RFR: 8310031: Parallel: Implement better work distribution for large object arrays in old gen [v24]

[Thomas Schatzl]

On Thu, 19 Oct 2023 20:06:59 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> Likely I do not completely understand what you are saying but this would be my explanation why the `align_down` for `clear_length` is needed.
>> `T := old_gen->object_space()->top()` is not necessarily card aligned at scavenge start. We must not clear the card for `T` if an object was copied there because it was promoted and it has a reference to a young object on that card.
>
> Example
> 
> We cannot clear card n containing old gen top T because we won't scan the promoted
> objects on card n and we can only clear cards if we scan all objects on them afterwards.
> 
> 
>   card n-1             card n               card n+1
>  +--------------------+--------------------+--------------------
>  |                    |        .  Promoted |
>  |                    |        .  Objects  |
>  |                    |        .           |
>  +--------------------+--------------------+--------------------
>                                ^
>                                |
>                                T
>                       old gen top at scavenge start
>                     / end of last stripe

Okay, I think I understood this now. I would probably just fill up the last card before scavenge, but this explanation makes sense. Please add a comment there for the next person.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14846#discussion_r1366158560