RFR: 8338977: Parallel: Improve heap resizing heuristics [v13]

[Zhengyu Gu]

On Sun, 6 Jul 2025 21:00:42 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> src/hotspot/share/gc/parallel/parallelScavengeHeap.cpp line 381:
>> 
>>> 379:   HeapWord* result = young_gen()->expand_and_allocate(size);
>>> 380: 
>>> 381:   if (result == nullptr && !is_tlab && !should_alloc_in_eden(size)) {
>> 
>> I feel that you changed intent of `should_alloc_in_eden()`. I believe the original intent is to prevent allocating large objects in eden, and here, seems to prevent allocating small objects in old gen.
>> 
>> What benefits do you get?
>
> I don't see how I changed the semantics.
> 
> On baseline, from `mem_allocate_work`:
> 
> 
> if (result != nullptr) {
>   return result;
> }
> 
> // If certain conditions hold, try allocating from the old gen.
> if (!is_tlab) {
>   result = mem_allocate_old_gen(size);
>   if (result != nullptr) {
>     return result;
>   }
> }
> 
> 
> and
> 
> 
> HeapWord* ParallelScavengeHeap::mem_allocate_old_gen(size_t size) {
>   if (!should_alloc_in_eden(size)) {
>     // Size is too big for eden.
>     return allocate_old_gen_and_record(size);
>   }
> 
>   return nullptr;
> }
> 
> 
> The original logic is essentially:
> 
> 
> if (result == nullptr && !is_tlab && !should_alloc_in_eden(size)) {
>   // allocate in old-gen
> }
> 
> which is the same as I put here.

Ah, okay.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/25000#discussion_r2188761974