RFR: JDK-8289524: Add JFR JIT restart event [v2]

[Markus Grönlund]

On Tue, 5 Jul 2022 13:47:31 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

>> The JIT compiler restarts (see restart_compiler in NMethodSweeper::sweep_code_cache) would be a helpful addition to the JFR events. Currently we log the JIT stop operations in JFR (EventCodeCacheFull) but no restart.
>
> Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Incorporate JIT compiler restart into EventSweepCodeCache

Hi again Mathias, based on your observation that this does not really align well with the threshold parameter for SweepCodeCache and is a complement event to EventCodeCacheFull, I believe your original suggestion is better (an instant event, only for "jit restart"). I was slightly tripped up with the concept of a  "JIT restart", because at this point the JIT is already stopped, as implied by EventCodeCacheFull. So "JIT restart" here contextually means, "JIT start", as in "start JITting code again". Do you know if we have an event that describes the CodeCache settings in terms of memory sizes so that the "freed memory" can be interpreted relative to it?

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PR: https://git.openjdk.org/jdk/pull/9334