RFR: 8287135: Calculation of jmm_GetMemoryUsage is wrong

[Mandy Chung]

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`.

Memory pool API does not support "nested" memory pools (or sub-memory pools).  JDK 18 memory pools are the following:

HEAP      G1 Eden Space
HEAP      G1 Old Gen
HEAP      G1 Survivor Space
NON_HEAP  CodeHeap 'non-nmethods'
NON_HEAP  Metaspace
NON_HEAP  CodeHeap 'profiled nmethods'
NON_HEAP  Compressed Class Space
NON_HEAP  CodeHeap 'non-profiled nmethods'


If `Metaspace` memory pool contains the compressed class space, then I agree with David that the current non-heap memory pools need to be examined. The meta space should be separated into multiple non-heap memory pools without overlap.

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