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

[Stefan Karlsson]

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
>>>