it difficult for gc notification listeners to determine the phase of the cycle that emitted the notification. We are proposing a change to allow collectors to define specific values for the `gcAction` to make it easier for notification listeners to classify the gc phase responsible for the notification.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove trailing whitespace

Marked as reviewed by stefank (Reviewer).

I see that I dropped the ZGC changes when I rebased my proposed patch. Thanks for fixing that.

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PR Review: https://git.openjdk.org/jdk/pull/13785#pullrequestreview-1413812475
PR Comment: https://git.openjdk.org/jdk/pull/13785#issuecomment-1535329183