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

[Erik Österlund]

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