RFR/C: 8218922: SA: Enable best-effort implementation of live regions iteration for ZGC
stefan.karlsson at oracle.com
Mon Feb 18 09:37:45 UTC 2019
On 2019-02-18 10:35, Erik Österlund wrote:
> 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:
>> 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.
>> 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.
>>> 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
>>> 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
>>> 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.
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