RFR: 8252105: parallel heap inspection for ZCollectedHeap [v2]

[Per Liden]

On Fri, 11 Sep 2020 07:24:20 GMT, Lin Zang <lzang at openjdk.org> wrote:

>> Hi @pliden,
>>   Thanks for your comments, I will try to merge ZHeapIterator and ZHeapParIterator and update the pr then.
>> -Lin
>
> Hi @pliden,
>    I updated the PR which merge the changed code into ZHeapIterator.
>    May I ask your help to review it ? Thanks.
> -Lin

Hi @linzang,

I started reviewing your patch in more detail. However, I ended up with a large number of major comments and I started
to feel that it would probably be more productive to create an alternative patch for this, so I did.

https://github.com/openjdk/jdk/compare/master...pliden:8252105_parallel_object_iterator

Some of the main differences compared to your proposed patch:
* Root scanning is also now done in parallel
* Same code paths regardless of number of worker threads used
* Arrays are chunked for better work stealing/balance
* A terminator is used for better work stealing/balance
* All of the functionality is isolated (as much as possible) to ZHeapIterator

I think the easiest way forward is probably if you close this PR, and I'll open a new one.

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PR: https://git.openjdk.java.net/jdk/pull/103