Determining "GC" memory area size

[Aleksey Shipilev]

On 11/10/2017 06:06 PM, Glyn Normington wrote:
> Can anyone here tell me how the GC memory area size is determined? If there
> is documentation, so much the better as we'd prefer not to depend on code
> details that might flux arbitrarily.

NMT reports all allocations from GC code (with mtGC tag) as "GC". This includes, notably, Java heap
itself, and all auxiliary GC data structures. If you grep the OpenJDK source for "mtGC", you can get
the idea what allocations are tagged. Also, "detailed" NMT mode would give you allocation stacks,
which can also give some insight what those particular allocated bits are coming from.

The size of GC data structures is dependent on GC in question (and, quite probably, depends on GC
implementation *version*, with swings back and forth), and thus is not trivially deducible. For
simpler collectors, like Serial and Parallel, it would probably be dominated by Card Table (1/512-th
of heap size). In G1, it is most probably mark bitmaps (2/64-th of heap size), plus other
fine-grained remembered sets (which might get large, but maybe there is a high bound?). Etc.

Thanks,
-Aleksey