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

Erik Österlund erik.osterlund at oracle.com
Mon Feb 18 09:35:15 UTC 2019

Hi Stefan,

Looks good!


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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