RFR: 8241361: ZGC: Implement memory related JFR events

[Stefan Karlsson]

Updated webrevs:
  https://cr.openjdk.java.net/~stefank/8241361/webrev.04.delta/
  https://cr.openjdk.java.net/~stefank/8241361/webrev.04/

Changes after some testing and discussions with Per:

1) For some reason it's required by JFR testing that the .jfc files list 
the stackTrace value. Fixed that.
2) Only generate ZUncommit when we actually have uncommitted memory.
3) Send relocation set group events for large pages.
4) Turn off both logging and event generation when medium pages have 
been disabled.

Thanks,
StefanK

On 2020-03-20 14:43, Stefan Karlsson wrote:
> Hi all,
> 
> Please review this patch to add some memory related JFR events to ZGC.
> 
> https://cr.openjdk.java.net/~stefank/8241361/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8241361
> 
> Added events:
> 
> ZAllocationStall - Record when we run out of heap memory and the Java 
> threads stall, waiting for the GC to free up memory.
> 
> ZPageAllocation - Updated the existing event to also record the duration 
> of the event. Updated the event to only be reported if the allocation 
> takes longer than 1 ms.
> 
> ZPageCacheFlush - Record when the page cache needs to be flushed. This 
> usually happens when we run out of a specific page size and have to 
> detach the physical and virtual memory to materialize a new ZPage. We 
> also flush pages when we uncommit memory.
> 
> ZRelocationSet - Record information about the selected relocation set.
> 
> ZUncommit - Record when we uncommit and hand back memory to the OS.
> 
> The patch also contains some small cosmetic changes to existing events, 
> whitespace fixes.