RFR: JDK-8321565: [REDO] Heap dump does not contain virtual Thread stack references

[Alex Menkov]

On Tue, 12 Dec 2023 21:59:12 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

>> Prior to the virtual thread changes what thread is performing the merge? After these changes what thread would do it if not included in the VM operation?
>> 
>> The assert is correct - we should not do the merge at the safepoint (or even in the VMThread).
>
>> Prior to the virtual thread changes what thread is performing the merge?
> 
> Prior the change merge is performed on either AttachListener thread (when it's the current thread - special case for tools like jcmd) or on VMThread (if the current thread is not AttachListener thread).
> Note also that prior the change segmented dumping was conditional - if GC supports multiple workers and if the caller requests parallel dump. HeapDumpAfterFullGC/HeapDumpBeforeFullGC request dumping in 1 thread, so there was no merge in the scenarios.
> 
>> After these changes what thread would do it if not included in the VM operation?
> 
> On the current thread. As I wrote before I 'm not sure if it can cause some unexpected consequences in some cases.
> I tried to do the change - tier1 passed (TestReduceAllocationAndHeapDump.java test calls Runtime.gc() with HeapDumpAfterFullGC, merge is performed on main java thread)

Sorry, I was wrong. TestReduceAllocationAndHeapDump.java failed on the assert.
I.e. merge is performed on main java thread, but VM is at safepoint

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PR Review Comment: https://git.openjdk.org/jdk/pull/17040#discussion_r1424634783