[ruby-core:106298] [Ruby master Feature#18364] Add GC.stat_size_pool for Variable Width Allocation

Issue #18364 has been updated by peterzhu2118 (Peter Zhu).


Thanks for the TruffleRuby and JRuby info @eregon! Maybe we could extend `GC.stat` to include size pool info in an array. For example:

```
pp GC.stat
{:count=>21,
 :heap_allocated_pages=>99,
...
 :size_pools=>
  [
   {:slot_size=>40,
    :heap_allocatable_pages=>4,
    :heap_eden_pages=>40,
    :heap_eden_slots=>1234,
    :heap_tomb_pages=>0,
    :heap_tomb_slots=>0},
   {:slot_size=>80,
    :heap_allocatable_pages=>80,
    :heap_eden_pages=>14,
    :heap_eden_slots=>123,
    :heap_tomb_pages=>0,
    :heap_tomb_slots=>0}
   ...
  ],
...
}
```

However, I think I think that this would be a breaking change for those that assume all the values in the hash are of type Numeric.

----------------------------------------
Feature #18364: Add GC.stat_size_pool for Variable Width Allocation
https://bugs.ruby-lang.org/issues/18364#change-94924

* Author: peterzhu2118 (Peter Zhu)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
# 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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