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 283:

> 281:   }
> 282:   size_t eden_size = size - (2 * survivor_size);
> 283:   assert(eden_size > 0 && survivor_size <= eden_size, "just checking");

Is it guaranteed that `size > 2 * survivor_size`?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27513#discussion_r2381545364