RFR: 8252105: Parallel heap inspection for ZCollectedHeap

[Per Liden]

On Mon, 5 Oct 2020 18:20:37 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> This patch changes the ZHeapIterator to also support parallel iteration, for use in
>> ZCollectedHeap::parallel_object_iterator(). I initially looked at using the heap walking logic in ZMark, instead of
>> also implementing it in ZHeapIterator. That's probably something we want to do at some point, but that would require
>> some major restructuring of ZMark. In the mean time I've kept the heap walking logic in ZHeapIterator.  The
>> ZHeapIterator now implements ParallelObjectIterator, and the same code paths is used for both serial and parallel
>> iteration. Arrays are chunked using ObjArrayTask to reduce mark stack space. A terminator is used for better work
>> stealing/balance.  Tested using `jmap -histo` and various benchmarks with `-XX:+ZVerifyObjects` and
>> `-XX:+VerifyDuringGC` enabled.
>
> src/hotspot/share/gc/z/zHeapIterator.cpp line 55:
> 
>> 53:   ZHeapIteratorArrayQueue* const _array_queue;
>> 54:   const uint                     _worker_id;
>> 55:   ZStatTimerDisable              _timer_disable;
> 
> Why `ZStatTimerDisable`? Which timer(s) is it suppressing? I see that there's another one in `ZHeapIterator` as well.

It's suppressing uses of `ZStatTimer` in the root iterators.

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PR: https://git.openjdk.java.net/jdk/pull/507