RFR: 8319876: Reduce memory consumption of VM_ThreadDump::doit [v2]

[David Holmes]

On Fri, 10 Nov 2023 10:02:31 GMT, Long Yang <duke at openjdk.org> wrote:

>> I would like to fix this.
>> 
>> Create 1000 threads, and the stack depth of each thread is 1000. 
>> After running jmx.dumpAllThreads(true, true), the RSS reaches 5.2GiB. 
>> After optimization, the RSS is 250MiB.
>> 
>> I would appreciate it if anyone could review this.
>
> Long Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   move ResourceMark to before start_vf

Yes please explain the problem and the fix better. It is far from clear that every ResourceArea allocation that is made from this code can be safely discarded when this method returns. Which objects are the ones consuming so much space, and which can be erased? Maybe we should be avoiding these allocations rather than cleaning them up more promptly. ??

src/hotspot/share/services/threadService.cpp line 698:

> 696:                         RegisterMap::ProcessFrames::include,
> 697:                         RegisterMap::WalkContinuation::skip);
> 698:     ResourceMark rm;

Nit: Use `rm(VMThread::vm_thread());` to avoid the need to call `Thread::current()`.

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PR Review: https://git.openjdk.org/jdk/pull/16598#pullrequestreview-1726445999
PR Review Comment: https://git.openjdk.org/jdk/pull/16598#discussion_r1390558638