8356968: JFR: Compilation event should be enabled for all compilations by default

[shipilev]

In the field, we are using -XX:+PrintCompilation to track compiler performance. Alternatively, -Xlog:jit* prints the same. JFR has a the related jdk.Compilation event that gives us even richer diagnostics. Yet, that event is set at a very high threshold (1000ms), which skips almost all compilations! This threshold is set to such a high value from the beginning.

It is fairly normal to have lots of compilations in 100+ ms range individually, and their sum impact is what we are after. Also, the compilations are normally quite rare, and there are a couple of thousands of compiles in most workloads, and they only happen sporadically. This means, the event count without any threshold is not high.

Therefore, it would be convenient to make sure that basic Compilation event is enabled unconditionally, e.g. by dropping the default threshold to 0.

See more logs in the bug itself.

Additional testing:

• Ad-hoc tests with printing compilation events
• Linux x86_64 server fastdebug, jdk_jfr

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Issue

• JDK-8356968: JFR: Compilation event should be enabled for all compilations by default (Enhancement - P4)

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[mlbridge]

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[egahlin]

The overhead of the event is low, I believe, but I think the concern was that too many events were generated with little value to the end user. The buffers will be filled, and other, more important events will be pushed out. That said, there are probably other JVM events, i.e., low-level GC or runtime events, that are on by default, which may be less interesting than the compilation event.

jfr summary lists the number of events and how much space they take up.

Maybe we should reprioritize them?

[shipilev]

The overhead of the event is low, I believe, but I think the concern was that too many events were generated with little value to the end user.

I can see that. But the neat part about compilation events is that we are supposed to have a limited number of them. As system goes into steady state, compilations are supposed to become very rare. And if they are not, that is actually a signal something is really off, and we do want to know :)

jfr summary lists the number of events and how much space they take up.

I think current overheads are fair.

For example, on javac HelloWorld workload, jdk.Compilation is about 1K events, about 25K in total (~ 25 bytes/event), on-par with system process, flags, settings events. Takes ~7% of the dump on javac Hello World, and that is likely a high estimate, as longer apps would likely start dumping much more non-compilation events. Sounds to me doing a ThreadDump 3 times already takes more space than the sum total of compilation events.

 $ ls -lah comp.jfr
-rw-rw-r-- 1 shade shade 365K May 14 14:00 comp.jfr

$ jfr summary comp.jfr | sort -k 3 -n -r | head -n 10
 jdk.CheckPoint                             14        134299
 jdk.Metadata                                1        107126
 jdk.SystemProcess                         546         32949
 jdk.Compilation                           946         23678
 jdk.ThreadDump                              2         18679
 jdk.BooleanFlag                           492         14607
 jdk.ActiveSetting                         356          8846
 jdk.ModuleExport                          504          5414
 jdk.LongFlag                              136          4237
 jdk.InitialEnvironmentVariable             73          3077
...