RFR: 8287135: Calculation of jmm_GetMemoryUsage is wrong

[David Holmes]

On Mon, 23 May 2022 07:28:41 GMT, Yi Yang <yyang at openjdk.org> wrote:

> It seems that calculation of MemoryMXBean.getNonHeapMemoryUsage(jmm_GetMemoryUsage) is wrong.
> 
> Currently, `NonHeapUsage=CodeCache+Metaspace(ClassTypeSpace+NonClassTypeSpace)+CompressedClassSpace(ClassTypeSpace)`
> 
> ==> CodeHeap 'non-nmethods' 1532544 (Used)
> ==> CodeHeap 'profiled nmethods' 0
> ==> CodeHeap 'non-profiled nmethods' 13952
> ==> Metaspace 506696
> ==> Compressed Class Space 43312
> init = 7667712(7488K) used = 2096504(2047K) committed = 8454144(8256K) max = -1(-1K)
> 
> In this way, getNonHeapMemoryUsage is larger than it ought to be, it should be `NonHeapUsage = CodeCache + Metaspace`.

The basic problem is that we have two non-heap pools:
- `MetaspacePool`
  - consists of `ClassType` and `NonClassType` parts
- `CompressedKlassSpacePool`

but the `CompressedKlassSpacePool` is actually the "ClassType" part of the `MetaspacePool`!

I think the right fix is to just convert the `MetaspacePool` into `NonClassMetaspacePool` and only report the non-class values.

AFAICS this will only be visible via the MXBean.

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PR: https://git.openjdk.java.net/jdk/pull/8831