RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v3]

[Stefan Karlsson]

On Mon, 9 Sep 2024 12:49:05 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>>> If we get a promotion failure in the young gen, we are leaving the dead objects marked as forwarded. 
>> 
>> True; need to do sth like `obj->init_mark();` for the non-self-forwarded case. The postcondition is that no forwarded objs in eden/from.
>
> ParallelGC actually doesn't use bitmaps, it pushes all forwarded objs to preserved-marks-table, and uses that to find forwarded objects, which is why we can't remove the preserved-marks table in ParallelGC (IOW, after this patch, the preserved-marks-stuff in Parallel scavenger is *only* used to find forwarded objects. We might want to think about more efficient solutions for this).

(Just to clarify if others are reading this)

Right, what I referred to above was how we found the object to forward, which is done via the bitmaps:

          while (cur_addr < region_end) {
            cur_addr = mark_bitmap()->find_obj_beg(cur_addr, region_end);

If the Parallel Old collector didn't do that, but instead parsed the heap like Serial does, then the Parallel Young collector would also have to fix the from space copies of moved objects when when it hits a promotion failure, just like Serial does. This was just meant to point out the differences between the two collectors and why the young GC code is different.

I realize that in earlier comments I called the from-space copy of the objects "dead objects", but they are not dead they are just the stale objects that are discoverable because of promotion failure keeping the eden and from spaces.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750480983