RFR: 8355756: G1HeapSizingPolicy::full_collection_resize_amount should consider allocation size [v2]

[Thomas Schatzl]

On Wed, 30 Apr 2025 10:15:30 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

>> Hi,
>> 
>> Please review this change to account for pending allocations when deciding how much to shrink the heap after a full gc. Otherwise, we shrink the heap only to trigger an expansion to satisfy the allocation request that triggered the full gc.
>> 
>> Testing: Tier 1-3
>
> Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Thomas Review

Changes requested by tschatzl (Reviewer).

src/hotspot/share/gc/g1/g1HeapSizingPolicy.cpp line 242:

> 240:     // Humongous objects are allocated in entire regions, we must calculate
> 241:     // required space in terms of full regions, not just the object size.
> 242:     allocation_word_size = align_up(allocation_word_size, G1HeapRegion::GrainWords);

Maybe it is worth adding a (static?) helper function for the padding to humongous object size because this would be the third time this is done in `G1CollectedHeap.cpp`.

Also I would prefer if the parts of the whole calculation were done before the actual sum in similar style to existing code:

I.e. something like:


if (_g1h->is_humongous(...)) {
  allocation_word_size = _g1h->pad_to_humongous(...);
}

const size_t used_after_gc = 
                  summand_1 +
                  summand_2 +
                  ... +
                  allocation_word_size.

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

PR Review: https://git.openjdk.org/jdk/pull/24944#pullrequestreview-2807000400
PR Review Comment: https://git.openjdk.org/jdk/pull/24944#discussion_r2068631730