Hi,

Per cu. observation

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When switched from JDK 11 to JDK 21.0.6.0.7-1.relex1 build of JDK it appeared that there's significantly higher rate of FullGC (full compaction) phases occurring in comparison to JDK 11 (which rarely happened)

Typical manifestation of this behaviour is exhausted heap and failure to allocate new region:

Attempt heap expansion (allocation request failed). Allocation request: 1284800B
[2025-06-03T01:32:00.589+0300][debug][gc,ergo,heap ] Expand the heap. requested expansion amount: 33554432B expansion amount: 33554432B
[2025-06-03T01:32:00.589+0300][debug][gc,ergo,heap ] Did not expand the heap (heap already fully expanded)
[2025-06-03T01:32:00.589+0300][info ][gc,ergo ] Attempting full compaction

While available Eden regions is 0 despite there're 400Mb of free heap space with 2Tb of Xms/Xmx (G1ReservePercent set to 20 though):
[2025-06-03T01:32:00.588+0300][info ][gc,heap ] GC(448) Eden regions: 1074->0(0)

Execution path that being utilised is this one:

https://github.com/openjdk/jdk/blob/6cfd4057dce9262f54e71a3930e16da84aa0d9f1/src/hotspot/share/gc/g1/g1CollectedHeap.cpp#L893

 
We identified the root cause of this issue - that's the JVM bug:
https://bugs.openjdk.org/browse/JDK-8352765
There's a fix for this bug available in Java 25:
https://github.com/openjdk/jdk/pull/24257

~~~