MetaspaceChunkFreeListSummary in JFR

[Thomas Stüfe]

Hi,

We have EventMetaspaceChunkFreeListSummary, which is an event collecting
intimate information about the metaspace chunk freelist.

It shows up in JMC in the Event Browser under
JVM->GC->Metaspace->MetaspaceChunkFreeListSummary .

I'd argue that its usefulness in its current form is limited and could cut
down.

For one, in this detail it is probably only useful to VM developers who
know what Metaspace does and how it is implemented.

To gauge the amount of waste (the delta between metaspace used for useful
stuff and getting wasted) it is not useful since the freelists are only
part of the wastage that can happen. Also, for that purpose we have the
"Metaspace Summary" event.

And for someone who is really interested in the deeper implementation of
Metaspace there are better analysis tools (jcmd VM.metaspace).

With the upcoming JDK-8221173, things get more foggy, since chunks in the
freelist can become uncommitted and therefore do not contribute to RSS;
hence these numbers will tell us even less.

If we keep this event, I'd suggest to cut it down to just "# chunks" and
"total size". And when  JDK-8221173 hits, we may extend this to "total
reserved size" and "total committed size".

Any opinions?

Thanks! Thomas