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

[Stefan Karlsson]

Thanks for taking a look at this!

StefanK

On 2019-02-14 10:58, Erik Österlund wrote:
> Hi Stefan,
> 
> I think this makes things better. I like the cleanups and I like the 
> partial support for heap parsing. I think we should bring this in, and 
> it looks good to me.
> 
> Of course we can always improve this further at some point in the future 
> to deal with broken Klass pointers better.
> 
> Thanks,
> /Erik
> 
> On 2019-02-13 15:52, 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