[ruby-core:106299] [Ruby master Feature#18364] Add GC.stat_size_pool for Variable Width Allocation

Issue #18364 has been updated by Eregon (Benoit Daloze).


I would avoid extending `GC.stat`, I think it is good it only contains `Symbols` to `Integer`, and there is probably code relying on that.
Also `GC.stat(key)` seems relatively performance-sensitive as it's used in Rails for every request.

I think a new method is good.
Maybe GC.heap_stats/pool_stats/stat_pool` or so?

I think it would be useful if this new method when given no arguments return a Hash of all heaps/memory pools, so it also serves as an easy way to list heaps/memory pools.
And with an argument, only the stats for that specific heap/memory pool, and with 2 arguments the given stat for that memory pool.

That way, whether a heap/memory pool is identified by a size or a name it would work in both cases.

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Feature #18364: Add GC.stat_size_pool for Variable Width Allocation
https://bugs.ruby-lang.org/issues/18364#change-94925

* Author: peterzhu2118 (Peter Zhu)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
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# GitHub PR: https://github.com/ruby/ruby/pull/5177

We're proposing an API to get statistics for size pools for Variable Width Allocation similar to `GC.stat`. This will make it easier for us (and other developers) to tune VWA.

Before 3.1 release, we plan to keep this method hidden from the documentation using `:nodoc:` since it is not useful when not using VWA.

For example:

```ruby
# Get stats for size pool 2
puts GC.stat_size_pool(2)
#=> {:slot_size=>160, :heap_allocatable_pages=>80, :heap_eden_pages=>14, :heap_eden_slots=>1424, :heap_tomb_pages=>0, :heap_tomb_slots=>0}
puts GC.stat_size_pool(2, :heap_eden_pages)
#=> 14
```

We aim to keep the keys in the outputted hash the same as the keys used in `GC.stat`.

We chose to implement a new method instead of re-using an existing API (`GC.stat`) because the keys returned by `GC.stat_size_pool` will not be the same as `GC.stat`. We believe that having `GC.stat` return different shapes of hashes based on its arguments is confusing.




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