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

Stefan Karlsson stefan.karlsson at oracle.com
Mon Feb 18 09:37:45 UTC 2019

Thanks, Erik.


On 2019-02-18 10:35, Erik Österlund wrote:
> Hi Stefan,
> Looks good!
> Thanks,
> /Erik
> On 2019-02-15 20:25, Stefan Karlsson wrote:
>> Testing showed that the re-enabling of the retiring of TLABs was 
>> broken. This has been fixed with this patch:
>> http://cr.openjdk.java.net/~stefank/8218922/webrev.03.delta
>> http://cr.openjdk.java.net/~stefank/8218922/webrev.03
>> During mark start we change what's considered the "good" view of the 
>> heap, and if -XX:+ZUnmapBadViews is turned on we actually unmap the 
>> previous good view. This happens before TLABs are retired and filler 
>> objects get written. This used to work when we didn't create filler 
>> objects for TLABs, but know that we do, we need to ensure that the 
>> TLAB pointers have the correct colors, otherwise we'll end up crashing 
>> when -XX:+ZUnmapBadViews are used.
>> With this fix, the patches passes tier1,tier2, and tier3 testing.
>> Thanks,
>> StefanK
>> 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

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