RFR: 8317755: G1: Periodic GC interval should test for the last whole heap GC [v2]

[Thomas Schatzl]

On Wed, 18 Oct 2023 16:19:08 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> I believe that having periodic Garbage Collections (whether they be young, concurrent, full or mixed) may be something that user applications need in certain scenarios. However, the current JVM lacks a way for Java to invoke a specific kind of GC cycle. If the WhiteBox API (or something equivalent) were available in Java, a user application could easily implement periodic GCs, 

The user can already do this to some degree with `System.gc()` and `-XX:+ExplicitGCInvokesConcurrent`.

I think the point @shipilev (and @gctony) make is that there are resources (not only Java heap memory) that ought to be cleaned up "in time" to keep resource usage and performance "optimal". The end user has even less insight into this than the VM - being "idle" is only a convenient time (for the user) to do that, so making the end user doing random stuff does not make anyone happy.

This is corroborated by experience from me working with end users actually doing that (i.e. force a whole heap analysis externally). The solution was to tell them to not do that because they were spamming these to an extent very detrimental to performance - because they did not know better (it did not help that at that time a forced concurrent cycle interrupted an ongoing one, so it happened that effectively they went into a full gc sometimes).

Both ZGC and Shenandoah have this regular cleanup (and CMS also provided the option) based on time. Not the best solution, but workable.

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PR Comment: https://git.openjdk.org/jdk/pull/16107#issuecomment-1770279786