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

[Stefan Karlsson]

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
>>