RFR: 8254012: NMT: MetaspaceSnapshot::snapshot uses wrong enum

[Yumin Qi]

On Wed, 14 Oct 2020 10:49:10 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> The change looks good // not Reviewer though.
>> Thanks for fixing!
>
>> > > I wonder why this bug has not been discovered until now. It seems like the old code would not work at all when
>> > > compressed class space is disabled. Do we have any test cases for it? Or, is this feature actually used?
>> > 
>> > 
>> > MetaspaceSnapshot predefine the _reserved_in_bytes (and other two, _commited_in_byte, _used_in_bytes) with dimension of
>> > MetadataTypeCount, the array of index ClassType is a valid slot. In this bug, if CCP is disabled, the snapshot for
>> > ClassType gets updated with zero, but non class type data is not update in snapshot. There is no output for class type
>> > data when CCP is disabled. While the original data recording is correct (at allocation) the snapshot did not get it. I
>> > will do some tests.
>> 
>> This is confusing to be sure. The part that is broken by this bug is using the basline..diff functionality if
>> compressed class space is off. All other metaspace numbers are still okay because they are printed via other code
>> paths.  Reproduction:
>> 
>> ```
>> >java -XX:-UseCompressedClassPointers -XX:NativeMemoryTracking=summary de.stuefe.repros.metaspace.JustLoadAndLoad &
>> > jcmd JustLoadAndLoad VM.native_memory baseline
>> 7518:
>> Baseline succeeded
>> > jcmd JustLoadAndLoad VM.native_memory summary.diff
>> 7518:
>> 
>> Native Memory Tracking:
>> 
>> Total: reserved=5015640KB +163200KB, committed=543088KB +161924KB
>> <skip>
>> -                     Class (reserved=427146KB +161234KB, committed=425458KB +159954KB)
>>                             (classes #6011 +1185)
>>                             (  instance classes #5706 +1185, array classes #305)
>>                             (malloc=9354KB +3538KB #41433 +14309)
>>                             (mmap: reserved=417792KB +157696KB, committed=416104KB +156416KB)
>>                             (  Metadata:   )   << no increase shown
>>                             (    reserved=0KB, committed=0KB)
>>                             (    used=0KB)
>>                             (    free=0KB)
>>                             (    waste=0KB =-nan%)
>> ```
>> 
>> And no, there are no tests for that. This is an old bug.
> 
> Not sure how Oracle handles this usually, but maybe it makes sense to reformulate the JBS issue and describe above
> effect? Or, alternatively, open a new one as a duplicate to this bug?
> When people look into the JBS for this specific NMT bug they may not notice the harmless test error.

@tstuefe @reinrich
Thanks for review!
I am not sure if JBS issue changed what the PR will react --- Skara requests exact match for the issue. File a new bug
with correct description as a dup of this one is OK I think.

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PR: https://git.openjdk.java.net/jdk/pull/645