RFR: 8238665: Add JFR event for direct memory statistics

[Denghui Dong]

Hi Erik,

Thanks for the review. See answers embedded.

New webrev: http://cr.openjdk.java.net/~ddong/8238665/webrev.01/ <http://cr.openjdk.java.net/~ddong/8238665/webrev.01/>

> On Feb 11, 2020, at 1:52 AM, Erik Gahlin <erik.gahlin at oracle.com> wrote:
> 
> Hi Denghiu,
> 
> Can you explain when this event can be useful, i.e. when troubleshooting x etc.? This makes it easier to determine if the event should be on by default, if it is worth the increased startup cost that the instrumentation creates and what a reasonable period for the event should be.
> 
Many Java middleware(e.g. Netty) and applications use direct memory for achieving higher performance,  if java process use too much direct memory, some exceptions will occur, such as oom-killer. Hence, it’s necessary to monitor it,  and we use JMX to monitor it in our production environment currently.

> src/jdk.jfr/share/classes/jdk/jfr/events/DirectMemoryStatisticsEvent.java:
> 
> @Label("TotalCapacity") -> @Label("Total Capacity")
> @Label("MemoryUsed") -> @Label("Memory Used”)
> 
Updated

> What is meant by 'limit'? Could we have a better field name and/or description that explains what this fields contains, because it is not obvious from that name. Fields that contains bytes, should have the DataAmount; annotation.
> 
> What is the value if -XX:MaxDirectMemorySize has not been set?
> 
limit -> maxCapacity

 if -XX:MaxDirectMemorySize has not been set, the value of Runtime.getRuntime().maxMemory() will be used.

> Can you make the event class final.
> 
Updated

> @StackTrace(false) should not be needed.
> 
Removed

> test/jdk/jdk/jfr/event/runtime/TestDirectMemoryStatisticsEvent.java:
> 
> Use try-with-resources on the Recording object.
> 
Updated

Thanks
Denghui Dong

> Thanks
> Erik
> 
> On 2020-02-07 06:24, Denghui Dong wrote:
>> Hi,
>> 
>> Could I have a review of a change that adds JFR event for direct memory statistics
>> 
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8238665
>> Webrev: http://cr.openjdk.java.net/~ddong/8238665/webrev.00/
>> Test: test/jdk/jdk/jfr/event/runtime/TestDirectMemoryStatisticsEvent.java
>> 
>> Thanks
>> Denghui Dong