RFR: 8366865: G1: Allocation GC Pauses Triggered after JVM has started shutdown

[Ivan Walulya]

On Thu, 11 Sep 2025 01:41:46 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Please review this patch to skip VM_GC_Collect_Operations  if initiated after the VM shutdown process has begun. We add a _is_shutting_down flag to CollectedHeap, which is set while holding the Heap_lock. This ensures mutual exclusion with VM_GC_Collect_Operations, which also require the Heap_lock.
>> 
>> Skipping VM_GC_Collect_Operation would otherwise cause allocation requests to fail (resulting in OutOfMemoryError) if requesting daemon threads were allowed to continue, we instead block these threads on a monitor. They remain stalled until they are terminated as part of the VM shutdown sequence.
>> 
>> Testing: Tier 1-7
>
> src/hotspot/share/gc/shared/collectedHeap.cpp line 616:
> 
>> 614:   // If the VM is shutting down, we may have skipped VM_CollectForAllocation.
>> 615:   // To avoid returning nullptr (which could cause premature OOME), we stall
>> 616:   // allocation requests here until the VM shutdown is complete.
> 
> If I understand the shutdown sequence correctly, I don't think you can do this without risking a hang. The thread doing the shutdown can potentially execute code that requires allocation after `is_shutting_down()` returns true. This is due to the JVMTI events posted from `before_exit`:
> 
> if (JvmtiExport::should_post_thread_life()) {
>     JvmtiExport::post_thread_end(thread);
>   }
> 
>   // Always call even when there are not JVMTI environments yet, since environments
>   // may be attached late and JVMTI must track phases of VM execution
>   JvmtiExport::post_vm_death();
> 
> I think if you can't GC during shutdown then you have to simply let the allocation fail.

Thanks, I completely overlooked the JVMTI callbacks. It’s probably better to stall with a timeout and then return an allocation failure.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27190#discussion_r2339377413