Determining "GC" memory area size

[Zhengyu Gu]

On 11/10/2017 01:47 PM, Aleksey Shipilev wrote:
> 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.

Java heap actually is *not* tagged as GC memory, but Java heap.

-Zhengyu

> 
> 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
> 
>