RFR/C: 8218922: SA: Enable best-effort implementation of live regions iteration for ZGC

[Kevin Walls]

Hi Stefan,

This is great - if I want to try it out, what order do the changes 
apply, does the 8218746 webrev apply to create ZExternalBitMap.java and 
then the 8218922 webrev?

Thanks
Kevin


On 14/02/2019 17:12, Stefan Karlsson wrote:
> Hi again,
>
> I've separated the live regions iteration refactoring into this patch:
> https://cr.openjdk.java.net/~stefank/8219003/webrev.01/
>
> And use this RFE for the ZGC specific parts:
> https://cr.openjdk.java.net/~stefank/8218922/webrev.02/
>
> Thanks,
> StefanK
>
> On 2019-02-14 14:39, Stefan Karlsson wrote:
>> Hi Yasumasa,
>>
>> On 2019-02-14 14:11, Yasumasa Suenaga wrote:
>>> Hi Stefan,
>>>
>>>> Maybe this is enough to enable a bit more SA debugging capabilities 
>>>> when
>>>> running with ZGC? What do you think, should we bring in this change?
>>>
>>> I think it should be brought this in.
>>> I filed same issue as JDK-8207843, but I've not yet worked.
>>> So I will close it as duplicate of your change.
>>>
>>>
>>>> To be able to implement this more cleanly I've restructured the live
>>>> region collection, and pushed GC specific code into the specific GCs.
>>>> There are some extra usage of generics to make the code a bit 
>>>> easier to
>>>> read and develop.
>>>
>>> IMHO refactoring for live region collection and ZGC related changes 
>>> should
>>> be separated. What do you think?
>>
>> Yes. I think that would be good. I'll separate this out into two 
>> changes.
>>
>>>
>>>
>>> Your change looks good to me.
>>> BTW, did you check `jhsdb jmap --binaryheap` with this change?
>>>
>>
>> Yes, when testing this I ran all tests in serviceability/sa and 
>> manually tested the command above.
>>
>> While testing this I also had this patch applied to enable SA hprof 
>> for ZGC:
>> http://cr.openjdk.java.net/~stefank/8218970/webrev.01/
>>
>> And used this patch to turn off the ZGC filtering in the tests:
>> http://cr.openjdk.java.net/~stefank/8218978/webrev.01/
>>
>> I'm currently rerunning the tests to see that the latest changes 
>> didn't break anything.
>>
>> Thanks,
>> StefanK
>>>
>>> Thanks,
>>>
>>> Yasumasa
>>>
>>>
>>>
>>> On 2019/02/13 23:52, Stefan Karlsson wrote:
>>>> Hi all,
>>>>
>>>> Please review / comment on this patch to enable a best-effort live 
>>>> heap
>>>> region iteration implementation in ZGC.
>>>>
>>>> http://cr.openjdk.java.net/~stefank/8218922/webrev.01/
>>>> https://bugs.openjdk.java.net/browse/JDK-8218922
>>>>
>>>> The SA has functionally that relies on live heap region information 
>>>> from
>>>> the GCs. This is problematic when dead objects are left in the 
>>>> heap, and
>>>> their classes have been unloaded.
>>>>
>>>> Because of this ZGC has so far not implemented this feature. 
>>>> However, we
>>>> could provide a best-effort implementation that most likely will 
>>>> work if
>>>> classes are not unloaded (or class unloading is turned off), and
>>>> otherwise might fail to fully parse and report all live heap regions.
>>>>
>>>> For active, non-relocating pages the patch simply returns [start, top)
>>>> and for pages being actively relocated, it reports regions containing
>>>> the non-forwarded objects, the other objects are either dead or 
>>>> could be
>>>> found in one of the to-regions.
>>>>
>>>> With this patch I'm able to get heap histograms with ZGC.
>>>>
>>>> Maybe this is enough to enable a bit more SA debugging capabilities 
>>>> when
>>>> running with ZGC? What do you think, should we bring in this change?
>>>>
>>>> To be able to implement this more cleanly I've restructured the live
>>>> region collection, and pushed GC specific code into the specific GCs.
>>>> There are some extra usage of generics to make the code a bit 
>>>> easier to
>>>> read and develop.
>>>>
>>>> Thanks,
>>>> StefanK
>>>>
>