RFR/C: 8218922: SA: Enable best-effort implementation of live regions iteration for ZGC
stefan.karlsson at oracle.com
Wed Feb 13 14:52:05 UTC 2019
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 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.
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