RFR: 8368740: Serial: Swap eden and survivor spaces position in young generation

[Francesco Andreuzzi]

On Fri, 26 Sep 2025 07:33:19 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Change the relative position of eden and survior spaces in young-gen so that eden can be expanded to fulfill allocation request in heap-near-full scenario. What is on master doesn't allow eden expansion if there is live objs inside from/to after full-gc.
> 
> Test: tier1-8

src/hotspot/share/gc/serial/defNewGeneration.cpp line 426:

> 424:       return;
> 425:     }
> 426:   } else {

How about `else if (current_young_gen_size_bytes > desired_young_gen_size_bytes)` here? `VirtualSpace::shrink_by(size_t)` seems to do some work even when `delta_bytes = 0`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27513#discussion_r2381571226