RFR: 8351142: Add JFR monitor deflation and statistics events

[Erik Gahlin]

On Tue, 4 Mar 2025 14:47:09 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> We already have JFR JavaMonitorInflate event, which tells when the monitor is inflated. We are missing JavaMonitorDeflate event, which would tell us when the monitor is deflated. This makes it hard to see the monitor lifecycle, and/or estimate the population of currently inflated monitors. I believe we should add JavaMonitorDeflate event. It would also be useful to have the statistics for the number of currently used/deflating monitors. Deflation event alone would require post-processing to investigate this, so it would be good to have the statistics event as well.
> 
> This would also replace two of the RT counters that are going away in [JDK-8348829](https://bugs.openjdk.org/browse/JDK-8348829).
> 
> Monitor deflation is done asynchronously in `MonitorDeflationThread`, so the additional overhead of recording the deflation events would likely be performance neutral. We still only enable the statistics event by default to be on a safer side.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `jdk_jfr`

src/hotspot/share/jfr/metadata/metadata.xml line 124:

> 122:   </Event>
> 123: 
> 124:   <Event name="JavaMonitorStatistics" category="Java Application" label="Java Monitor Statistics" startTime="false">

For consistency with other statistical events, the category for JavaMonitorStatistics should be "Java Application, Statistics" 

The event should probably be periodic, so users can set an interval to reduce the number of events, with a default period of "everyChunk", so it is emitted at least at the beginning and end of a recording.

src/hotspot/share/jfr/metadata/metadata.xml line 125:

> 123: 
> 124:   <Event name="JavaMonitorStatistics" category="Java Application" label="Java Monitor Statistics" startTime="false">
> 125:     <Field type="ulong" name="totalCount" label="Monitors In Use" description="Number of in-use monitors"/>

The label should be 'Monitor in Use' (lowercase 'i').

Here is the style guideline if you're wondering.
https://docs.oracle.com/en/java/javase/21/docs/api/jdk.jfr/jdk/jfr/Label.html

test/jdk/jdk/jfr/event/runtime/TestJavaMonitorDeflateEvent.java line 82:

> 80:             waitThread.join();
> 81:             // Let deflater thread run.
> 82:             Thread.sleep(3000);

I see that you took code from the MonitorInflate test. It's a really old test. A RecordingStream would be a more suitable as you can avoid using Thread.sleep() and the TestThread. I don't think a file needs to be dumped if the events are printed to standard out. Something like this:


String lockClassName = lock.getClass().getName();
List<RecordedEvent> events = new CopyOnWriteArrayList<>();
try (RecordingStream rs = new RecordingStream()) {
   rs.enable(EVENT_NAME).withoutThreshold();
   rs.onEvent(EVENT_NAME, e -> {
        RecordedClass clazz = e.getType(FIELD_KLASS_NAME);
        if (clazz.getName().equals(lockClassName)) {
           rs.close();
        }
   });
   rs.startAsync(); 
   ...
   synchronized (lock) {
   ...
   }
   ...
   rs.awaitTermination();
   System.out.println(events);
   RecordedEvent event = events.get(0);
   Events.assertField(event, FIELD_ADDRESS).notEqual(0L);
}

test/jdk/jdk/jfr/event/runtime/TestJavaMonitorStatisticsEvent.java line 60:

> 58:         Recording recording = new Recording();
> 59:         recording.enable(EVENT_NAME).withThreshold(Duration.ofMillis(0));
> 60:         final Lock lock = new Lock();

If the event is periodic, you can set:

`recording.enable(EVENT_NAME).with("period", "everyChunk");`

and use the following instead of isAnyFound:

List<RecordedEvent> events = Events.fromRecording(recording);
Events.hasEvents(events);

There's no need to dump to failed.jfr. Events.fromRecording will create a file that can be inspected in case the test fails. try-with-resources would be nice to have.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/23900#discussion_r1979685743
PR Review Comment: https://git.openjdk.org/jdk/pull/23900#discussion_r1979647552
PR Review Comment: https://git.openjdk.org/jdk/pull/23900#discussion_r1979758837
PR Review Comment: https://git.openjdk.org/jdk/pull/23900#discussion_r1979896549