[master] RFR: 8347711: [Lilliput] Parallel GC support for compact identity hashcode [v4]

[Roman Kennke]

On Mon, 7 Apr 2025 13:47:18 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> > > How does this algorithm deal with objects larger than region size?
> > 
> > 
> > Currently not particularily well: it doesn't move them at all (because they don't fit), and it _also_ doesn't move other objects past them. There's a TODO to make it possible to move objects around large objects (don't have to be > region-sized, also something like 1/2 region sized objects may have difficulty to move). I'll address that in a follow-up PR.
> 
> Are you talking about parallel specific? Can you explain why region size > object size > 1/2 region size scenario? Thanks.

I only used 1/2 region size as an example. It can affect any object: if there is not enough space in the target region, and there is no other preceding target region in the queue, we might not be able to move the object, or only move the object within its region. It's only that the problem becomes worse the larger the object is, and for objects > region-size it's currently guaranteed that we can not move it. Being able to move objects around large/un-movable objects would help a lot to reduce gaps.

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PR Comment: https://git.openjdk.org/lilliput/pull/195#issuecomment-2783733550