RFR: 8352765: G1CollectedHeap::expand_and_allocate() may fail to allocate even after heap expansion succeeds

[Thomas Schatzl]

On Wed, 26 Mar 2025 15:05:46 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

> Hi,
> 
> Please review this change to ensure that G1 provisions for at least one Eden region after a GC when computing the young length target.
> 
> The issue reported in the CR occurs at the end of a GC, after successfully expanding the heap, an allocation fails because `policy()->should_allocate_mutator_region()` returns false. This happens because the computation did not properly account for young regions already allocated as survivor regions, leading to an Eden region target of zero.
> 
> With this change, we factor in the young regions that have already been allocated as survivor regions and ensure that at least one region is targeted for Eden.
> 
> Testing: Tier 1-3
>               Reproducer in the CR.

Changes requested by tschatzl (Reviewer).

src/hotspot/share/gc/g1/g1Policy.cpp line 360:

> 358:       log_trace(gc, ergo, heap)("Young target length: Fully eat into reserve "
> 359:                                 "receiving young %u receiving additional eden %u",
> 360:                                 receiving_eden,

the log message reads "receiving young" still.

src/hotspot/share/gc/g1/g1Policy.cpp line 383:

> 381:                                 "receiving additional eden %u",
> 382:                                 free_outside_reserve, receiving_within_reserve,
> 383:                                 receiving_eden, receiving_additional_eden);

Log message needs update.

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PR Review: https://git.openjdk.org/jdk/pull/24257#pullrequestreview-2721033969
PR Review Comment: https://git.openjdk.org/jdk/pull/24257#discussion_r2016346138
PR Review Comment: https://git.openjdk.org/jdk/pull/24257#discussion_r2016347443