RFR: 8359110: Log accumulated GC and process CPU time upon VM exit [v3]

[Thomas Schatzl]

On Fri, 13 Jun 2025 10:45:15 GMT, Jonas Norlinder <duke at openjdk.org> wrote:

> > The argument that "the user could forget specifying it", is somewhat weak imo - in that case one could argue why there are those labels, and I kind of doubt that GC cpu usage at the end only is that important to have for everyone every time.
> 
> I disagree, it is equally important as reporting pre and post compaction heap usage like we do now with -Xlog:gc. Users who are not experts in GC may underestimate the CPU cost of GC at a given heap max. Even experts in academia tend to run with too small heap. I maintain my position that adding it at the end is crucial.

I do not disagree about the usefulness of the message (it is - I even liked the nominal output), I only somewhat disagree about making a message purposefully printed at VM exit, to state the cpu usage at exit, not having the "exit" label.
(Regarding the nominal logging: my points were that I thought it was a waste to use three messages, and the presentation format)

We do not need to agree on everything 100%.

> Additionally, if we want to we can also add capabilities to track it continuously with JFR and/or MXBeans. But that may introduce a performance penalty as sampling may not be free so I want to still keep logging it at the end as a base case.

That's fine.

Fwiw, there has even been interest from me (https://bugs.openjdk.org/browse/JDK-8349476) about regularly printing these statistics at even higher detail.

However as far as I understand, there are jstat/perf counters already for that, and they are in use (in industry). There is the jcmd command (even before that) that prints per-thread cpu usage for some time now - one could filter out the interesting threads manually...

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PR Comment: https://git.openjdk.org/jdk/pull/25779#issuecomment-2970035555