[ruby-core:112440] [Ruby master Feature#19435] Expose counts for each GC reason in GC.stat

Issue #19435 has been updated by Eregon (Benoit Daloze).


byroot (Jean Boussier) wrote in #note-3:
> Our initial patch had those but I removed it to limit the number of extra keys, and because it's not a cause you should see in production, ever, aside from pre-fork memory optimization etc. 

It shouldn't doesn't mean it doesn't unfortunately.
I remember some horrible hack in oj which disabled and enabled the GC around a piece of C code for instance.
So I think it's very valuable to add that and it makes a lot of sense with other keys added here.
If it's low all good, if it's high, it is worth investigating, some gem is probably misbehaving.
Also this is a key I would happily add to TruffleRuby and I suspect JRuby would add it too (it's a very bad idea to force GC on JVM, that's very slow and destroys GC heuristics).

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Feature #19435: Expose counts for each GC reason in GC.stat
https://bugs.ruby-lang.org/issues/19435#change-101884

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
### Context

We recently tuned the GC settings on our monolith application because we were seeing some very long GC pauses (multiple seconds) during some requests.

Very early we realized that we could know how often the GC was triggered, and how long it was taking, but we had no information as to why, hence no good way
to know which specific configuration to tune. As of today, the only way to get this information is to compile Ruby with debug counters, but that's not really
accessible for most users, and not very suitable to be deployed in production.

So we patched our Ruby to expose counters for each specific reason in `GC.stat` and this data was extremely valuable.

For instance we discovered that the number 1 cause of major GC was `shady` objects, which allowed us to both better tune or GC and to drive some
targeted patches to Ruby.


### Proposal

We'd like to merge the patch we used on our Ruby build. It expose 8 new keys in `GC.stat`:

- `:major_gc_nofree_count`
- `:major_gc_oldgen_count`
- `:major_gc_shady_count`
- `:major_gc_newobj_count`
- `:major_gc_malloc_count`
- `:major_gc_oldmalloc_count`
- `:minor_gc_newobj_count`
- `:minor_gc_malloc_count`

Some very uncommon reasons like `force` etc are ignored as they're not valuable.

Also note that sometimes multiple conditions can be met to trigger GC, in such case we my increment several counters, so the sum of `major_gc_*_count` can be higher than `major_gc_count`.


Proposed patch: https://github.com/ruby/ruby/pull/7250



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