RFR: JDK-8215264 add parallel heap inspection support for jmap histo(G1)(Internet mail)

[linzang(臧琳)]

So sorry the number in this title is wrong. please ignore it !
so sorry about making this mistake. 
will re post with correct number. 
--------------
Lin
>Re-post this RFR with enhancement number to make it trackable.
>webrev: http://cr.openjdk.java.net/~lzang/jmap-8214535/8215264/webrev_01/
>bug: https://bugs.openjdk.java.net/browse/JDK-8215624
>CSR: https://bugs.openjdk.java.net/browse/JDK-8239290
> 
>Thanks!
>--------------
>Lin
>>Hi Lin,
>>
>>Could you, please, re-post your RFR with the right enhancement number in
>>the message subject?
>>It will be more trackable this way.
>>
>>Thanks,
>>Serguei
>>
>>
>>On 2/17/20 10:29 PM, linzang(臧琳) wrote:
>>> Dear David,
>>>        Thanks a lot!
>>>       I have updated the refined code to http://cr.openjdk.java.net/~lzang/jmap-8214535/8215264/webrev_01/.
>>>        IMHO the parallel heap inspection can be extended to all kinds of heap as long as the heap layout can support parallel iteration.
>>>        Maybe we can firstly use this webrev to discuss how to implement it, because I am not sure my current implementation is an appropriate way to communicate with collectedHeap, then we can extend the solution to other kinds of heap.
>>>    
>>> Thanks,
>>> --------------
>>> Lin
>>>> Hi Lin,
>>>>
>>>> Adding in hotspot-gc-dev as they need to see how this interacts with GC
>>>> worker threads, and whether it needs to be extended beyond G1.
>>>>
>>>> I happened to spot one nit when browsing:
>>>>
>>>> src/hotspot/share/gc/shared/collectedHeap.hpp
>>>>
>>>> +   virtual bool run_par_heap_inspect_task(KlassInfoTable* cit,
>>>> +                                          BoolObjectClosure* filter,
>>>> +                                          size_t* missed_count,
>>>> +                                          size_t thread_num) {
>>>> +     return NULL;
>>>>
>>>> s/NULL/false/
>>>>
>>>> Cheers,
>>>> David
>>>>
>>>> On 18/02/2020 2:15 pm, linzang(臧琳) wrote:
>>>>> Dear All,
>>>>>         May I ask your help to review the follow changes:
>>>>>         webrev:
>>>>> http://cr.openjdk.java.net/~lzang/jmap-8214535/8215264/webrev_00/
>>>>>      bug: https://bugs.openjdk.java.net/browse/JDK-8215624
>>>>>      related CSR: https://bugs.openjdk.java.net/browse/JDK-8239290
>>>>>         This patch enable parallel heap inspection of G1 for jmap histo.
>>>>>         my simple test shown it can speed up 2x of jmap -histo with
>>>>> parallelThreadNum set to 2 for heap at ~500M on 4-core platform.
>>>>>
>>>>> ------------------------------------------------------------------------
>>>>> BRs,
>>>>> Lin
>>> >
>>